Make the Store API more type-safe

Most functions now take a StorePath argument rather than a Path (which is just an alias for std::string). The StorePath constructor ensures that the path is syntactically correct (i.e. it looks like <store-dir>/<base32-hash>-<name>). Similarly, functions like buildPaths() now take a StorePathWithOutputs, rather than abusing Path by adding a '!<outputs>' suffix. Note that the StorePath type is implemented in Rust. This involves some hackery to allow Rust values to be used directly in C++, via a helper type whose destructor calls the Rust type's drop() function. The main issue is the dynamic nature of C++ move semantics: after we have moved a Rust value, we should not call the drop function on the original value. So when we move a value, we set the original value to bitwise zero, and the destructor only calls drop() if the value is not bitwise zero. This should be sufficient for most types. Also lots of minor cleanups to the C++ API to make it more modern (e.g. using std::optional and std::string_view in some places).
2019-12-05 19:11:09 +01:00 · 2019-12-05 19:11:09 +01:00 · bbe97dff8b
parent ebd89999c2
commit bbe97dff8b
98 changed files with 2638 additions and 2880 deletions
--- a/nix-rust/Cargo.lock
+++ b/nix-rust/Cargo.lock
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 "checksum rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)" = "6d71dacdc3c88c1fde3885a3be3fbab9f35724e6ce99467f7d9c5026132184ca"
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@ -1153,41 +422,12 @@ dependencies = [
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--- a/nix-rust/Cargo.toml
+++ b/nix-rust/Cargo.toml
@ -11,12 +11,12 @@ crate-type = ["cdylib"]
 [dependencies]
 tar = "0.4"
 libc = "0.2"
-futures-preview = { version = "=0.3.0-alpha.19" }
+#futures-preview = { version = "=0.3.0-alpha.19" }
-hyper = "0.13.0-alpha.4"
+#hyper = "0.13.0-alpha.4"
-http = "0.1"
+#http = "0.1"
-tokio = { version = "0.2.0-alpha.6", default-features = false, features = ["rt-full"] }
+#tokio = { version = "0.2.0-alpha.6", default-features = false, features = ["rt-full"] }
 lazy_static = "1.4"
-byteorder = "1.3"
+#byteorder = "1.3"
 [dev-dependencies]
 hex = "0.3"
--- a/nix-rust/local.mk
+++ b/nix-rust/local.mk
@ -37,7 +37,9 @@ clean: clean-rust
 clean-rust:
 	$(suppress) rm -rfv nix-rust/target
 ifneq ($(OS), Darwin)
 check: rust-tests
 rust-tests:
-	cd nix-rust && CARGO_HOME=$$(if [[ -d vendor ]]; then echo vendor; fi) RUSTC_BOOTSTRAP=1 cargo test --release $$(if [[ -d vendor ]]; then echo -Z offline; fi)
+	cd nix-rust && CARGO_HOME=$$(if [[ -d vendor ]]; then echo vendor; fi) cargo test --release $$(if [[ -d vendor ]]; then echo --offline; fi)
 endif
--- a/nix-rust/src/c.rs
+++ b/nix-rust/src/c.rs
@ -1,6 +1,7 @@
 use super::{
    error,
    foreign::{self, CBox},
    store::StorePath,
    util,
 };
@ -13,43 +14,72 @@ pub extern "C" fn unpack_tarfile(
 }
 #[no_mangle]
-pub extern "C" fn rust_test() {
+pub unsafe extern "C" fn ffi_String_new(s: &str, out: *mut String) {
-    use crate::store::{self, Store};
+    // FIXME: check whether 's' is valid UTF-8?
-    use std::path::Path;
+    out.write(s.to_string())
-    use tokio::runtime::Runtime;
+}
-
+
-    let fut = async move {
+#[no_mangle]
-        let store: Box<dyn Store> = Box::new(store::BinaryCacheStore::new(
+pub unsafe extern "C" fn ffi_String_drop(self_: *mut String) {
-            "http://cache.nixos.org".to_string(),
+    std::ptr::drop_in_place(self_);
-        ));
+}
-
+
-        let path = store
+#[no_mangle]
-            .parse_store_path(&Path::new(
+pub extern "C" fn ffi_StorePath_new(
-                "/nix/store/7h7qgvs4kgzsn8a6rb273saxyqh4jxlz-konsole-18.12.3",
+    path: &str,
-            ))
+    store_dir: &str,
-            .unwrap();
+) -> Result<StorePath, error::CppException> {
-
+    StorePath::new(std::path::Path::new(path), store_dir).map_err(|err| err.into())
-        /*
+}
-        let info = store.query_path_info(&path).await.unwrap();
+
-
+#[no_mangle]
-        eprintln!("INFO = {:?}", info);
+pub extern "C" fn ffi_StorePath_new2(
-         */
+    hash: &[u8; crate::store::path::STORE_PATH_HASH_BYTES],
-
+    name: &str,
-        let closure = store
+) -> Result<StorePath, error::CppException> {
-            .compute_path_closure(vec![path].into_iter().collect())
+    StorePath::from_parts(*hash, name).map_err(|err| err.into())
-            .await
+}
-            .unwrap();
+
-
+#[no_mangle]
-        eprintln!("CLOSURE = {:?}", closure.len());
+pub extern "C" fn ffi_StorePath_fromBaseName(
-    };
+    base_name: &str,
-
+) -> Result<StorePath, error::CppException> {
-    let rt = Runtime::new().unwrap();
+    StorePath::new_from_base_name(base_name).map_err(|err| err.into())
-
+}
-    rt.block_on(fut);
+
-
+#[no_mangle]
-    /*
+pub unsafe extern "C" fn ffi_StorePath_drop(self_: *mut StorePath) {
-    let file = std::fs::File::open("test.nar").unwrap();
+    std::ptr::drop_in_place(self_);
-
+}
-    crate::nar::parse(&mut std::io::BufReader::new(file)).unwrap();
+
-    */
+#[no_mangle]
 pub extern "C" fn ffi_StorePath_to_string(self_: &StorePath) -> String {
    format!("{}", self_)
 }
 #[no_mangle]
 pub extern "C" fn ffi_StorePath_less_than(a: &StorePath, b: &StorePath) -> bool {
    a < b
 }
 #[no_mangle]
 pub extern "C" fn ffi_StorePath_eq(a: &StorePath, b: &StorePath) -> bool {
    a == b
 }
 #[no_mangle]
 pub extern "C" fn ffi_StorePath_clone(self_: &StorePath) -> StorePath {
    self_.clone()
 }
 #[no_mangle]
 pub extern "C" fn ffi_StorePath_name(self_: &StorePath) -> &str {
    self_.name.name()
 }
 #[no_mangle]
 pub extern "C" fn ffi_StorePath_hash_data(
    self_: &StorePath,
 ) -> &[u8; crate::store::path::STORE_PATH_HASH_BYTES] {
    self_.hash.hash()
 }
--- a/nix-rust/src/error.rs
+++ b/nix-rust/src/error.rs
@ -19,6 +19,7 @@ pub enum Error {
    BadNarField(String),
    BadExecutableField,
    IOError(std::io::Error),
    #[cfg(unused)]
    HttpError(hyper::error::Error),
    Misc(String),
    Foreign(CppException),
@ -30,6 +31,7 @@ impl From<std::io::Error> for Error {
    }
 }
 #[cfg(unused)]
 impl From<hyper::error::Error> for Error {
    fn from(err: hyper::error::Error) -> Self {
        Error::HttpError(err)
@ -58,6 +60,7 @@ impl fmt::Display for Error {
            Error::BadNarField(s) => write!(f, "unrecognized NAR field '{}'", s),
            Error::BadExecutableField => write!(f, "bad 'executable' field in NAR"),
            Error::IOError(err) => write!(f, "I/O error: {}", err),
            #[cfg(unused)]
            Error::HttpError(err) => write!(f, "HTTP error: {}", err),
            Error::Foreign(_) => write!(f, "<C++ exception>"), // FIXME
            Error::Misc(s) => write!(f, "{}", s),
--- a/nix-rust/src/lib.rs
+++ b/nix-rust/src/lib.rs
@ -12,6 +12,7 @@ extern crate proptest;
 mod c;
 mod error;
 mod foreign;
 #[cfg(unused)]
 mod nar;
 mod store;
 mod util;
--- a/nix-rust/src/store/mod.rs
+++ b/nix-rust/src/store/mod.rs
@ -1,9 +1,17 @@
 pub mod path;
 #[cfg(unused)]
 mod binary_cache_store;
-mod path;
+#[cfg(unused)]
 mod path_info;
 #[cfg(unused)]
 mod store;
 pub use binary_cache_store::BinaryCacheStore;
 pub use path::{StorePath, StorePathHash, StorePathName};
 #[cfg(unused)]
 pub use binary_cache_store::BinaryCacheStore;
 #[cfg(unused)]
 pub use path_info::PathInfo;
 #[cfg(unused)]
 pub use store::Store;
--- a/nix-rust/src/store/path.rs
+++ b/nix-rust/src/store/path.rs
@ -23,6 +23,13 @@ impl StorePath {
        )
    }
    pub fn from_parts(hash: [u8; STORE_PATH_HASH_BYTES], name: &str) -> Result<Self, Error> {
        Ok(StorePath {
            hash: StorePathHash(hash),
            name: StorePathName::new(name)?,
        })
    }
    pub fn new_from_base_name(base_name: &str) -> Result<Self, Error> {
        if base_name.len() < STORE_PATH_HASH_CHARS + 1
            || base_name.as_bytes()[STORE_PATH_HASH_CHARS] != '-' as u8
@ -43,7 +50,7 @@ impl fmt::Display for StorePath {
    }
 }
-#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug)]
 pub struct StorePathHash([u8; STORE_PATH_HASH_BYTES]);
 impl StorePathHash {
@ -55,6 +62,10 @@ impl StorePathHash {
        bytes.copy_from_slice(&v[0..STORE_PATH_HASH_BYTES]);
        Ok(Self(bytes))
    }
    pub fn hash<'a>(&'a self) -> &'a [u8; STORE_PATH_HASH_BYTES] {
        &self.0
    }
 }
 impl fmt::Display for StorePathHash {
@ -63,6 +74,21 @@ impl fmt::Display for StorePathHash {
    }
 }
 impl Ord for StorePathHash {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        // Historically we've sorted store paths by their base32
        // serialization, but our base32 encodes bytes in reverse
        // order. So compare them in reverse order as well.
        self.0.iter().rev().cmp(other.0.iter().rev())
    }
 }
 impl PartialOrd for StorePathHash {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
 }
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Debug)]
 pub struct StorePathName(String);
@ -93,6 +119,10 @@ impl StorePathName {
        Ok(Self(s.to_string()))
    }
    pub fn name<'a>(&'a self) -> &'a str {
        &self.0
    }
 }
 impl fmt::Display for StorePathName {
--- a/perl/lib/Nix/Store.xs
+++ b/perl/lib/Nix/Store.xs
@ -59,7 +59,7 @@ void setVerbosity(int level)
 int isValidPath(char * path)
    CODE:
        try {
-            RETVAL = store()->isValidPath(path);
+            RETVAL = store()->isValidPath(store()->parseStorePath(path));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -70,9 +70,8 @@ int isValidPath(char * path)
 SV * queryReferences(char * path)
    PPCODE:
        try {
-            PathSet paths = store()->queryPathInfo(path)->references;
+            for (auto & i : store()->queryPathInfo(store()->parseStorePath(path))->references)
-            for (PathSet::iterator i = paths.begin(); i != paths.end(); ++i)
+                XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(i).c_str(), 0)));
                XPUSHs(sv_2mortal(newSVpv(i->c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -81,7 +80,7 @@ SV * queryReferences(char * path)
 SV * queryPathHash(char * path)
    PPCODE:
        try {
-            auto s = store()->queryPathInfo(path)->narHash.to_string();
+            auto s = store()->queryPathInfo(store()->parseStorePath(path))->narHash.to_string();
            XPUSHs(sv_2mortal(newSVpv(s.c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
@ -91,9 +90,9 @@ SV * queryPathHash(char * path)
 SV * queryDeriver(char * path)
    PPCODE:
        try {
-            auto deriver = store()->queryPathInfo(path)->deriver;
+            auto info = store()->queryPathInfo(store()->parseStorePath(path));
-            if (deriver == "") XSRETURN_UNDEF;
+            if (!info->deriver) XSRETURN_UNDEF;
-            XPUSHs(sv_2mortal(newSVpv(deriver.c_str(), 0)));
+            XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(*info->deriver).c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -102,18 +101,18 @@ SV * queryDeriver(char * path)
 SV * queryPathInfo(char * path, int base32)
    PPCODE:
        try {
-            auto info = store()->queryPathInfo(path);
+            auto info = store()->queryPathInfo(store()->parseStorePath(path));
-            if (info->deriver == "")
+            if (info->deriver)
                XPUSHs(&PL_sv_undef);
            else
-                XPUSHs(sv_2mortal(newSVpv(info->deriver.c_str(), 0)));
+                XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(*info->deriver).c_str(), 0)));
            auto s = info->narHash.to_string(base32 ? Base32 : Base16);
            XPUSHs(sv_2mortal(newSVpv(s.c_str(), 0)));
            mXPUSHi(info->registrationTime);
            mXPUSHi(info->narSize);
            AV * arr = newAV();
-            for (PathSet::iterator i = info->references.begin(); i != info->references.end(); ++i)
+            for (auto & i : info->references)
-                av_push(arr, newSVpv(i->c_str(), 0));
+                av_push(arr, newSVpv(store()->printStorePath(i).c_str(), 0));
            XPUSHs(sv_2mortal(newRV((SV *) arr)));
        } catch (Error & e) {
            croak("%s", e.what());
@ -123,8 +122,8 @@ SV * queryPathInfo(char * path, int base32)
 SV * queryPathFromHashPart(char * hashPart)
    PPCODE:
        try {
-            Path path = store()->queryPathFromHashPart(hashPart);
+            auto path = store()->queryPathFromHashPart(hashPart);
-            XPUSHs(sv_2mortal(newSVpv(path.c_str(), 0)));
+            XPUSHs(sv_2mortal(newSVpv(path ? store()->printStorePath(*path).c_str() : "", 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -133,11 +132,11 @@ SV * queryPathFromHashPart(char * hashPart)
 SV * computeFSClosure(int flipDirection, int includeOutputs, ...)
    PPCODE:
        try {
-            PathSet paths;
+            StorePathSet paths;
            for (int n = 2; n < items; ++n)
-                store()->computeFSClosure(SvPV_nolen(ST(n)), paths, flipDirection, includeOutputs);
+                store()->computeFSClosure(store()->parseStorePath(SvPV_nolen(ST(n))), paths, flipDirection, includeOutputs);
-            for (PathSet::iterator i = paths.begin(); i != paths.end(); ++i)
+            for (auto & i : paths)
-                XPUSHs(sv_2mortal(newSVpv(i->c_str(), 0)));
+                XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(i).c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -146,11 +145,11 @@ SV * computeFSClosure(int flipDirection, int includeOutputs, ...)
 SV * topoSortPaths(...)
    PPCODE:
        try {
-            PathSet paths;
+            StorePathSet paths;
-            for (int n = 0; n < items; ++n) paths.insert(SvPV_nolen(ST(n)));
+            for (int n = 0; n < items; ++n) paths.insert(store()->parseStorePath(SvPV_nolen(ST(n))));
-            Paths sorted = store()->topoSortPaths(paths);
+            auto sorted = store()->topoSortPaths(paths);
-            for (Paths::iterator i = sorted.begin(); i != sorted.end(); ++i)
+            for (auto & i : sorted)
-                XPUSHs(sv_2mortal(newSVpv(i->c_str(), 0)));
+                XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(i).c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -159,7 +158,7 @@ SV * topoSortPaths(...)
 SV * followLinksToStorePath(char * path)
    CODE:
        try {
-            RETVAL = newSVpv(store()->followLinksToStorePath(path).c_str(), 0);
+            RETVAL = newSVpv(store()->printStorePath(store()->followLinksToStorePath(path)).c_str(), 0);
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -170,8 +169,8 @@ SV * followLinksToStorePath(char * path)
 void exportPaths(int fd, ...)
    PPCODE:
        try {
-            Paths paths;
+            StorePathSet paths;
-            for (int n = 1; n < items; ++n) paths.push_back(SvPV_nolen(ST(n)));
+            for (int n = 1; n < items; ++n) paths.insert(store()->parseStorePath(SvPV_nolen(ST(n))));
            FdSink sink(fd);
            store()->exportPaths(paths, sink);
        } catch (Error & e) {
@ -275,8 +274,8 @@ int checkSignature(SV * publicKey_, SV * sig_, char * msg)
 SV * addToStore(char * srcPath, int recursive, char * algo)
    PPCODE:
        try {
-            Path path = store()->addToStore(baseNameOf(srcPath), srcPath, recursive, parseHashType(algo));
+            auto path = store()->addToStore(std::string(baseNameOf(srcPath)), srcPath, recursive, parseHashType(algo));
-            XPUSHs(sv_2mortal(newSVpv(path.c_str(), 0)));
+            XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(path).c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -286,8 +285,8 @@ SV * makeFixedOutputPath(int recursive, char * algo, char * hash, char * name)
    PPCODE:
        try {
            Hash h(hash, parseHashType(algo));
-            Path path = store()->makeFixedOutputPath(recursive, h, name);
+            auto path = store()->makeFixedOutputPath(recursive, h, name);
-            XPUSHs(sv_2mortal(newSVpv(path.c_str(), 0)));
+            XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(path).c_str(), 0)));
        } catch (Error & e) {
            croak("%s", e.what());
        }
@ -298,35 +297,35 @@ SV * derivationFromPath(char * drvPath)
        HV *hash;
    CODE:
        try {
-            Derivation drv = store()->derivationFromPath(drvPath);
+            Derivation drv = store()->derivationFromPath(store()->parseStorePath(drvPath));
            hash = newHV();
            HV * outputs = newHV();
-            for (DerivationOutputs::iterator i = drv.outputs.begin(); i != drv.outputs.end(); ++i)
+            for (auto & i : drv.outputs)
-                hv_store(outputs, i->first.c_str(), i->first.size(), newSVpv(i->second.path.c_str(), 0), 0);
+                hv_store(outputs, i.first.c_str(), i.first.size(), newSVpv(store()->printStorePath(i.second.path).c_str(), 0), 0);
            hv_stores(hash, "outputs", newRV((SV *) outputs));
            AV * inputDrvs = newAV();
-            for (DerivationInputs::iterator i = drv.inputDrvs.begin(); i != drv.inputDrvs.end(); ++i)
+            for (auto & i : drv.inputDrvs)
-                av_push(inputDrvs, newSVpv(i->first.c_str(), 0)); // !!! ignores i->second
+                av_push(inputDrvs, newSVpv(store()->printStorePath(i.first).c_str(), 0)); // !!! ignores i->second
            hv_stores(hash, "inputDrvs", newRV((SV *) inputDrvs));
            AV * inputSrcs = newAV();
-            for (PathSet::iterator i = drv.inputSrcs.begin(); i != drv.inputSrcs.end(); ++i)
+            for (auto & i : drv.inputSrcs)
-                av_push(inputSrcs, newSVpv(i->c_str(), 0));
+                av_push(inputSrcs, newSVpv(store()->printStorePath(i).c_str(), 0));
            hv_stores(hash, "inputSrcs", newRV((SV *) inputSrcs));
            hv_stores(hash, "platform", newSVpv(drv.platform.c_str(), 0));
            hv_stores(hash, "builder", newSVpv(drv.builder.c_str(), 0));
            AV * args = newAV();
-            for (Strings::iterator i = drv.args.begin(); i != drv.args.end(); ++i)
+            for (auto & i : drv.args)
-                av_push(args, newSVpv(i->c_str(), 0));
+                av_push(args, newSVpv(i.c_str(), 0));
            hv_stores(hash, "args", newRV((SV *) args));
            HV * env = newHV();
-            for (StringPairs::iterator i = drv.env.begin(); i != drv.env.end(); ++i)
+            for (auto & i : drv.env)
-                hv_store(env, i->first.c_str(), i->first.size(), newSVpv(i->second.c_str(), 0), 0);
+                hv_store(env, i.first.c_str(), i.first.size(), newSVpv(i.second.c_str(), 0), 0);
            hv_stores(hash, "env", newRV((SV *) env));
            RETVAL = newRV_noinc((SV *)hash);
@ -340,7 +339,7 @@ SV * derivationFromPath(char * drvPath)
 void addTempRoot(char * storePath)
    PPCODE:
        try {
-            store()->addTempRoot(storePath);
+            store()->addTempRoot(store()->parseStorePath(storePath));
        } catch (Error & e) {
            croak("%s", e.what());
        }
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@ -88,7 +88,7 @@ static int _main(int argc, char * * argv)
            return 0;
        }
-        string drvPath;
+        std::optional<StorePath> drvPath;
        string storeUri;
        while (true) {
@ -100,7 +100,7 @@ static int _main(int argc, char * * argv)
            auto amWilling = readInt(source);
            auto neededSystem = readString(source);
-            source >> drvPath;
+            drvPath = store->parseStorePath(readString(source));
            auto requiredFeatures = readStrings<std::set<std::string>>(source);
             auto canBuildLocally = amWilling
@ -236,26 +236,27 @@ connected:
        {
            Activity act(*logger, lvlTalkative, actUnknown, fmt("copying dependencies to '%s'", storeUri));
-            copyPaths(store, ref<Store>(sshStore), inputs, NoRepair, NoCheckSigs, substitute);
+            copyPaths(store, ref<Store>(sshStore), store->parseStorePathSet(inputs), NoRepair, NoCheckSigs, substitute);
        }
        uploadLock = -1;
-        BasicDerivation drv(readDerivation(store->realStoreDir + "/" + baseNameOf(drvPath)));
+        BasicDerivation drv(readDerivation(*store, store->realStoreDir + "/" + std::string(drvPath->to_string())));
-        drv.inputSrcs = inputs;
+        drv.inputSrcs = store->parseStorePathSet(inputs);
-        auto result = sshStore->buildDerivation(drvPath, drv);
+        auto result = sshStore->buildDerivation(*drvPath, drv);
        if (!result.success())
-            throw Error("build of '%s' on '%s' failed: %s", drvPath, storeUri, result.errorMsg);
+            throw Error("build of '%s' on '%s' failed: %s", store->printStorePath(*drvPath), storeUri, result.errorMsg);
-        PathSet missing;
+        StorePathSet missing;
        for (auto & path : outputs)
-            if (!store->isValidPath(path)) missing.insert(path);
+            if (!store->isValidPath(store->parseStorePath(path))) missing.insert(store->parseStorePath(path));
        if (!missing.empty()) {
            Activity act(*logger, lvlTalkative, actUnknown, fmt("copying outputs from '%s'", storeUri));
-            store->locksHeld.insert(missing.begin(), missing.end()); /* FIXME: ugly */
+            for (auto & i : missing)
                store->locksHeld.insert(store->printStorePath(i)); /* FIXME: ugly */
            copyPaths(ref<Store>(sshStore), store, missing, NoRepair, NoCheckSigs, NoSubstitute);
        }
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@ -43,6 +43,19 @@ static char * dupString(const char * s)
 }
 static char * dupStringWithLen(const char * s, size_t size)
 {
    char * t;
 #if HAVE_BOEHMGC
    t = GC_STRNDUP(s, size);
 #else
    t = strndup(s, size);
 #endif
    if (!t) throw std::bad_alloc();
    return t;
 }
 static void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v)
 {
    checkInterrupt();
@ -330,10 +343,10 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
            auto path = r.second;
            if (store->isInStore(r.second)) {
-                PathSet closure;
+                StorePathSet closure;
-                store->computeFSClosure(store->toStorePath(r.second), closure);
+                store->computeFSClosure(store->parseStorePath(store->toStorePath(r.second)), closure);
                for (auto & path : closure)
-                    allowedPaths->insert(path);
+                    allowedPaths->insert(store->printStorePath(path));
            } else
                allowedPaths->insert(r.second);
        }
@ -550,9 +563,11 @@ void mkString(Value & v, const char * s)
 }
-Value & mkString(Value & v, const string & s, const PathSet & context)
+Value & mkString(Value & v, std::string_view s, const PathSet & context)
 {
-    mkString(v, s.c_str());
+    v.type = tString;
    v.string.s = dupStringWithLen(s.data(), s.size());
    v.string.context = 0;
    if (!context.empty()) {
        size_t n = 0;
        v.string.context = (const char * *)
@ -1639,15 +1654,16 @@ string EvalState::copyPathToStore(PathSet & context, const Path & path)
        throwEvalError("file names are not allowed to end in '%1%'", drvExtension);
    Path dstPath;
-    if (srcToStore[path] != "")
+    auto i = srcToStore.find(path);
-        dstPath = srcToStore[path];
+    if (i != srcToStore.end())
        dstPath = store->printStorePath(i->second);
    else {
-        dstPath = settings.readOnlyMode
+        auto p = settings.readOnlyMode
-            ? store->computeStorePathForPath(baseNameOf(path), checkSourcePath(path)).first
+            ? store->computeStorePathForPath(std::string(baseNameOf(path)), checkSourcePath(path)).first
-            : store->addToStore(baseNameOf(path), checkSourcePath(path), true, htSHA256, defaultPathFilter, repair);
+            : store->addToStore(std::string(baseNameOf(path)), checkSourcePath(path), true, htSHA256, defaultPathFilter, repair);
-        srcToStore[path] = dstPath;
+        dstPath = store->printStorePath(p);
-        printMsg(lvlChatty, format("copied source '%1%' -> '%2%'")
+        srcToStore.insert_or_assign(path, std::move(p));
-            % path % dstPath);
+        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, dstPath);
    }
    context.insert(dstPath);
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@ -17,6 +17,7 @@ namespace nix {
 class Store;
 class EvalState;
 struct StorePath;
 enum RepairFlag : bool;
@ -42,14 +43,14 @@ struct Env
 };
-Value & mkString(Value & v, const string & s, const PathSet & context = PathSet());
+Value & mkString(Value & v, std::string_view s, const PathSet & context = PathSet());
 void copyContext(const Value & v, PathSet & context);
 /* Cache for calls to addToStore(); maps source paths to the store
   paths. */
-typedef std::map<Path, Path> SrcToStore;
+typedef std::map<Path, StorePath> SrcToStore;
 std::ostream & operator << (std::ostream & str, const Value & v);
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@ -19,27 +19,27 @@ DrvInfo::DrvInfo(EvalState & state, const string & attrPath, Bindings * attrs)
 DrvInfo::DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPathWithOutputs)
    : state(&state), attrs(nullptr), attrPath("")
 {
-    auto spec = parseDrvPathWithOutputs(drvPathWithOutputs);
+    auto [drvPath, selectedOutputs] = store->parseDrvPathWithOutputs(drvPathWithOutputs);
-    drvPath = spec.first;
+    this->drvPath = store->printStorePath(drvPath);
    auto drv = store->derivationFromPath(drvPath);
-    name = storePathToName(drvPath);
+    name = drvPath.name();
-    if (spec.second.size() > 1)
+    if (selectedOutputs.size() > 1)
        throw Error("building more than one derivation output is not supported, in '%s'", drvPathWithOutputs);
    outputName =
-        spec.second.empty()
+        selectedOutputs.empty()
-        ? get(drv.env, "outputName", "out")
+        ? get(drv.env, "outputName").value_or("out")
-        : *spec.second.begin();
+        : *selectedOutputs.begin();
    auto i = drv.outputs.find(outputName);
    if (i == drv.outputs.end())
-        throw Error("derivation '%s' does not have output '%s'", drvPath, outputName);
+        throw Error("derivation '%s' does not have output '%s'", store->printStorePath(drvPath), outputName);
-    outPath = i->second.path;
+    outPath = store->printStorePath(i->second.path);
 }
--- a/src/libexpr/local.mk
+++ b/src/libexpr/local.mk
@ -6,7 +6,7 @@ libexpr_DIR := $(d)
 libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(d)/primops/*.cc) $(d)/lexer-tab.cc $(d)/parser-tab.cc
-libexpr_LIBS = libutil libstore
+libexpr_LIBS = libutil libstore libnixrust
 libexpr_LDFLAGS =
 ifneq ($(OS), FreeBSD)
--- a/src/libexpr/names.cc
+++ b/src/libexpr/names.cc
@ -16,14 +16,14 @@ DrvName::DrvName()
   a letter.  The `version' part is the rest (excluding the separating
   dash).  E.g., `apache-httpd-2.0.48' is parsed to (`apache-httpd',
   '2.0.48'). */
-DrvName::DrvName(const string & s) : hits(0)
+DrvName::DrvName(std::string_view s) : hits(0)
 {
-    name = fullName = s;
+    name = fullName = std::string(s);
    for (unsigned int i = 0; i < s.size(); ++i) {
        /* !!! isalpha/isdigit are affected by the locale. */
        if (s[i] == '-' && i + 1 < s.size() && !isalpha(s[i + 1])) {
-            name = string(s, 0, i);
+            name = s.substr(0, i);
-            version = string(s, i + 1);
+            version = s.substr(i + 1);
            break;
        }
    }
--- a/src/libexpr/names.hh
+++ b/src/libexpr/names.hh
@ -15,7 +15,7 @@ struct DrvName
    unsigned int hits;
    DrvName();
-    DrvName(const string & s);
+    DrvName(std::string_view s);
    bool matches(DrvName & n);
 private:
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@ -44,29 +44,28 @@ std::pair<string, string> decodeContext(const string & s)
 InvalidPathError::InvalidPathError(const Path & path) :
-    EvalError(format("path '%1%' is not valid") % path), path(path) {}
+    EvalError("path '%s' is not valid", path), path(path) {}
 void EvalState::realiseContext(const PathSet & context)
 {
-    PathSet drvs;
+    std::vector<StorePathWithOutputs> drvs;
    for (auto & i : context) {
        std::pair<string, string> decoded = decodeContext(i);
-        Path ctx = decoded.first;
+        auto ctx = store->parseStorePath(decoded.first);
        assert(store->isStorePath(ctx));
        if (!store->isValidPath(ctx))
-            throw InvalidPathError(ctx);
+            throw InvalidPathError(store->printStorePath(ctx));
-        if (!decoded.second.empty() && nix::isDerivation(ctx)) {
+        if (!decoded.second.empty() && ctx.isDerivation()) {
-            drvs.insert(decoded.first + "!" + decoded.second);
+            drvs.push_back(StorePathWithOutputs{ctx.clone(), {decoded.second}});
            /* Add the output of this derivation to the allowed
               paths. */
            if (allowedPaths) {
-                auto drv = store->derivationFromPath(decoded.first);
+                auto drv = store->derivationFromPath(store->parseStorePath(decoded.first));
                DerivationOutputs::iterator i = drv.outputs.find(decoded.second);
                if (i == drv.outputs.end())
                    throw Error("derivation '%s' does not have an output named '%s'", decoded.first, decoded.second);
-                allowedPaths->insert(i->second.path);
+                allowedPaths->insert(store->printStorePath(i->second.path));
            }
        }
    }
@ -74,10 +73,11 @@ void EvalState::realiseContext(const PathSet & context)
    if (drvs.empty()) return;
    if (!evalSettings.enableImportFromDerivation)
-        throw EvalError(format("attempted to realize '%1%' during evaluation but 'allow-import-from-derivation' is false") % *(drvs.begin()));
+        throw EvalError("attempted to realize '%1%' during evaluation but 'allow-import-from-derivation' is false",
            store->printStorePath(drvs.begin()->path));
    /* For performance, prefetch all substitute info. */
-    PathSet willBuild, willSubstitute, unknown;
+    StorePathSet willBuild, willSubstitute, unknown;
    unsigned long long downloadSize, narSize;
    store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
    store->buildPaths(drvs);
@ -100,8 +100,9 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
    Path realPath = state.checkSourcePath(state.toRealPath(path, context));
-    if (state.store->isStorePath(path) && state.store->isValidPath(path) && isDerivation(path)) {
+    // FIXME
-        Derivation drv = readDerivation(realPath);
+    if (state.store->isStorePath(path) && state.store->isValidPath(state.store->parseStorePath(path)) && isDerivation(path)) {
        Derivation drv = readDerivation(*state.store, realPath);
        Value & w = *state.allocValue();
        state.mkAttrs(w, 3 + drv.outputs.size());
        Value * v2 = state.allocAttr(w, state.sDrvPath);
@ -115,7 +116,7 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
        for (const auto & o : drv.outputs) {
            v2 = state.allocAttr(w, state.symbols.create(o.first));
-            mkString(*v2, o.second.path, {"!" + o.first + "!" + path});
+            mkString(*v2, state.store->printStorePath(o.second.path), {"!" + o.first + "!" + path});
            outputsVal->listElems()[outputs_index] = state.allocValue();
            mkString(*(outputsVal->listElems()[outputs_index++]), o.first);
        }
@ -676,24 +677,24 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
           runs. */
        if (path.at(0) == '=') {
            /* !!! This doesn't work if readOnlyMode is set. */
-            PathSet refs;
+            StorePathSet refs;
-            state.store->computeFSClosure(string(path, 1), refs);
+            state.store->computeFSClosure(state.store->parseStorePath(std::string_view(path).substr(1)), refs);
            for (auto & j : refs) {
-                drv.inputSrcs.insert(j);
+                drv.inputSrcs.insert(j.clone());
-                if (isDerivation(j))
+                if (j.isDerivation())
-                    drv.inputDrvs[j] = state.store->queryDerivationOutputNames(j);
+                    drv.inputDrvs[j.clone()] = state.store->queryDerivationOutputNames(j);
            }
        }
        /* Handle derivation outputs of the form ‘!<name>!<path>’. */
        else if (path.at(0) == '!') {
            std::pair<string, string> ctx = decodeContext(path);
-            drv.inputDrvs[ctx.first].insert(ctx.second);
+            drv.inputDrvs[state.store->parseStorePath(ctx.first)].insert(ctx.second);
        }
        /* Otherwise it's a source file. */
        else
-            drv.inputSrcs.insert(path);
+            drv.inputSrcs.insert(state.store->parseStorePath(path));
    }
    /* Do we have all required attributes? */
@ -703,10 +704,8 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
        throw EvalError(format("required attribute 'system' missing, at %1%") % posDrvName);
    /* Check whether the derivation name is valid. */
    checkStoreName(drvName);
    if (isDerivation(drvName))
-        throw EvalError(format("derivation names are not allowed to end in '%1%', at %2%")
+        throw EvalError("derivation names are not allowed to end in '%s', at %s", drvExtension, posDrvName);
            % drvExtension % posDrvName);
    if (outputHash) {
        /* Handle fixed-output derivations. */
@ -716,52 +715,51 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
        HashType ht = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
        Hash h(*outputHash, ht);
-        Path outPath = state.store->makeFixedOutputPath(outputHashRecursive, h, drvName);
+        auto outPath = state.store->makeFixedOutputPath(outputHashRecursive, h, drvName);
-        if (!jsonObject) drv.env["out"] = outPath;
+        if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);
-        drv.outputs["out"] = DerivationOutput(outPath,
+        drv.outputs.insert_or_assign("out", DerivationOutput(std::move(outPath),
                (outputHashRecursive ? "r:" : "") + printHashType(h.type),
-            h.to_string(Base16, false));
+                h.to_string(Base16, false)));
    }
    else {
-        /* Construct the "masked" store derivation, which is the final
+        /* Compute a hash over the "masked" store derivation, which is
-           one except that in the list of outputs, the output paths
+           the final one except that in the list of outputs, the
-           are empty, and the corresponding environment variables have
+           output paths are empty strings, and the corresponding
-           an empty value.  This ensures that changes in the set of
+           environment variables have an empty value.  This ensures
-           output names do get reflected in the hash. */
+           that changes in the set of output names do get reflected in
           the hash. */
        for (auto & i : outputs) {
            if (!jsonObject) drv.env[i] = "";
            drv.outputs[i] = DerivationOutput("", "", "");
        }
-        /* Use the masked derivation expression to compute the output
+        Hash h = hashDerivationModulo(*state.store, Derivation(drv), true);
           path. */
        Hash h = hashDerivationModulo(*state.store, drv);
-        for (auto & i : drv.outputs)
+        for (auto & i : outputs) {
-            if (i.second.path == "") {
+            auto outPath = state.store->makeOutputPath(i, h, drvName);
-                Path outPath = state.store->makeOutputPath(i.first, h, drvName);
+            if (!jsonObject) drv.env[i] = state.store->printStorePath(outPath);
-                if (!jsonObject) drv.env[i.first] = outPath;
+            drv.outputs.insert_or_assign(i,
-                i.second.path = outPath;
+                DerivationOutput(std::move(outPath), "", ""));
        }
    }
    /* Write the resulting term into the Nix store directory. */
-    Path drvPath = writeDerivation(state.store, drv, drvName, state.repair);
+    auto drvPath = writeDerivation(state.store, drv, drvName, state.repair);
    auto drvPathS = state.store->printStorePath(drvPath);
-    printMsg(lvlChatty, format("instantiated '%1%' -> '%2%'")
+    printMsg(lvlChatty, "instantiated '%1%' -> '%2%'", drvName, drvPathS);
        % drvName % drvPath);
    /* Optimisation, but required in read-only mode! because in that
       case we don't actually write store derivations, so we can't
       read them later. */
-    drvHashes[drvPath] = hashDerivationModulo(*state.store, drv);
+    drvHashes.insert_or_assign(drvPath.clone(),
        hashDerivationModulo(*state.store, Derivation(drv), false));
    state.mkAttrs(v, 1 + drv.outputs.size());
-    mkString(*state.allocAttr(v, state.sDrvPath), drvPath, {"=" + drvPath});
+    mkString(*state.allocAttr(v, state.sDrvPath), drvPathS, {"=" + drvPathS});
    for (auto & i : drv.outputs) {
        mkString(*state.allocAttr(v, state.symbols.create(i.first)),
-            i.second.path, {"!" + i.first + "!" + drvPath});
+            state.store->printStorePath(i.second.path), {"!" + i.first + "!" + drvPathS});
    }
    v.attrs->sort();
 }
@ -814,7 +812,7 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
        throw EvalError(format("path '%1%' is not in the Nix store, at %2%") % path % pos);
    Path path2 = state.store->toStorePath(path);
    if (!settings.readOnlyMode)
-        state.store->ensurePath(path2);
+        state.store->ensurePath(state.store->parseStorePath(path2));
    context.insert(path2);
    mkString(v, path, context);
 }
@ -1010,17 +1008,17 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
    string name = state.forceStringNoCtx(*args[0], pos);
    string contents = state.forceString(*args[1], context, pos);
-    PathSet refs;
+    StorePathSet refs;
    for (auto path : context) {
        if (path.at(0) != '/')
            throw EvalError(format("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%") % name % pos);
-        refs.insert(path);
+        refs.insert(state.store->parseStorePath(path));
    }
-    Path storePath = settings.readOnlyMode
+    auto storePath = state.store->printStorePath(settings.readOnlyMode
        ? state.store->computeStorePathForText(name, contents, refs)
-        : state.store->addTextToStore(name, contents, refs, state.repair);
+        : state.store->addTextToStore(name, contents, refs, state.repair));
    /* Note: we don't need to add `context' to the context of the
       result, since `storePath' itself has references to the paths
@ -1060,21 +1058,18 @@ static void addPath(EvalState & state, const Pos & pos, const string & name, con
        return state.forceBool(res, pos);
    }) : defaultPathFilter;
-    Path expectedStorePath;
+    std::optional<StorePath> expectedStorePath;
-    if (expectedHash) {
+    if (expectedHash)
-        expectedStorePath =
+        expectedStorePath = state.store->makeFixedOutputPath(recursive, expectedHash, name);
            state.store->makeFixedOutputPath(recursive, expectedHash, name);
    }
    Path dstPath;
-    if (!expectedHash || !state.store->isValidPath(expectedStorePath)) {
+    if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
-        dstPath = settings.readOnlyMode
+        dstPath = state.store->printStorePath(settings.readOnlyMode
            ? state.store->computeStorePathForPath(name, path, recursive, htSHA256, filter).first
-            : state.store->addToStore(name, path, recursive, htSHA256, filter, state.repair);
+            : state.store->addToStore(name, path, recursive, htSHA256, filter, state.repair));
-        if (expectedHash && expectedStorePath != dstPath) {
+        if (expectedHash && expectedStorePath != state.store->parseStorePath(dstPath))
-            throw Error(format("store path mismatch in (possibly filtered) path added from '%1%'") % path);
+            throw Error("store path mismatch in (possibly filtered) path added from '%s'", path);
        }
    } else
-        dstPath = expectedStorePath;
+        dstPath = state.store->printStorePath(*expectedStorePath);
    mkString(v, dstPath, {dstPath});
 }
@ -1091,7 +1086,7 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
    if (args[0]->type != tLambda)
        throw TypeError(format("first argument in call to 'filterSource' is not a function but %1%, at %2%") % showType(*args[0]) % pos);
-    addPath(state, pos, baseNameOf(path), path, args[0], true, Hash(), v);
+    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], true, Hash(), v);
 }
 static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value & v)
@ -2151,7 +2146,7 @@ void EvalState::createBaseEnv()
    }
    if (!evalSettings.pureEval) {
-        mkString(v, settings.thisSystem);
+        mkString(v, settings.thisSystem.get());
        addConstant("__currentSystem", v);
    }
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@ -148,7 +148,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
        if (!state.store->isStorePath(i.name))
            throw EvalError("Context key '%s' is not a store path, at %s", i.name, i.pos);
        if (!settings.readOnlyMode)
-            state.store->ensurePath(i.name);
+            state.store->ensurePath(state.store->parseStorePath(i.name));
        state.forceAttrs(*i.value, *i.pos);
        auto iter = i.value->attrs->find(sPath);
        if (iter != i.value->attrs->end()) {
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@ -69,7 +69,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
                return files.count(file);
            };
-            gitInfo.storePath = store->addToStore("source", uri, true, htSHA256, filter);
+            gitInfo.storePath = store->printStorePath(store->addToStore("source", uri, true, htSHA256, filter));
            return gitInfo;
        }
@ -156,7 +156,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
        gitInfo.storePath = json["storePath"];
-        if (store->isValidPath(gitInfo.storePath)) {
+        if (store->isValidPath(store->parseStorePath(gitInfo.storePath))) {
            gitInfo.revCount = json["revCount"];
            return gitInfo;
        }
@ -176,7 +176,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
    unpackTarfile(*source, tmpDir);
-    gitInfo.storePath = store->addToStore(name, tmpDir);
+    gitInfo.storePath = store->printStorePath(store->addToStore(name, tmpDir));
    gitInfo.revCount = std::stoull(runProgram("git", true, { "-C", cacheDir, "rev-list", "--count", gitInfo.rev }));
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@ -63,7 +63,7 @@ HgInfo exportMercurial(ref<Store> store, const std::string & uri,
                return files.count(file);
            };
-            hgInfo.storePath = store->addToStore("source", uri, true, htSHA256, filter);
+            hgInfo.storePath = store->printStorePath(store->addToStore("source", uri, true, htSHA256, filter));
            return hgInfo;
        }
@ -134,7 +134,7 @@ HgInfo exportMercurial(ref<Store> store, const std::string & uri,
        hgInfo.storePath = json["storePath"];
-        if (store->isValidPath(hgInfo.storePath)) {
+        if (store->isValidPath(store->parseStorePath(hgInfo.storePath))) {
            printTalkative("using cached Mercurial store path '%s'", hgInfo.storePath);
            return hgInfo;
        }
@ -150,7 +150,7 @@ HgInfo exportMercurial(ref<Store> store, const std::string & uri,
    deletePath(tmpDir + "/.hg_archival.txt");
-    hgInfo.storePath = store->addToStore(name, tmpDir);
+    hgInfo.storePath = store->printStorePath(store->addToStore(name, tmpDir));
    nlohmann::json json;
    json["storePath"] = hgInfo.storePath;
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@ -38,7 +38,12 @@ public:
        return s < s2.s;
    }
-    operator const string & () const
+    operator const std::string & () const
    {
        return *s;
    }
    operator const std::string_view () const
    {
        return *s;
    }
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@ -33,25 +33,25 @@ void printGCWarning()
 }
-void printMissing(ref<Store> store, const PathSet & paths, Verbosity lvl)
+void printMissing(ref<Store> store, const std::vector<StorePathWithOutputs> & paths, Verbosity lvl)
 {
    unsigned long long downloadSize, narSize;
-    PathSet willBuild, willSubstitute, unknown;
+    StorePathSet willBuild, willSubstitute, unknown;
    store->queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
    printMissing(store, willBuild, willSubstitute, unknown, downloadSize, narSize, lvl);
 }
-void printMissing(ref<Store> store, const PathSet & willBuild,
+void printMissing(ref<Store> store, const StorePathSet & willBuild,
-    const PathSet & willSubstitute, const PathSet & unknown,
+    const StorePathSet & willSubstitute, const StorePathSet & unknown,
    unsigned long long downloadSize, unsigned long long narSize, Verbosity lvl)
 {
    if (!willBuild.empty()) {
        printMsg(lvl, "these derivations will be built:");
-        Paths sorted = store->topoSortPaths(willBuild);
+        auto sorted = store->topoSortPaths(willBuild);
        reverse(sorted.begin(), sorted.end());
        for (auto & i : sorted)
-            printMsg(lvl, fmt("  %s", i));
+            printMsg(lvl, fmt("  %s", store->printStorePath(i)));
    }
    if (!willSubstitute.empty()) {
@ -59,14 +59,14 @@ void printMissing(ref<Store> store, const PathSet & willBuild,
                downloadSize / (1024.0 * 1024.0),
                narSize / (1024.0 * 1024.0)));
        for (auto & i : willSubstitute)
-            printMsg(lvl, fmt("  %s", i));
+            printMsg(lvl, fmt("  %s", store->printStorePath(i)));
    }
    if (!unknown.empty()) {
        printMsg(lvl, fmt("don't know how to build these paths%s:",
                (settings.readOnlyMode ? " (may be caused by read-only store access)" : "")));
        for (auto & i : unknown)
-            printMsg(lvl, fmt("  %s", i));
+            printMsg(lvl, fmt("  %s", store->printStorePath(i)));
    }
 }
@ -237,7 +237,7 @@ bool LegacyArgs::processArgs(const Strings & args, bool finish)
 void parseCmdLine(int argc, char * * argv,
    std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg)
 {
-    parseCmdLine(baseNameOf(argv[0]), argvToStrings(argc, argv), parseArg);
+    parseCmdLine(std::string(baseNameOf(argv[0])), argvToStrings(argc, argv), parseArg);
 }
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@ -3,6 +3,7 @@
 #include "util.hh"
 #include "args.hh"
 #include "common-args.hh"
 #include "path.hh"
 #include <signal.h>
@ -37,11 +38,15 @@ void printVersion(const string & programName);
 void printGCWarning();
 class Store;
 struct StorePathWithOutputs;
-void printMissing(ref<Store> store, const PathSet & paths, Verbosity lvl = lvlInfo);
+void printMissing(
    ref<Store> store,
    const std::vector<StorePathWithOutputs> & paths,
    Verbosity lvl = lvlInfo);
-void printMissing(ref<Store> store, const PathSet & willBuild,
+void printMissing(ref<Store> store, const StorePathSet & willBuild,
-    const PathSet & willSubstitute, const PathSet & unknown,
+    const StorePathSet & willSubstitute, const StorePathSet & unknown,
    unsigned long long downloadSize, unsigned long long narSize, Verbosity lvl = lvlInfo);
 string getArg(const string & opt,
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@ -91,19 +91,18 @@ std::shared_ptr<std::string> BinaryCacheStore::getFile(const std::string & path)
    return sink.s;
 }
-Path BinaryCacheStore::narInfoFileFor(const Path & storePath)
+std::string BinaryCacheStore::narInfoFileFor(const StorePath & storePath)
 {
-    assertStorePath(storePath);
+    return storePathToHash(printStorePath(storePath)) + ".narinfo";
    return storePathToHash(storePath) + ".narinfo";
 }
 void BinaryCacheStore::writeNarInfo(ref<NarInfo> narInfo)
 {
    auto narInfoFile = narInfoFileFor(narInfo->path);
-    upsertFile(narInfoFile, narInfo->to_string(), "text/x-nix-narinfo");
+    upsertFile(narInfoFile, narInfo->to_string(*this), "text/x-nix-narinfo");
-    auto hashPart = storePathToHash(narInfo->path);
+    auto hashPart = storePathToHash(printStorePath(narInfo->path));
    {
        auto state_(state.lock());
@ -126,8 +125,8 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, const ref<std::str
            if (ref != info.path)
                queryPathInfo(ref);
        } catch (InvalidPath &) {
-            throw Error(format("cannot add '%s' to the binary cache because the reference '%s' is not valid")
+            throw Error("cannot add '%s' to the binary cache because the reference '%s' is not valid",
-                % info.path % ref);
+                printStorePath(info.path), printStorePath(ref));
        }
    assert(nar->compare(0, narMagic.size(), narMagic) == 0);
@ -138,14 +137,14 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, const ref<std::str
    narInfo->narHash = hashString(htSHA256, *nar);
    if (info.narHash && info.narHash != narInfo->narHash)
-        throw Error(format("refusing to copy corrupted path '%1%' to binary cache") % info.path);
+        throw Error("refusing to copy corrupted path '%1%' to binary cache", printStorePath(info.path));
    auto accessor_ = std::dynamic_pointer_cast<RemoteFSAccessor>(accessor);
    auto narAccessor = makeNarAccessor(nar);
    if (accessor_)
-        accessor_->addToCache(info.path, *nar, narAccessor);
+        accessor_->addToCache(printStorePath(info.path), *nar, narAccessor);
    /* Optionally write a JSON file containing a listing of the
       contents of the NAR. */
@ -162,7 +161,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, const ref<std::str
            }
        }
-        upsertFile(storePathToHash(info.path) + ".ls", jsonOut.str(), "application/json");
+        upsertFile(storePathToHash(printStorePath(info.path)) + ".ls", jsonOut.str(), "application/json");
    }
    /* Compress the NAR. */
@ -174,10 +173,10 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, const ref<std::str
    narInfo->fileSize = narCompressed->size();
    auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1).count();
-    printMsg(lvlTalkative, format("copying path '%1%' (%2% bytes, compressed %3$.1f%% in %4% ms) to binary cache")
+    printMsg(lvlTalkative, "copying path '%1%' (%2% bytes, compressed %3$.1f%% in %4% ms) to binary cache",
-        % narInfo->path % narInfo->narSize
+        printStorePath(narInfo->path), narInfo->narSize,
-        % ((1.0 - (double) narCompressed->size() / nar->size()) * 100.0)
+        ((1.0 - (double) narCompressed->size() / nar->size()) * 100.0),
-        % duration);
+        duration);
    narInfo->url = "nar/" + narInfo->fileHash.to_string(Base32, false) + ".nar"
        + (compression == "xz" ? ".xz" :
@ -254,14 +253,14 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, const ref<std::str
    stats.narWriteCompressionTimeMs += duration;
    /* Atomically write the NAR info file.*/
-    if (secretKey) narInfo->sign(*secretKey);
+    if (secretKey) narInfo->sign(*this, *secretKey);
    writeNarInfo(narInfo);
    stats.narInfoWrite++;
 }
-bool BinaryCacheStore::isValidPathUncached(const Path & storePath)
+bool BinaryCacheStore::isValidPathUncached(const StorePath & storePath)
 {
    // FIXME: this only checks whether a .narinfo with a matching hash
    // part exists. So ‘f4kb...-foo’ matches ‘f4kb...-bar’, even
@ -269,7 +268,7 @@ bool BinaryCacheStore::isValidPathUncached(const Path & storePath)
    return fileExists(narInfoFileFor(storePath));
 }
-void BinaryCacheStore::narFromPath(const Path & storePath, Sink & sink)
+void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
 {
    auto info = queryPathInfo(storePath).cast<const NarInfo>();
@ -295,12 +294,13 @@ void BinaryCacheStore::narFromPath(const Path & storePath, Sink & sink)
    stats.narReadBytes += narSize;
 }
-void BinaryCacheStore::queryPathInfoUncached(const Path & storePath,
+void BinaryCacheStore::queryPathInfoUncached(const StorePath & storePath,
    Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept
 {
    auto uri = getUri();
    auto storePathS = printStorePath(storePath);
    auto act = std::make_shared<Activity>(*logger, lvlTalkative, actQueryPathInfo,
-        fmt("querying info about '%s' on '%s'", storePath, uri), Logger::Fields{storePath, uri});
+        fmt("querying info about '%s' on '%s'", storePathS, uri), Logger::Fields{storePathS, uri});
    PushActivity pact(act->id);
    auto narInfoFile = narInfoFileFor(storePath);
@ -326,7 +326,7 @@ void BinaryCacheStore::queryPathInfoUncached(const Path & storePath,
        }});
 }
-Path BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
+StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
    bool recursive, HashType hashAlgo, PathFilter & filter, RepairFlag repair)
 {
    // FIXME: some cut&paste from LocalStore::addToStore().
@ -345,20 +345,18 @@ Path BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
        h = hashString(hashAlgo, s);
    }
-    ValidPathInfo info;
+    ValidPathInfo info(makeFixedOutputPath(recursive, h, name));
    info.path = makeFixedOutputPath(recursive, h, name);
    addToStore(info, sink.s, repair, CheckSigs, nullptr);
-    return info.path;
+    return std::move(info.path);
 }
-Path BinaryCacheStore::addTextToStore(const string & name, const string & s,
+StorePath BinaryCacheStore::addTextToStore(const string & name, const string & s,
-    const PathSet & references, RepairFlag repair)
+    const StorePathSet & references, RepairFlag repair)
 {
-    ValidPathInfo info;
+    ValidPathInfo info(computeStorePathForText(name, s, references));
-    info.path = computeStorePathForText(name, s, references);
+    info.references = cloneStorePathSet(references);
    info.references = references;
    if (repair || !isValidPath(info.path)) {
        StringSink sink;
@ -366,7 +364,7 @@ Path BinaryCacheStore::addTextToStore(const string & name, const string & s,
        addToStore(info, sink.s, repair, CheckSigs, nullptr);
    }
-    return info.path;
+    return std::move(info.path);
 }
 ref<FSAccessor> BinaryCacheStore::getFSAccessor()
@ -374,7 +372,7 @@ ref<FSAccessor> BinaryCacheStore::getFSAccessor()
    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()), localNarCache);
 }
-void BinaryCacheStore::addSignatures(const Path & storePath, const StringSet & sigs)
+void BinaryCacheStore::addSignatures(const StorePath & storePath, const StringSet & sigs)
 {
    /* Note: this is inherently racy since there is no locking on
       binary caches. In particular, with S3 this unreliable, even
@ -390,24 +388,22 @@ void BinaryCacheStore::addSignatures(const Path & storePath, const StringSet & s
    writeNarInfo(narInfo);
 }
-std::shared_ptr<std::string> BinaryCacheStore::getBuildLog(const Path & path)
+std::shared_ptr<std::string> BinaryCacheStore::getBuildLog(const StorePath & path)
 {
-    Path drvPath;
+    auto drvPath = path.clone();
-    if (isDerivation(path))
+    if (!path.isDerivation()) {
        drvPath = path;
    else {
        try {
            auto info = queryPathInfo(path);
            // FIXME: add a "Log" field to .narinfo
-            if (info->deriver == "") return nullptr;
+            if (!info->deriver) return nullptr;
-            drvPath = info->deriver;
+            drvPath = info->deriver->clone();
        } catch (InvalidPath &) {
            return nullptr;
        }
    }
-    auto logPath = "log/" + baseNameOf(drvPath);
+    auto logPath = "log/" + std::string(baseNameOf(printStorePath(drvPath)));
    debug("fetching build log from binary cache '%s/%s'", getUri(), logPath);
--- a/src/libstore/binary-cache-store.hh
+++ b/src/libstore/binary-cache-store.hh
@ -65,18 +65,18 @@ private:
    std::string narMagic;
-    std::string narInfoFileFor(const Path & storePath);
+    std::string narInfoFileFor(const StorePath & storePath);
    void writeNarInfo(ref<NarInfo> narInfo);
 public:
-    bool isValidPathUncached(const Path & path) override;
+    bool isValidPathUncached(const StorePath & path) override;
-    void queryPathInfoUncached(const Path & path,
+    void queryPathInfoUncached(const StorePath & path,
        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override;
-    Path queryPathFromHashPart(const string & hashPart) override
+    std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override
    { unsupported("queryPathFromHashPart"); }
    bool wantMassQuery() override { return wantMassQuery_; }
@ -85,27 +85,27 @@ public:
        RepairFlag repair, CheckSigsFlag checkSigs,
        std::shared_ptr<FSAccessor> accessor) override;
-    Path addToStore(const string & name, const Path & srcPath,
+    StorePath addToStore(const string & name, const Path & srcPath,
        bool recursive, HashType hashAlgo,
        PathFilter & filter, RepairFlag repair) override;
-    Path addTextToStore(const string & name, const string & s,
+    StorePath addTextToStore(const string & name, const string & s,
-        const PathSet & references, RepairFlag repair) override;
+        const StorePathSet & references, RepairFlag repair) override;
-    void narFromPath(const Path & path, Sink & sink) override;
+    void narFromPath(const StorePath & path, Sink & sink) override;
-    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+    BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
        BuildMode buildMode) override
    { unsupported("buildDerivation"); }
-    void ensurePath(const Path & path) override
+    void ensurePath(const StorePath & path) override
    { unsupported("ensurePath"); }
    ref<FSAccessor> getFSAccessor() override;
-    void addSignatures(const Path & storePath, const StringSet & sigs) override;
+    void addSignatures(const StorePath & storePath, const StringSet & sigs) override;
-    std::shared_ptr<std::string> getBuildLog(const Path & path) override;
+    std::shared_ptr<std::string> getBuildLog(const StorePath & path) override;
    int getPriority() override { return priority; }
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@ -24,7 +24,7 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
    Path storePath = getAttr("out");
    auto mainUrl = getAttr("url");
-    bool unpack = get(drv.env, "unpack", "") == "1";
+    bool unpack = get(drv.env, "unpack").value_or("") == "1";
    /* Note: have to use a fresh downloader here because we're in
       a forked process. */
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@ -260,14 +260,8 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    switch (op) {
    case wopIsValidPath: {
-        /* 'readStorePath' could raise an error leading to the connection
+        auto path = store->parseStorePath(readString(from));
           being closed.  To be able to recover from an invalid path error,
           call 'startWork' early, and do 'assertStorePath' afterwards so
           that the 'Error' exception handler doesn't close the
           connection.  */
        Path path = readString(from);
        logger->startWork();
        store->assertStorePath(path);
        bool result = store->isValidPath(path);
        logger->stopWork();
        to << result;
@ -275,34 +269,36 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopQueryValidPaths: {
-        PathSet paths = readStorePaths<PathSet>(*store, from);
+        auto paths = readStorePaths<StorePathSet>(*store, from);
        logger->startWork();
-        PathSet res = store->queryValidPaths(paths);
+        auto res = store->queryValidPaths(paths);
        logger->stopWork();
-        to << res;
+        writeStorePaths(*store, to, res);
        break;
    }
    case wopHasSubstitutes: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
-        PathSet res = store->querySubstitutablePaths({path});
+        StorePathSet paths; // FIXME
        paths.insert(path.clone());
        auto res = store->querySubstitutablePaths(paths);
        logger->stopWork();
-        to << (res.find(path) != res.end());
+        to << (res.count(path) != 0);
        break;
    }
    case wopQuerySubstitutablePaths: {
-        PathSet paths = readStorePaths<PathSet>(*store, from);
+        auto paths = readStorePaths<StorePathSet>(*store, from);
        logger->startWork();
-        PathSet res = store->querySubstitutablePaths(paths);
+        auto res = store->querySubstitutablePaths(paths);
        logger->stopWork();
-        to << res;
+        writeStorePaths(*store, to, res);
        break;
    }
    case wopQueryPathHash: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
        auto hash = store->queryPathInfo(path)->narHash;
        logger->stopWork();
@ -314,23 +310,24 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    case wopQueryReferrers:
    case wopQueryValidDerivers:
    case wopQueryDerivationOutputs: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
-        PathSet paths;
+        StorePathSet paths;
        if (op == wopQueryReferences)
-            paths = store->queryPathInfo(path)->references;
+            for (auto & i : store->queryPathInfo(path)->references)
                paths.insert(i.clone());
        else if (op == wopQueryReferrers)
            store->queryReferrers(path, paths);
        else if (op == wopQueryValidDerivers)
            paths = store->queryValidDerivers(path);
        else paths = store->queryDerivationOutputs(path);
        logger->stopWork();
-        to << paths;
+        writeStorePaths(*store, to, paths);
        break;
    }
    case wopQueryDerivationOutputNames: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
        StringSet names;
        names = store->queryDerivationOutputNames(path);
@ -340,20 +337,20 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopQueryDeriver: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
-        auto deriver = store->queryPathInfo(path)->deriver;
+        auto info = store->queryPathInfo(path);
        logger->stopWork();
-        to << deriver;
+        to << (info->deriver ? store->printStorePath(*info->deriver) : "");
        break;
    }
    case wopQueryPathFromHashPart: {
-        string hashPart = readString(from);
+        auto hashPart = readString(from);
        logger->startWork();
-        Path path = store->queryPathFromHashPart(hashPart);
+        auto path = store->queryPathFromHashPart(hashPart);
        logger->stopWork();
-        to << path;
+        to << (path ? store->printStorePath(*path) : "");
        break;
    }
@ -383,26 +380,26 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
        logger->startWork();
        if (!savedRegular.regular) throw Error("regular file expected");
-        Path path = store->addToStoreFromDump(recursive ? *savedNAR.data : savedRegular.s, baseName, recursive, hashAlgo);
+        auto path = store->addToStoreFromDump(recursive ? *savedNAR.data : savedRegular.s, baseName, recursive, hashAlgo);
        logger->stopWork();
-        to << path;
+        to << store->printStorePath(path);
        break;
    }
    case wopAddTextToStore: {
        string suffix = readString(from);
        string s = readString(from);
-        PathSet refs = readStorePaths<PathSet>(*store, from);
+        auto refs = readStorePaths<StorePathSet>(*store, from);
        logger->startWork();
-        Path path = store->addTextToStore(suffix, s, refs, NoRepair);
+        auto path = store->addTextToStore(suffix, s, refs, NoRepair);
        logger->stopWork();
-        to << path;
+        to << store->printStorePath(path);
        break;
    }
    case wopExportPath: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        readInt(from); // obsolete
        logger->startWork();
        TunnelSink sink(to);
@ -415,15 +412,19 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    case wopImportPaths: {
        logger->startWork();
        TunnelSource source(from, to);
-        Paths paths = store->importPaths(source, nullptr,
+        auto paths = store->importPaths(source, nullptr,
            trusted ? NoCheckSigs : CheckSigs);
        logger->stopWork();
-        to << paths;
+        Strings paths2;
        for (auto & i : paths) paths2.push_back(store->printStorePath(i));
        to << paths2;
        break;
    }
    case wopBuildPaths: {
-        PathSet drvs = readStorePaths<PathSet>(*store, from);
+        std::vector<StorePathWithOutputs> drvs;
        for (auto & s : readStrings<Strings>(from))
            drvs.push_back(store->parseDrvPathWithOutputs(s));
        BuildMode mode = bmNormal;
        if (GET_PROTOCOL_MINOR(clientVersion) >= 15) {
            mode = (BuildMode) readInt(from);
@ -441,7 +442,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopBuildDerivation: {
-        Path drvPath = readStorePath(*store, from);
+        auto drvPath = store->parseStorePath(readString(from));
        BasicDerivation drv;
        readDerivation(from, *store, drv);
        BuildMode buildMode = (BuildMode) readInt(from);
@ -455,7 +456,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopEnsurePath: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
        store->ensurePath(path);
        logger->stopWork();
@ -464,7 +465,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopAddTempRoot: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
        store->addTempRoot(path);
        logger->stopWork();
@ -502,7 +503,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
        for (auto & [target, links] : roots)
            for (auto & link : links)
-                to << link << target;
+                to << link << store->printStorePath(target);
        break;
    }
@ -510,7 +511,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    case wopCollectGarbage: {
        GCOptions options;
        options.action = (GCOptions::GCAction) readInt(from);
-        options.pathsToDelete = readStorePaths<PathSet>(*store, from);
+        options.pathsToDelete = readStorePaths<StorePathSet>(*store, from);
        from >> options.ignoreLiveness >> options.maxFreed;
        // obsolete fields
        readInt(from);
@ -568,44 +569,52 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopQuerySubstitutablePathInfo: {
-        Path path = absPath(readString(from));
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
        SubstitutablePathInfos infos;
-        store->querySubstitutablePathInfos({path}, infos);
+        StorePathSet paths;
        paths.insert(path.clone()); // FIXME
        store->querySubstitutablePathInfos(paths, infos);
        logger->stopWork();
-        SubstitutablePathInfos::iterator i = infos.find(path);
+        auto i = infos.find(path);
        if (i == infos.end())
            to << 0;
        else {
-            to << 1 << i->second.deriver << i->second.references << i->second.downloadSize << i->second.narSize;
+            to << 1
               << (i->second.deriver ? store->printStorePath(*i->second.deriver) : "");
            writeStorePaths(*store, to, i->second.references);
            to << i->second.downloadSize
               << i->second.narSize;
        }
        break;
    }
    case wopQuerySubstitutablePathInfos: {
-        PathSet paths = readStorePaths<PathSet>(*store, from);
+        auto paths = readStorePaths<StorePathSet>(*store, from);
        logger->startWork();
        SubstitutablePathInfos infos;
        store->querySubstitutablePathInfos(paths, infos);
        logger->stopWork();
        to << infos.size();
        for (auto & i : infos) {
-            to << i.first << i.second.deriver << i.second.references
+            to << store->printStorePath(i.first)
-               << i.second.downloadSize << i.second.narSize;
+               << (i.second.deriver ? store->printStorePath(*i.second.deriver) : "");
            writeStorePaths(*store, to, i.second.references);
            to << i.second.downloadSize << i.second.narSize;
        }
        break;
    }
    case wopQueryAllValidPaths: {
        logger->startWork();
-        PathSet paths = store->queryAllValidPaths();
+        auto paths = store->queryAllValidPaths();
        logger->stopWork();
-        to << paths;
+        writeStorePaths(*store, to, paths);
        break;
    }
    case wopQueryPathInfo: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        std::shared_ptr<const ValidPathInfo> info;
        logger->startWork();
        try {
@ -617,8 +626,10 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
        if (info) {
            if (GET_PROTOCOL_MINOR(clientVersion) >= 17)
                to << 1;
-            to << info->deriver << info->narHash.to_string(Base16, false) << info->references
+            to << (info->deriver ? store->printStorePath(*info->deriver) : "")
-               << info->registrationTime << info->narSize;
+               << info->narHash.to_string(Base16, false);
            writeStorePaths(*store, to, info->references);
            to << info->registrationTime << info->narSize;
            if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
                to << info->ultimate
                   << info->sigs
@ -651,7 +662,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopAddSignatures: {
-        Path path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        StringSet sigs = readStrings<StringSet>(from);
        logger->startWork();
        if (!trusted)
@ -663,22 +674,21 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopNarFromPath: {
-        auto path = readStorePath(*store, from);
+        auto path = store->parseStorePath(readString(from));
        logger->startWork();
        logger->stopWork();
-        dumpPath(path, to);
+        dumpPath(store->printStorePath(path), to);
        break;
    }
    case wopAddToStoreNar: {
        bool repair, dontCheckSigs;
-        ValidPathInfo info;
+        ValidPathInfo info(store->parseStorePath(readString(from)));
-        info.path = readStorePath(*store, from);
+        auto deriver = readString(from);
-        from >> info.deriver;
+        if (deriver != "")
-        if (!info.deriver.empty())
+            info.deriver = store->parseStorePath(deriver);
            store->assertStorePath(info.deriver);
        info.narHash = Hash(readString(from), htSHA256);
-        info.references = readStorePaths<PathSet>(*store, from);
+        info.references = readStorePaths<StorePathSet>(*store, from);
        from >> info.registrationTime >> info.narSize >> info.ultimate;
        info.sigs = readStrings<StringSet>(from);
        from >> info.ca >> repair >> dontCheckSigs;
@ -709,13 +719,18 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
    }
    case wopQueryMissing: {
-        PathSet targets = readStorePaths<PathSet>(*store, from);
+        std::vector<StorePathWithOutputs> targets;
        for (auto & s : readStrings<Strings>(from))
            targets.push_back(store->parseDrvPathWithOutputs(s));
        logger->startWork();
-        PathSet willBuild, willSubstitute, unknown;
+        StorePathSet willBuild, willSubstitute, unknown;
        unsigned long long downloadSize, narSize;
        store->queryMissing(targets, willBuild, willSubstitute, unknown, downloadSize, narSize);
        logger->stopWork();
-        to << willBuild << willSubstitute << unknown << downloadSize << narSize;
+        writeStorePaths(*store, to, willBuild);
        writeStorePaths(*store, to, willSubstitute);
        writeStorePaths(*store, to, unknown);
        to << downloadSize << narSize;
        break;
    }
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@ -21,17 +21,39 @@ void DerivationOutput::parseHashInfo(bool & recursive, Hash & hash) const
    HashType hashType = parseHashType(algo);
    if (hashType == htUnknown)
-        throw Error(format("unknown hash algorithm '%1%'") % algo);
+        throw Error("unknown hash algorithm '%s'", algo);
    hash = Hash(this->hash, hashType);
 }
-Path BasicDerivation::findOutput(const string & id) const
+BasicDerivation::BasicDerivation(const BasicDerivation & other)
    : platform(other.platform)
    , builder(other.builder)
    , args(other.args)
    , env(other.env)
 {
    for (auto & i : other.outputs)
        outputs.insert_or_assign(i.first,
            DerivationOutput(i.second.path.clone(), std::string(i.second.hashAlgo), std::string(i.second.hash)));
    for (auto & i : other.inputSrcs)
        inputSrcs.insert(i.clone());
 }
 Derivation::Derivation(const Derivation & other)
    : BasicDerivation(other)
 {
    for (auto & i : other.inputDrvs)
        inputDrvs.insert_or_assign(i.first.clone(), i.second);
 }
 const StorePath & BasicDerivation::findOutput(const string & id) const
 {
    auto i = outputs.find(id);
    if (i == outputs.end())
-        throw Error(format("derivation has no output '%1%'") % id);
+        throw Error("derivation has no output '%s'", id);
    return i->second.path;
 }
@ -42,18 +64,17 @@ bool BasicDerivation::isBuiltin() const
 }
-Path writeDerivation(ref<Store> store,
+StorePath writeDerivation(ref<Store> store,
    const Derivation & drv, const string & name, RepairFlag repair)
 {
-    PathSet references;
+    auto references = cloneStorePathSet(drv.inputSrcs);
    references.insert(drv.inputSrcs.begin(), drv.inputSrcs.end());
    for (auto & i : drv.inputDrvs)
-        references.insert(i.first);
+        references.insert(i.first.clone());
    /* Note that the outputs of a derivation are *not* references
       (that can be missing (of course) and should not necessarily be
       held during a garbage collection). */
    string suffix = name + drvExtension;
-    string contents = drv.unparse();
+    string contents = drv.unparse(*store, false);
    return settings.readOnlyMode
        ? store->computeStorePathForText(suffix, contents, references)
        : store->addTextToStore(suffix, contents, references, repair);
@ -121,7 +142,7 @@ static StringSet parseStrings(std::istream & str, bool arePaths)
 }
-static Derivation parseDerivation(const string & s)
+static Derivation parseDerivation(const Store & store, const string & s)
 {
    Derivation drv;
    istringstream_nocopy str(s);
@ -129,13 +150,12 @@ static Derivation parseDerivation(const string & s)
    /* Parse the list of outputs. */
    while (!endOfList(str)) {
-        DerivationOutput out;
+        expect(str, "("); std::string id = parseString(str);
-        expect(str, "("); string id = parseString(str);
+        expect(str, ","); auto path = store.parseStorePath(parsePath(str));
-        expect(str, ","); out.path = parsePath(str);
+        expect(str, ","); auto hashAlgo = parseString(str);
-        expect(str, ","); out.hashAlgo = parseString(str);
+        expect(str, ","); auto hash = parseString(str);
        expect(str, ","); out.hash = parseString(str);
        expect(str, ")");
-        drv.outputs[id] = out;
+        drv.outputs.emplace(id, DerivationOutput(std::move(path), std::move(hashAlgo), std::move(hash)));
    }
    /* Parse the list of input derivations. */
@ -144,11 +164,11 @@ static Derivation parseDerivation(const string & s)
        expect(str, "(");
        Path drvPath = parsePath(str);
        expect(str, ",[");
-        drv.inputDrvs[drvPath] = parseStrings(str, false);
+        drv.inputDrvs.insert_or_assign(store.parseStorePath(drvPath), parseStrings(str, false));
        expect(str, ")");
    }
-    expect(str, ",["); drv.inputSrcs = parseStrings(str, true);
+    expect(str, ",["); drv.inputSrcs = store.parseStorePathSet(parseStrings(str, true));
    expect(str, ","); drv.platform = parseString(str);
    expect(str, ","); drv.builder = parseString(str);
@ -171,25 +191,24 @@ static Derivation parseDerivation(const string & s)
 }
-Derivation readDerivation(const Path & drvPath)
+Derivation readDerivation(const Store & store, const Path & drvPath)
 {
    try {
-        return parseDerivation(readFile(drvPath));
+        return parseDerivation(store, readFile(drvPath));
    } catch (FormatError & e) {
        throw Error(format("error parsing derivation '%1%': %2%") % drvPath % e.msg());
    }
 }
-Derivation Store::derivationFromPath(const Path & drvPath)
+Derivation Store::derivationFromPath(const StorePath & drvPath)
 {
    assertStorePath(drvPath);
    ensurePath(drvPath);
    auto accessor = getFSAccessor();
    try {
-        return parseDerivation(accessor->readFile(drvPath));
+        return parseDerivation(*this, accessor->readFile(printStorePath(drvPath)));
    } catch (FormatError & e) {
-        throw Error(format("error parsing derivation '%1%': %2%") % drvPath % e.msg());
+        throw Error("error parsing derivation '%s': %s", printStorePath(drvPath), e.msg());
    }
 }
@ -220,33 +239,56 @@ static void printStrings(string & res, ForwardIterator i, ForwardIterator j)
 }
-string Derivation::unparse() const
+string Derivation::unparse(const Store & store, bool maskOutputs,
    std::map<std::string, StringSet> * actualInputs) const
 {
    string s;
    s.reserve(65536);
    s += "Derive([";
    StringSet maskedOutputs;
    if (maskOutputs) {
        bool first = true;
        maskedOutputs = tokenizeString<StringSet>(get(env, "outputs").value_or("out"), " ");
        for (auto & i : maskedOutputs) {
            if (first) first = false; else s += ',';
            s += '('; printString(s, i);
            s += ",\"\",\"\",\"\")";
        }
    } else {
        bool first = true;
        for (auto & i : outputs) {
            if (first) first = false; else s += ',';
            s += '('; printString(s, i.first);
-        s += ','; printString(s, i.second.path);
+            s += ','; printString(s, store.printStorePath(i.second.path));
            s += ','; printString(s, i.second.hashAlgo);
            s += ','; printString(s, i.second.hash);
            s += ')';
        }
    }
    s += "],[";
-    first = true;
+    bool first = true;
-    for (auto & i : inputDrvs) {
+    if (actualInputs) {
        for (auto & i : *actualInputs) {
            if (first) first = false; else s += ',';
            s += '('; printString(s, i.first);
            s += ','; printStrings(s, i.second.begin(), i.second.end());
            s += ')';
        }
    } else {
        for (auto & i : inputDrvs) {
            if (first) first = false; else s += ',';
            s += '('; printString(s, store.printStorePath(i.first));
            s += ','; printStrings(s, i.second.begin(), i.second.end());
            s += ')';
        }
    }
    s += "],";
-    printStrings(s, inputSrcs.begin(), inputSrcs.end());
+    auto paths = store.printStorePathSet(inputSrcs); // FIXME: slow
    printStrings(s, paths.begin(), paths.end());
    s += ','; printString(s, platform);
    s += ','; printString(s, builder);
@ -257,7 +299,7 @@ string Derivation::unparse() const
    for (auto & i : env) {
        if (first) first = false; else s += ',';
        s += '('; printString(s, i.first);
-        s += ','; printString(s, i.second);
+        s += ','; printString(s, maskOutputs && maskedOutputs.count(i.first) ? "" : i.second);
        s += ')';
    }
@ -267,6 +309,7 @@ string Derivation::unparse() const
 }
 // FIXME: remove
 bool isDerivation(const string & fileName)
 {
    return hasSuffix(fileName, drvExtension);
@ -304,7 +347,7 @@ DrvHashes drvHashes;
   paths have been replaced by the result of a recursive call to this
   function, and that for fixed-output derivations we return a hash of
   its output path. */
-Hash hashDerivationModulo(Store & store, Derivation drv)
+Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutputs)
 {
    /* Return a fixed hash for fixed-output derivations. */
    if (drv.isFixedOutput()) {
@ -312,42 +355,41 @@ Hash hashDerivationModulo(Store & store, Derivation drv)
        return hashString(htSHA256, "fixed:out:"
            + i->second.hashAlgo + ":"
            + i->second.hash + ":"
-            + i->second.path);
+            + store.printStorePath(i->second.path));
    }
    /* For other derivations, replace the inputs paths with recursive
       calls to this function.*/
-    DerivationInputs inputs2;
+    std::map<std::string, StringSet> inputs2;
    for (auto & i : drv.inputDrvs) {
-        Hash h = drvHashes[i.first];
+        auto h = drvHashes.find(i.first);
-        if (!h) {
+        if (h == drvHashes.end()) {
            assert(store.isValidPath(i.first));
-            Derivation drv2 = readDerivation(store.toRealPath(i.first));
+            h = drvHashes.insert_or_assign(i.first.clone(), hashDerivationModulo(store,
-            h = hashDerivationModulo(store, drv2);
+                readDerivation(store, store.toRealPath(store.printStorePath(i.first))), false)).first;
            drvHashes[i.first] = h;
        }
-        inputs2[h.to_string(Base16, false)] = i.second;
+        inputs2.insert_or_assign(h->second.to_string(Base16, false), i.second);
    }
    drv.inputDrvs = inputs2;
-    return hashString(htSHA256, drv.unparse());
+    return hashString(htSHA256, drv.unparse(store, maskOutputs, &inputs2));
 }
-DrvPathWithOutputs parseDrvPathWithOutputs(const string & s)
+StorePathWithOutputs Store::parseDrvPathWithOutputs(const std::string & s)
 {
    size_t n = s.find("!");
    return n == s.npos
-        ? DrvPathWithOutputs(s, std::set<string>())
+        ? StorePathWithOutputs{parseStorePath(s), std::set<string>()}
-        : DrvPathWithOutputs(string(s, 0, n), tokenizeString<std::set<string> >(string(s, n + 1), ","));
+        : StorePathWithOutputs{parseStorePath(std::string_view(s.data(), n)),
            tokenizeString<std::set<string>>(string(s, n + 1), ",")};
 }
-Path makeDrvPathWithOutputs(const Path & drvPath, const std::set<string> & outputs)
+std::string StorePathWithOutputs::to_string(const Store & store) const
 {
    return outputs.empty()
-        ? drvPath
+        ? store.printStorePath(path)
-        : drvPath + "!" + concatStringsSep(",", outputs);
+        : store.printStorePath(path) + "!" + concatStringsSep(",", outputs);
 }
@ -357,28 +399,28 @@ bool wantOutput(const string & output, const std::set<string> & wanted)
 }
-PathSet BasicDerivation::outputPaths() const
+StorePathSet BasicDerivation::outputPaths() const
 {
-    PathSet paths;
+    StorePathSet paths;
    for (auto & i : outputs)
-        paths.insert(i.second.path);
+        paths.insert(i.second.path.clone());
    return paths;
 }
-Source & readDerivation(Source & in, Store & store, BasicDerivation & drv)
+Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv)
 {
    drv.outputs.clear();
    auto nr = readNum<size_t>(in);
    for (size_t n = 0; n < nr; n++) {
        auto name = readString(in);
-        DerivationOutput o;
+        auto path = store.parseStorePath(readString(in));
-        in >> o.path >> o.hashAlgo >> o.hash;
+        auto hashAlgo = readString(in);
-        store.assertStorePath(o.path);
+        auto hash = readString(in);
-        drv.outputs[name] = o;
+        drv.outputs.emplace(name, DerivationOutput(std::move(path), std::move(hashAlgo), std::move(hash)));
    }
-    drv.inputSrcs = readStorePaths<PathSet>(store, in);
+    drv.inputSrcs = readStorePaths<StorePathSet>(store, in);
    in >> drv.platform >> drv.builder;
    drv.args = readStrings<Strings>(in);
@ -393,16 +435,16 @@ Source & readDerivation(Source & in, Store & store, BasicDerivation & drv)
 }
-Sink & operator << (Sink & out, const BasicDerivation & drv)
+void writeDerivation(Sink & out, const Store & store, const BasicDerivation & drv)
 {
    out << drv.outputs.size();
    for (auto & i : drv.outputs)
-        out << i.first << i.second.path << i.second.hashAlgo << i.second.hash;
+        out << i.first << store.printStorePath(i.second.path) << i.second.hashAlgo << i.second.hash;
-    out << drv.inputSrcs << drv.platform << drv.builder << drv.args;
+    writeStorePaths(store, out, drv.inputSrcs);
    out << drv.platform << drv.builder << drv.args;
    out << drv.env.size();
    for (auto & i : drv.env)
        out << i.first << i.second;
    return out;
 }
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@ -10,26 +10,18 @@
 namespace nix {
 /* Extension of derivations in the Nix store. */
 const string drvExtension = ".drv";
 /* Abstract syntax of derivations. */
 struct DerivationOutput
 {
-    Path path;
+    StorePath path;
-    string hashAlgo; /* hash used for expected hash computation */
+    std::string hashAlgo; /* hash used for expected hash computation */
-    string hash; /* expected hash, may be null */
+    std::string hash; /* expected hash, may be null */
-    DerivationOutput()
+    DerivationOutput(StorePath && path, std::string && hashAlgo, std::string && hash)
-    {
+        : path(std::move(path))
-    }
+        , hashAlgo(std::move(hashAlgo))
-    DerivationOutput(Path path, string hashAlgo, string hash)
+        , hash(std::move(hash))
-    {
+    { }
        this->path = path;
        this->hashAlgo = hashAlgo;
        this->hash = hash;
    }
    void parseHashInfo(bool & recursive, Hash & hash) const;
 };
@ -37,24 +29,26 @@ typedef std::map<string, DerivationOutput> DerivationOutputs;
 /* For inputs that are sub-derivations, we specify exactly which
   output IDs we are interested in. */
-typedef std::map<Path, StringSet> DerivationInputs;
+typedef std::map<StorePath, StringSet> DerivationInputs;
 typedef std::map<string, string> StringPairs;
 struct BasicDerivation
 {
    DerivationOutputs outputs; /* keyed on symbolic IDs */
-    PathSet inputSrcs; /* inputs that are sources */
+    StorePathSet inputSrcs; /* inputs that are sources */
    string platform;
    Path builder;
    Strings args;
    StringPairs env;
    BasicDerivation() { }
    explicit BasicDerivation(const BasicDerivation & other);
    virtual ~BasicDerivation() { };
    /* Return the path corresponding to the output identifier `id' in
       the given derivation. */
-    Path findOutput(const string & id) const;
+    const StorePath & findOutput(const std::string & id) const;
    bool isBuiltin() const;
@ -62,7 +56,7 @@ struct BasicDerivation
    bool isFixedOutput() const;
    /* Return the output paths of a derivation. */
-    PathSet outputPaths() const;
+    StorePathSet outputPaths() const;
 };
@ -71,7 +65,12 @@ struct Derivation : BasicDerivation
    DerivationInputs inputDrvs; /* inputs that are sub-derivations */
    /* Print a derivation. */
-    std::string unparse() const;
+    std::string unparse(const Store & store, bool maskOutputs,
        std::map<std::string, StringSet> * actualInputs = nullptr) const;
    Derivation() { }
    Derivation(Derivation && other) = default;
    explicit Derivation(const Derivation & other);
 };
@ -79,38 +78,29 @@ class Store;
 /* Write a derivation to the Nix store, and return its path. */
-Path writeDerivation(ref<Store> store,
+StorePath writeDerivation(ref<Store> store,
    const Derivation & drv, const string & name, RepairFlag repair = NoRepair);
 /* Read a derivation from a file. */
-Derivation readDerivation(const Path & drvPath);
+Derivation readDerivation(const Store & store, const Path & drvPath);
-/* Check whether a file name ends with the extension for
+// FIXME: remove
   derivations. */
 bool isDerivation(const string & fileName);
-Hash hashDerivationModulo(Store & store, Derivation drv);
+Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutputs);
 /* Memoisation of hashDerivationModulo(). */
-typedef std::map<Path, Hash> DrvHashes;
+typedef std::map<StorePath, Hash> DrvHashes;
 extern DrvHashes drvHashes; // FIXME: global, not thread-safe
 /* Split a string specifying a derivation and a set of outputs
   (/nix/store/hash-foo!out1,out2,...) into the derivation path and
   the outputs. */
 typedef std::pair<string, std::set<string> > DrvPathWithOutputs;
 DrvPathWithOutputs parseDrvPathWithOutputs(const string & s);
 Path makeDrvPathWithOutputs(const Path & drvPath, const std::set<string> & outputs);
 bool wantOutput(const string & output, const std::set<string> & wanted);
 struct Source;
 struct Sink;
-Source & readDerivation(Source & in, Store & store, BasicDerivation & drv);
+Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv);
-Sink & operator << (Sink & out, const BasicDerivation & drv);
+void writeDerivation(Sink & out, const Store & store, const BasicDerivation & drv);
 std::string hashPlaceholder(const std::string & outputName);
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@ -650,10 +650,10 @@ struct CurlDownloader : public Downloader
 #ifdef ENABLE_S3
                auto [bucketName, key, params] = parseS3Uri(request.uri);
-                std::string profile = get(params, "profile", "");
+                std::string profile = get(params, "profile").value_or("");
-                std::string region = get(params, "region", Aws::Region::US_EAST_1);
+                std::string region = get(params, "region").value_or(Aws::Region::US_EAST_1);
-                std::string scheme = get(params, "scheme", "");
+                std::string scheme = get(params, "scheme").value_or("");
-                std::string endpoint = get(params, "endpoint", "");
+                std::string endpoint = get(params, "endpoint").value_or("");
                S3Helper s3Helper(profile, region, scheme, endpoint);
@ -811,13 +811,13 @@ CachedDownloadResult Downloader::downloadCached(
        if (p != string::npos) name = string(url, p + 1);
    }
-    Path expectedStorePath;
+    std::optional<StorePath> expectedStorePath;
    if (request.expectedHash) {
        expectedStorePath = store->makeFixedOutputPath(request.unpack, request.expectedHash, name);
-        if (store->isValidPath(expectedStorePath)) {
+        if (store->isValidPath(*expectedStorePath)) {
            CachedDownloadResult result;
-            result.storePath = expectedStorePath;
+            result.storePath = store->printStorePath(*expectedStorePath);
-            result.path = store->toRealPath(expectedStorePath);
+            result.path = store->toRealPath(result.storePath);
            return result;
        }
    }
@ -832,7 +832,7 @@ CachedDownloadResult Downloader::downloadCached(
    PathLocks lock({fileLink}, fmt("waiting for lock on '%1%'...", fileLink));
-    Path storePath;
+    std::optional<StorePath> storePath;
    string expectedETag;
@ -841,9 +841,10 @@ CachedDownloadResult Downloader::downloadCached(
    CachedDownloadResult result;
    if (pathExists(fileLink) && pathExists(dataFile)) {
-        storePath = readLink(fileLink);
+        storePath = store->parseStorePath(readLink(fileLink));
-        store->addTempRoot(storePath);
+        // FIXME
-        if (store->isValidPath(storePath)) {
+        store->addTempRoot(*storePath);
        if (store->isValidPath(*storePath)) {
            auto ss = tokenizeString<vector<string>>(readFile(dataFile), "\n");
            if (ss.size() >= 3 && ss[0] == url) {
                time_t lastChecked;
@ -857,7 +858,7 @@ CachedDownloadResult Downloader::downloadCached(
                }
            }
        } else
-            storePath = "";
+            storePath.reset();
    }
    if (!skip) {
@ -870,65 +871,65 @@ CachedDownloadResult Downloader::downloadCached(
            result.etag = res.etag;
            if (!res.cached) {
                ValidPathInfo info;
                StringSink sink;
                dumpString(*res.data, sink);
                Hash hash = hashString(request.expectedHash ? request.expectedHash.type : htSHA256, *res.data);
-                info.path = store->makeFixedOutputPath(false, hash, name);
+                ValidPathInfo info(store->makeFixedOutputPath(false, hash, name));
                info.narHash = hashString(htSHA256, *sink.s);
                info.narSize = sink.s->size();
                info.ca = makeFixedOutputCA(false, hash);
                store->addToStore(info, sink.s, NoRepair, NoCheckSigs);
-                storePath = info.path;
+                storePath = info.path.clone();
            }
-            assert(!storePath.empty());
+            assert(storePath);
-            replaceSymlink(storePath, fileLink);
+            replaceSymlink(store->printStorePath(*storePath), fileLink);
            writeFile(dataFile, url + "\n" + res.etag + "\n" + std::to_string(time(0)) + "\n");
        } catch (DownloadError & e) {
-            if (storePath.empty()) throw;
+            if (!storePath) throw;
            warn("warning: %s; using cached result", e.msg());
            result.etag = expectedETag;
        }
    }
    if (request.unpack) {
-        Path unpackedLink = cacheDir + "/" + baseNameOf(storePath) + "-unpacked";
+        Path unpackedLink = cacheDir + "/" + ((std::string) storePath->to_string()) + "-unpacked";
        PathLocks lock2({unpackedLink}, fmt("waiting for lock on '%1%'...", unpackedLink));
-        Path unpackedStorePath;
+        std::optional<StorePath> unpackedStorePath;
        if (pathExists(unpackedLink)) {
-            unpackedStorePath = readLink(unpackedLink);
+            unpackedStorePath = store->parseStorePath(readLink(unpackedLink));
-            store->addTempRoot(unpackedStorePath);
+            // FIXME
-            if (!store->isValidPath(unpackedStorePath))
+            store->addTempRoot(*unpackedStorePath);
-                unpackedStorePath = "";
+            if (!store->isValidPath(*unpackedStorePath))
                unpackedStorePath.reset();
        }
-        if (unpackedStorePath.empty()) {
+        if (!unpackedStorePath) {
            printInfo("unpacking '%s'...", url);
            Path tmpDir = createTempDir();
            AutoDelete autoDelete(tmpDir, true);
-            unpackTarfile(store->toRealPath(storePath), tmpDir, baseNameOf(url));
+            unpackTarfile(store->toRealPath(store->printStorePath(*storePath)), tmpDir, std::string(baseNameOf(url)));
            auto members = readDirectory(tmpDir);
            if (members.size() != 1)
                throw nix::Error("tarball '%s' contains an unexpected number of top-level files", url);
            auto topDir = tmpDir + "/" + members.begin()->name;
            unpackedStorePath = store->addToStore(name, topDir, true, htSHA256, defaultPathFilter, NoRepair);
        }
-        replaceSymlink(unpackedStorePath, unpackedLink);
+        replaceSymlink(store->printStorePath(*unpackedStorePath), unpackedLink);
-        storePath = unpackedStorePath;
+        storePath = std::move(*unpackedStorePath);
    }
-    if (expectedStorePath != "" && storePath != expectedStorePath) {
+    if (expectedStorePath && *storePath != *expectedStorePath) {
        unsigned int statusCode = 102;
        Hash gotHash = request.unpack
-            ? hashPath(request.expectedHash.type, store->toRealPath(storePath)).first
+            ? hashPath(request.expectedHash.type, store->toRealPath(store->printStorePath(*storePath))).first
-            : hashFile(request.expectedHash.type, store->toRealPath(storePath));
+            : hashFile(request.expectedHash.type, store->toRealPath(store->printStorePath(*storePath)));
        throw nix::Error(statusCode, "hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
            url, request.expectedHash.to_string(), gotHash.to_string());
    }
-    result.storePath = storePath;
+    result.storePath = store->printStorePath(*storePath);
-    result.path = store->toRealPath(storePath);
+    result.path = store->toRealPath(result.storePath);
    return result;
 }
--- a/src/libstore/export-import.cc
+++ b/src/libstore/export-import.cc
@ -24,9 +24,9 @@ struct HashAndWriteSink : Sink
    }
 };
-void Store::exportPaths(const Paths & paths, Sink & sink)
+void Store::exportPaths(const StorePathSet & paths, Sink & sink)
 {
-    Paths sorted = topoSortPaths(PathSet(paths.begin(), paths.end()));
+    auto sorted = topoSortPaths(paths);
    std::reverse(sorted.begin(), sorted.end());
    std::string doneLabel("paths exported");
@ -42,7 +42,7 @@ void Store::exportPaths(const Paths & paths, Sink & sink)
    sink << 0;
 }
-void Store::exportPath(const Path & path, Sink & sink)
+void Store::exportPath(const StorePath & path, Sink & sink)
 {
    auto info = queryPathInfo(path);
@ -55,15 +55,21 @@ void Store::exportPath(const Path & path, Sink & sink)
       Don't complain if the stored hash is zero (unknown). */
    Hash hash = hashAndWriteSink.currentHash();
    if (hash != info->narHash && info->narHash != Hash(info->narHash.type))
-        throw Error(format("hash of path '%1%' has changed from '%2%' to '%3%'!") % path
+        throw Error("hash of path '%s' has changed from '%s' to '%s'!",
-            % info->narHash.to_string() % hash.to_string());
+            printStorePath(path), info->narHash.to_string(), hash.to_string());
-    hashAndWriteSink << exportMagic << path << info->references << info->deriver << 0;
+    hashAndWriteSink
        << exportMagic
        << printStorePath(path);
    writeStorePaths(*this, hashAndWriteSink, info->references);
    hashAndWriteSink
        << (info->deriver ? printStorePath(*info->deriver) : "")
        << 0;
 }
-Paths Store::importPaths(Source & source, std::shared_ptr<FSAccessor> accessor, CheckSigsFlag checkSigs)
+StorePaths Store::importPaths(Source & source, std::shared_ptr<FSAccessor> accessor, CheckSigsFlag checkSigs)
 {
-    Paths res;
+    StorePaths res;
    while (true) {
        auto n = readNum<uint64_t>(source);
        if (n == 0) break;
@ -77,16 +83,15 @@ Paths Store::importPaths(Source & source, std::shared_ptr<FSAccessor> accessor,
        if (magic != exportMagic)
            throw Error("Nix archive cannot be imported; wrong format");
-        ValidPathInfo info;
+        ValidPathInfo info(parseStorePath(readString(source)));
        info.path = readStorePath(*this, source);
        //Activity act(*logger, lvlInfo, format("importing path '%s'") % info.path);
-        info.references = readStorePaths<PathSet>(*this, source);
+        info.references = readStorePaths<StorePathSet>(*this, source);
-        info.deriver = readString(source);
+        auto deriver = readString(source);
-        if (info.deriver != "") assertStorePath(info.deriver);
+        if (deriver != "")
            info.deriver = parseStorePath(deriver);
        info.narHash = hashString(htSHA256, *tee.source.data);
        info.narSize = tee.source.data->size();
@ -97,7 +102,7 @@ Paths Store::importPaths(Source & source, std::shared_ptr<FSAccessor> accessor,
        addToStore(info, tee.source.data, NoRepair, checkSigs, accessor);
-        res.push_back(info.path);
+        res.push_back(info.path.clone());
    }
    return res;
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@ -85,12 +85,10 @@ void LocalStore::addIndirectRoot(const Path & path)
 }
-Path LocalFSStore::addPermRoot(const Path & _storePath,
+Path LocalFSStore::addPermRoot(const StorePath & storePath,
    const Path & _gcRoot, bool indirect, bool allowOutsideRootsDir)
 {
    Path storePath(canonPath(_storePath));
    Path gcRoot(canonPath(_gcRoot));
    assertStorePath(storePath);
    if (isInStore(gcRoot))
        throw Error(format(
@ -102,7 +100,7 @@ Path LocalFSStore::addPermRoot(const Path & _storePath,
           point to the Nix store. */
        if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
            throw Error(format("cannot create symlink '%1%'; already exists") % gcRoot);
-        makeSymlink(gcRoot, storePath);
+        makeSymlink(gcRoot, printStorePath(storePath));
        addIndirectRoot(gcRoot);
    }
@ -117,10 +115,10 @@ Path LocalFSStore::addPermRoot(const Path & _storePath,
                    % gcRoot % rootsDir);
        }
-        if (baseNameOf(gcRoot) == baseNameOf(storePath))
+        if (baseNameOf(gcRoot) == std::string(storePath.to_string()))
            writeFile(gcRoot, "");
        else
-            makeSymlink(gcRoot, storePath);
+            makeSymlink(gcRoot, printStorePath(storePath));
    }
    /* Check that the root can be found by the garbage collector.
@ -129,13 +127,12 @@ Path LocalFSStore::addPermRoot(const Path & _storePath,
       check if the root is in a directory in or linked from the
       gcroots directory. */
    if (settings.checkRootReachability) {
-        Roots roots = findRoots(false);
+        auto roots = findRoots(false);
-        if (roots[storePath].count(gcRoot) == 0)
+        if (roots[storePath.clone()].count(gcRoot) == 0)
            printError(
                format(
                "warning: '%1%' is not in a directory where the garbage collector looks for roots; "
-                    "therefore, '%2%' might be removed by the garbage collector")
+                "therefore, '%2%' might be removed by the garbage collector",
-                % gcRoot % storePath);
+                gcRoot, printStorePath(storePath));
    }
    /* Grab the global GC root, causing us to block while a GC is in
@ -147,7 +144,7 @@ Path LocalFSStore::addPermRoot(const Path & _storePath,
 }
-void LocalStore::addTempRoot(const Path & path)
+void LocalStore::addTempRoot(const StorePath & path)
 {
    auto state(_state.lock());
@ -188,7 +185,7 @@ void LocalStore::addTempRoot(const Path & path)
    debug(format("acquiring write lock on '%1%'") % fnTempRoots);
    lockFile(state->fdTempRoots.get(), ltWrite, true);
-    string s = path + '\0';
+    string s = printStorePath(path) + '\0';
    writeFull(state->fdTempRoots.get(), s);
    /* Downgrade to a read lock. */
@ -246,8 +243,7 @@ void LocalStore::findTempRoots(FDs & fds, Roots & tempRoots, bool censor)
        while ((end = contents.find((char) 0, pos)) != string::npos) {
            Path root(contents, pos, end - pos);
            debug("got temporary root '%s'", root);
-            assertStorePath(root);
+            tempRoots[parseStorePath(root)].emplace(censor ? censored : fmt("{temp:%d}", pid));
            tempRoots[root].emplace(censor ? censored : fmt("{temp:%d}", pid));
            pos = end + 1;
        }
@ -260,10 +256,11 @@ void LocalStore::findRoots(const Path & path, unsigned char type, Roots & roots)
 {
    auto foundRoot = [&](const Path & path, const Path & target) {
        Path storePath = toStorePath(target);
-        if (isStorePath(storePath) && isValidPath(storePath))
+        // FIXME
-            roots[storePath].emplace(path);
+        if (isStorePath(storePath) && isValidPath(parseStorePath(storePath)))
            roots[parseStorePath(storePath)].emplace(path);
        else
-            printInfo(format("skipping invalid root from '%1%' to '%2%'") % path % storePath);
+            printInfo("skipping invalid root from '%1%' to '%2%'", path, storePath);
    };
    try {
@ -299,9 +296,10 @@ void LocalStore::findRoots(const Path & path, unsigned char type, Roots & roots)
        }
        else if (type == DT_REG) {
-            Path storePath = storeDir + "/" + baseNameOf(path);
+            Path storePath = storeDir + "/" + std::string(baseNameOf(path));
-            if (isStorePath(storePath) && isValidPath(storePath))
+            // FIXME
-                roots[storePath].emplace(path);
+            if (isStorePath(storePath) && isValidPath(parseStorePath(storePath)))
                roots[parseStorePath(storePath)].emplace(path);
        }
    }
@ -309,7 +307,7 @@ void LocalStore::findRoots(const Path & path, unsigned char type, Roots & roots)
    catch (SysError & e) {
        /* We only ignore permanent failures. */
        if (e.errNo == EACCES || e.errNo == ENOENT || e.errNo == ENOTDIR)
-            printInfo(format("cannot read potential root '%1%'") % path);
+            printInfo("cannot read potential root '%1%'", path);
        else
            throw;
    }
@ -340,7 +338,9 @@ Roots LocalStore::findRoots(bool censor)
    return roots;
 }
-static void readProcLink(const string & file, Roots & roots)
+typedef std::unordered_map<Path, std::unordered_set<std::string>> UncheckedRoots;
 static void readProcLink(const string & file, UncheckedRoots & roots)
 {
    /* 64 is the starting buffer size gnu readlink uses... */
    auto bufsiz = ssize_t{64};
@ -369,7 +369,7 @@ static string quoteRegexChars(const string & raw)
    return std::regex_replace(raw, specialRegex, R"(\$&)");
 }
-static void readFileRoots(const char * path, Roots & roots)
+static void readFileRoots(const char * path, UncheckedRoots & roots)
 {
    try {
        roots[readFile(path)].emplace(path);
@ -381,7 +381,7 @@ static void readFileRoots(const char * path, Roots & roots)
 void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
 {
-    Roots unchecked;
+    UncheckedRoots unchecked;
    auto procDir = AutoCloseDir{opendir("/proc")};
    if (procDir) {
@ -466,16 +466,16 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
 #endif
    for (auto & [target, links] : unchecked) {
-        if (isInStore(target)) {
+        if (!isInStore(target)) continue;
-            Path path = toStorePath(target);
+        Path pathS = toStorePath(target);
-            if (isStorePath(path) && isValidPath(path)) {
+        if (!isStorePath(pathS)) continue;
-                debug(format("got additional root '%1%'") % path);
+        auto path = parseStorePath(pathS);
        if (!isValidPath(path)) continue;
        debug("got additional root '%1%'", pathS);
        if (censor)
-                    roots[path].insert(censored);
+            roots[path.clone()].insert(censored);
        else
-                    roots[path].insert(links.begin(), links.end());
+            roots[path.clone()].insert(links.begin(), links.end());
            }
        }
    }
 }
@ -485,18 +485,19 @@ struct GCLimitReached { };
 struct LocalStore::GCState
 {
-    GCOptions options;
+    const GCOptions & options;
    GCResults & results;
-    PathSet roots;
+    StorePathSet roots;
-    PathSet tempRoots;
+    StorePathSet tempRoots;
-    PathSet dead;
+    StorePathSet dead;
-    PathSet alive;
+    StorePathSet alive;
    bool gcKeepOutputs;
    bool gcKeepDerivations;
    unsigned long long bytesInvalidated;
    bool moveToTrash = true;
    bool shouldDelete;
-    GCState(GCResults & results_) : results(results_), bytesInvalidated(0) { }
+    GCState(const GCOptions & options, GCResults & results)
        : options(options), results(results), bytesInvalidated(0) { }
 };
@ -504,7 +505,7 @@ bool LocalStore::isActiveTempFile(const GCState & state,
    const Path & path, const string & suffix)
 {
    return hasSuffix(path, suffix)
-        && state.tempRoots.find(string(path, 0, path.size() - suffix.size())) != state.tempRoots.end();
+        && state.tempRoots.count(parseStorePath(string(path, 0, path.size() - suffix.size())));
 }
@ -522,16 +523,17 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
    unsigned long long size = 0;
-    if (isStorePath(path) && isValidPath(path)) {
+    // FIXME
-        PathSet referrers;
+    if (isStorePath(path) && isValidPath(parseStorePath(path))) {
-        queryReferrers(path, referrers);
+        StorePathSet referrers;
        queryReferrers(parseStorePath(path), referrers);
        for (auto & i : referrers)
-            if (i != path) deletePathRecursive(state, i);
+            if (printStorePath(i) != path) deletePathRecursive(state, printStorePath(i));
-        size = queryPathInfo(path)->narSize;
+        size = queryPathInfo(parseStorePath(path))->narSize;
-        invalidatePathChecked(path);
+        invalidatePathChecked(parseStorePath(path));
    }
-    Path realPath = realStoreDir + "/" + baseNameOf(path);
+    Path realPath = realStoreDir + "/" + std::string(baseNameOf(path));
    struct stat st;
    if (lstat(realPath.c_str(), &st)) {
@ -555,7 +557,7 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
        try {
            if (chmod(realPath.c_str(), st.st_mode | S_IWUSR) == -1)
                throw SysError(format("making '%1%' writable") % realPath);
-            Path tmp = trashDir + "/" + baseNameOf(path);
+            Path tmp = trashDir + "/" + std::string(baseNameOf(path));
            if (rename(realPath.c_str(), tmp.c_str()))
                throw SysError(format("unable to rename '%1%' to '%2%'") % realPath % tmp);
            state.bytesInvalidated += size;
@ -575,7 +577,7 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
 }
-bool LocalStore::canReachRoot(GCState & state, PathSet & visited, const Path & path)
+bool LocalStore::canReachRoot(GCState & state, StorePathSet & visited, const StorePath & path)
 {
    if (visited.count(path)) return false;
@ -584,41 +586,41 @@ bool LocalStore::canReachRoot(GCState & state, PathSet & visited, const Path & p
    if (state.dead.count(path)) return false;
    if (state.roots.count(path)) {
-        debug(format("cannot delete '%1%' because it's a root") % path);
+        debug("cannot delete '%1%' because it's a root", printStorePath(path));
-        state.alive.insert(path);
+        state.alive.insert(path.clone());
        return true;
    }
-    visited.insert(path);
+    visited.insert(path.clone());
-    if (!isStorePath(path) || !isValidPath(path)) return false;
+    //FIXME
    if (!isStorePath(printStorePath(path)) || !isValidPath(path)) return false;
-    PathSet incoming;
+    StorePathSet incoming;
    /* Don't delete this path if any of its referrers are alive. */
    queryReferrers(path, incoming);
    /* If keep-derivations is set and this is a derivation, then
       don't delete the derivation if any of the outputs are alive. */
-    if (state.gcKeepDerivations && isDerivation(path)) {
+    if (state.gcKeepDerivations && path.isDerivation()) {
-        PathSet outputs = queryDerivationOutputs(path);
+        for (auto & i : queryDerivationOutputs(path))
        for (auto & i : outputs)
            if (isValidPath(i) && queryPathInfo(i)->deriver == path)
-                incoming.insert(i);
+                incoming.insert(i.clone());
    }
    /* If keep-outputs is set, then don't delete this path if there
       are derivers of this path that are not garbage. */
    if (state.gcKeepOutputs) {
-        PathSet derivers = queryValidDerivers(path);
+        auto derivers = queryValidDerivers(path);
        for (auto & i : derivers)
-            incoming.insert(i);
+            incoming.insert(i.clone());
    }
    for (auto & i : incoming)
        if (i != path)
            if (canReachRoot(state, visited, i)) {
-                state.alive.insert(path);
+                state.alive.insert(path.clone());
                return true;
            }
@ -630,12 +632,13 @@ void LocalStore::tryToDelete(GCState & state, const Path & path)
 {
    checkInterrupt();
-    auto realPath = realStoreDir + "/" + baseNameOf(path);
+    auto realPath = realStoreDir + "/" + std::string(baseNameOf(path));
    if (realPath == linksDir || realPath == trashDir) return;
    //Activity act(*logger, lvlDebug, format("considering whether to delete '%1%'") % path);
-    if (!isStorePath(path) || !isValidPath(path)) {
+    // FIXME
    if (!isStorePath(path) || !isValidPath(parseStorePath(path))) {
        /* A lock file belonging to a path that we're building right
           now isn't garbage. */
        if (isActiveTempFile(state, path, ".lock")) return;
@ -650,16 +653,17 @@ void LocalStore::tryToDelete(GCState & state, const Path & path)
        if (isActiveTempFile(state, path, ".check")) return;
    }
-    PathSet visited;
+    StorePathSet visited;
-    if (canReachRoot(state, visited, path)) {
+    if (canReachRoot(state, visited, parseStorePath(path))) {
-        debug(format("cannot delete '%1%' because it's still reachable") % path);
+        debug("cannot delete '%s' because it's still reachable", path);
    } else {
        /* No path we visited was a root, so everything is garbage.
           But we only delete ‘path’ and its referrers here so that
           ‘nix-store --delete’ doesn't have the unexpected effect of
           recursing into derivations and outputs. */
-        state.dead.insert(visited.begin(), visited.end());
+        for (auto & i : visited)
            state.dead.insert(i.clone());
        if (state.shouldDelete)
            deletePathRecursive(state, path);
    }
@ -715,8 +719,7 @@ void LocalStore::removeUnusedLinks(const GCState & state)
 void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 {
-    GCState state(results);
+    GCState state(options, results);
    state.options = options;
    state.gcKeepOutputs = settings.gcKeepOutputs;
    state.gcKeepDerivations = settings.gcKeepDerivations;
@ -741,12 +744,12 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
    /* Find the roots.  Since we've grabbed the GC lock, the set of
       permanent roots cannot increase now. */
-    printError(format("finding garbage collector roots..."));
+    printError("finding garbage collector roots...");
    Roots rootMap;
    if (!options.ignoreLiveness)
        findRootsNoTemp(rootMap, true);
-    for (auto & i : rootMap) state.roots.insert(i.first);
+    for (auto & i : rootMap) state.roots.insert(i.first.clone());
    /* Read the temporary roots.  This acquires read locks on all
       per-process temporary root files.  So after this point no paths
@ -754,9 +757,10 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
    FDs fds;
    Roots tempRoots;
    findTempRoots(fds, tempRoots, true);
-    for (auto & root : tempRoots)
+    for (auto & root : tempRoots) {
-        state.tempRoots.insert(root.first);
+        state.tempRoots.insert(root.first.clone());
-    state.roots.insert(state.tempRoots.begin(), state.tempRoots.end());
+        state.roots.insert(root.first.clone());
    }
    /* After this point the set of roots or temporary roots cannot
       increase, since we hold locks on everything.  So everything
@ -768,7 +772,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
            createDirs(trashDir);
        } catch (SysError & e) {
            if (e.errNo == ENOSPC) {
-                printInfo(format("note: can't create trash directory: %1%") % e.msg());
+                printInfo("note: can't create trash directory: %s", e.msg());
                state.moveToTrash = false;
            }
        }
@ -780,22 +784,21 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
    if (options.action == GCOptions::gcDeleteSpecific) {
        for (auto & i : options.pathsToDelete) {
-            assertStorePath(i);
+            tryToDelete(state, printStorePath(i));
            tryToDelete(state, i);
            if (state.dead.find(i) == state.dead.end())
-                throw Error(format(
+                throw Error(
                    "cannot delete path '%1%' since it is still alive. "
                    "To find out why use: "
-                    "nix-store --query --roots"
+                    "nix-store --query --roots",
-                    ) % i);
+                    printStorePath(i));
        }
    } else if (options.maxFreed > 0) {
        if (state.shouldDelete)
-            printError(format("deleting garbage..."));
+            printError("deleting garbage...");
        else
-            printError(format("determining live/dead paths..."));
+            printError("determining live/dead paths...");
        try {
@ -815,7 +818,8 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                string name = dirent->d_name;
                if (name == "." || name == "..") continue;
                Path path = storeDir + "/" + name;
-                if (isStorePath(path) && isValidPath(path))
+                // FIXME
                if (isStorePath(path) && isValidPath(parseStorePath(path)))
                    entries.push_back(path);
                else
                    tryToDelete(state, path);
@ -840,12 +844,14 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
    }
    if (state.options.action == GCOptions::gcReturnLive) {
-        state.results.paths = state.alive;
+        for (auto & i : state.alive)
            state.results.paths.insert(printStorePath(i));
        return;
    }
    if (state.options.action == GCOptions::gcReturnDead) {
-        state.results.paths = state.dead;
+        for (auto & i : state.dead)
            state.results.paths.insert(printStorePath(i));
        return;
    }
@ -859,7 +865,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
    /* Clean up the links directory. */
    if (options.action == GCOptions::gcDeleteDead || options.action == GCOptions::gcDeleteSpecific) {
-        printError(format("deleting unused links..."));
+        printError("deleting unused links...");
        removeUnusedLinks(state);
    }
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@ -87,25 +87,27 @@ struct LegacySSHStore : public Store
        return uriScheme + host;
    }
-    void queryPathInfoUncached(const Path & path,
+    void queryPathInfoUncached(const StorePath & path,
        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override
    {
        try {
            auto conn(connections->get());
-            debug("querying remote host '%s' for info on '%s'", host, path);
+            debug("querying remote host '%s' for info on '%s'", host, printStorePath(path));
-            conn->to << cmdQueryPathInfos << PathSet{path};
+            conn->to << cmdQueryPathInfos << PathSet{printStorePath(path)};
            conn->to.flush();
-            auto info = std::make_shared<ValidPathInfo>();
+            auto p = readString(conn->from);
-            conn->from >> info->path;
+            if (p.empty()) return callback(nullptr);
-            if (info->path.empty()) return callback(nullptr);
+            auto info = std::make_shared<ValidPathInfo>(parseStorePath(p));
            assert(path == info->path);
            PathSet references;
-            conn->from >> info->deriver;
+            auto deriver = readString(conn->from);
-            info->references = readStorePaths<PathSet>(*this, conn->from);
+            if (deriver != "")
                info->deriver = parseStorePath(deriver);
            info->references = readStorePaths<StorePathSet>(*this, conn->from);
            readLongLong(conn->from); // download size
            info->narSize = readLongLong(conn->from);
@ -127,7 +129,7 @@ struct LegacySSHStore : public Store
        RepairFlag repair, CheckSigsFlag checkSigs,
        std::shared_ptr<FSAccessor> accessor) override
    {
-        debug("adding path '%s' to remote host '%s'", info.path, host);
+        debug("adding path '%s' to remote host '%s'", printStorePath(info.path), host);
        auto conn(connections->get());
@ -135,10 +137,11 @@ struct LegacySSHStore : public Store
            conn->to
                << cmdAddToStoreNar
-                << info.path
+                << printStorePath(info.path)
-                << info.deriver
+                << (info.deriver ? printStorePath(*info.deriver) : "")
-                << info.narHash.to_string(Base16, false)
+                << info.narHash.to_string(Base16, false);
-                << info.references
+            writeStorePaths(*this, conn->to, info.references);
            conn->to
                << info.registrationTime
                << info.narSize
                << info.ultimate
@ -165,9 +168,10 @@ struct LegacySSHStore : public Store
            }
            conn->to
                << exportMagic
-                << info.path
+                << printStorePath(info.path);
-                << info.references
+            writeStorePaths(*this, conn->to, info.references);
-                << info.deriver
+            conn->to
                << (info.deriver ? printStorePath(*info.deriver) : "")
                << 0
                << 0;
            conn->to.flush();
@ -175,39 +179,40 @@ struct LegacySSHStore : public Store
        }
        if (readInt(conn->from) != 1)
-            throw Error("failed to add path '%s' to remote host '%s', info.path, host");
+            throw Error("failed to add path '%s' to remote host '%s'", printStorePath(info.path), host);
    }
-    void narFromPath(const Path & path, Sink & sink) override
+    void narFromPath(const StorePath & path, Sink & sink) override
    {
        auto conn(connections->get());
-        conn->to << cmdDumpStorePath << path;
+        conn->to << cmdDumpStorePath << printStorePath(path);
        conn->to.flush();
        copyNAR(conn->from, sink);
    }
-    Path queryPathFromHashPart(const string & hashPart) override
+    std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override
    { unsupported("queryPathFromHashPart"); }
-    Path addToStore(const string & name, const Path & srcPath,
+    StorePath addToStore(const string & name, const Path & srcPath,
        bool recursive, HashType hashAlgo,
        PathFilter & filter, RepairFlag repair) override
    { unsupported("addToStore"); }
-    Path addTextToStore(const string & name, const string & s,
+    StorePath addTextToStore(const string & name, const string & s,
-        const PathSet & references, RepairFlag repair) override
+        const StorePathSet & references, RepairFlag repair) override
    { unsupported("addTextToStore"); }
-    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+    BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
        BuildMode buildMode) override
    {
        auto conn(connections->get());
        conn->to
            << cmdBuildDerivation
-            << drvPath
+            << printStorePath(drvPath);
-            << drv
+        writeDerivation(conn->to, *this, drv);
        conn->to
            << settings.maxSilentTime
            << settings.buildTimeout;
        if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 2)
@ -230,11 +235,11 @@ struct LegacySSHStore : public Store
        return status;
    }
-    void ensurePath(const Path & path) override
+    void ensurePath(const StorePath & path) override
    { unsupported("ensurePath"); }
-    void computeFSClosure(const PathSet & paths,
+    void computeFSClosure(const StorePathSet & paths,
-        PathSet & out, bool flipDirection = false,
+        StorePathSet & out, bool flipDirection = false,
        bool includeOutputs = false, bool includeDerivers = false) override
    {
        if (flipDirection || includeDerivers) {
@ -246,16 +251,15 @@ struct LegacySSHStore : public Store
        conn->to
            << cmdQueryClosure
-            << includeOutputs
+            << includeOutputs;
-            << paths;
+        writeStorePaths(*this, conn->to, paths);
        conn->to.flush();
-        auto res = readStorePaths<PathSet>(*this, conn->from);
+        for (auto & i : readStorePaths<StorePathSet>(*this, conn->from))
-
+            out.insert(i.clone());
        out.insert(res.begin(), res.end());
    }
-    PathSet queryValidPaths(const PathSet & paths,
+    StorePathSet queryValidPaths(const StorePathSet & paths,
        SubstituteFlag maybeSubstitute = NoSubstitute) override
    {
        auto conn(connections->get());
@ -263,11 +267,11 @@ struct LegacySSHStore : public Store
        conn->to
            << cmdQueryValidPaths
            << false // lock
-            << maybeSubstitute
+            << maybeSubstitute;
-            << paths;
+        writeStorePaths(*this, conn->to, paths);
        conn->to.flush();
-        return readStorePaths<PathSet>(*this, conn->from);
+        return readStorePaths<StorePathSet>(*this, conn->from);
    }
    void connect() override
--- a/src/libstore/local-binary-cache-store.cc
+++ b/src/libstore/local-binary-cache-store.cc
@ -44,15 +44,15 @@ protected:
        }
    }
-    PathSet queryAllValidPaths() override
+    StorePathSet queryAllValidPaths() override
    {
-        PathSet paths;
+        StorePathSet paths;
        for (auto & entry : readDirectory(binaryCacheDir)) {
            if (entry.name.size() != 40 ||
                !hasSuffix(entry.name, ".narinfo"))
                continue;
-            paths.insert(storeDir + "/" + entry.name.substr(0, entry.name.size() - 8));
+            paths.insert(parseStorePath(storeDir + "/" + entry.name.substr(0, entry.name.size() - 8)));
        }
        return paths;
--- a/src/libstore/local-fs-store.cc
+++ b/src/libstore/local-fs-store.cc
@ -21,7 +21,7 @@ struct LocalStoreAccessor : public FSAccessor
    Path toRealPath(const Path & path)
    {
        Path storePath = store->toStorePath(path);
-        if (!store->isValidPath(storePath))
+        if (!store->isValidPath(store->parseStorePath(storePath)))
            throw InvalidPath(format("path '%1%' is not a valid store path") % storePath);
        return store->getRealStoreDir() + std::string(path, store->storeDir.size());
    }
@ -77,34 +77,32 @@ ref<FSAccessor> LocalFSStore::getFSAccessor()
            std::dynamic_pointer_cast<LocalFSStore>(shared_from_this())));
 }
-void LocalFSStore::narFromPath(const Path & path, Sink & sink)
+void LocalFSStore::narFromPath(const StorePath & path, Sink & sink)
 {
    if (!isValidPath(path))
-        throw Error(format("path '%s' is not valid") % path);
+        throw Error("path '%s' is not valid", printStorePath(path));
-    dumpPath(getRealStoreDir() + std::string(path, storeDir.size()), sink);
+    dumpPath(getRealStoreDir() + std::string(printStorePath(path), storeDir.size()), sink);
 }
 const string LocalFSStore::drvsLogDir = "drvs";
-std::shared_ptr<std::string> LocalFSStore::getBuildLog(const Path & path_)
+std::shared_ptr<std::string> LocalFSStore::getBuildLog(const StorePath & path_)
 {
-    auto path(path_);
+    auto path = path_.clone();
-    assertStorePath(path);
+    if (!path.isDerivation()) {
    if (!isDerivation(path)) {
        try {
-            path = queryPathInfo(path)->deriver;
+            auto info = queryPathInfo(path);
            if (!info->deriver) return nullptr;
            path = info->deriver->clone();
        } catch (InvalidPath &) {
            return nullptr;
        }
        if (path == "") return nullptr;
    }
-    string baseName = baseNameOf(path);
+    auto baseName = std::string(baseNameOf(printStorePath(path)));
    for (int j = 0; j < 2; j++) {
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@ -541,42 +541,36 @@ void canonicalisePathMetaData(const Path & path, uid_t fromUid)
 }
-void LocalStore::checkDerivationOutputs(const Path & drvPath, const Derivation & drv)
+void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivation & drv)
 {
-    string drvName = storePathToName(drvPath);
+    assert(drvPath.isDerivation());
-    assert(isDerivation(drvName));
+    std::string drvName(drvPath.name());
    drvName = string(drvName, 0, drvName.size() - drvExtension.size());
    if (drv.isFixedOutput()) {
        DerivationOutputs::const_iterator out = drv.outputs.find("out");
        if (out == drv.outputs.end())
-            throw Error(format("derivation '%1%' does not have an output named 'out'") % drvPath);
+            throw Error("derivation '%s' does not have an output named 'out'", printStorePath(drvPath));
        bool recursive; Hash h;
        out->second.parseHashInfo(recursive, h);
-        Path outPath = makeFixedOutputPath(recursive, h, drvName);
+        auto outPath = makeFixedOutputPath(recursive, h, drvName);
        StringPairs::const_iterator j = drv.env.find("out");
-        if (out->second.path != outPath || j == drv.env.end() || j->second != outPath)
+        if (out->second.path != outPath || j == drv.env.end() || parseStorePath(j->second) != outPath)
-            throw Error(format("derivation '%1%' has incorrect output '%2%', should be '%3%'")
+            throw Error("derivation '%s' has incorrect output '%s', should be '%s'",
-                % drvPath % out->second.path % outPath);
+                printStorePath(drvPath), printStorePath(out->second.path), printStorePath(outPath));
    }
    else {
-        Derivation drvCopy(drv);
+        Hash h = hashDerivationModulo(*this, drv, true);
        for (auto & i : drvCopy.outputs) {
            i.second.path = "";
            drvCopy.env[i.first] = "";
        }
        Hash h = hashDerivationModulo(*this, drvCopy);
        for (auto & i : drv.outputs) {
-            Path outPath = makeOutputPath(i.first, h, drvName);
+            auto outPath = makeOutputPath(i.first, h, drvName);
            StringPairs::const_iterator j = drv.env.find(i.first);
-            if (i.second.path != outPath || j == drv.env.end() || j->second != outPath)
+            if (i.second.path != outPath || j == drv.env.end() || parseStorePath(j->second) != outPath)
-                throw Error(format("derivation '%1%' has incorrect output '%2%', should be '%3%'")
+                throw Error("derivation '%s' has incorrect output '%s', should be '%s'",
-                    % drvPath % i.second.path % outPath);
+                    printStorePath(drvPath), printStorePath(i.second.path), printStorePath(outPath));
        }
    }
 }
@ -585,16 +579,15 @@ void LocalStore::checkDerivationOutputs(const Path & drvPath, const Derivation &
 uint64_t LocalStore::addValidPath(State & state,
    const ValidPathInfo & info, bool checkOutputs)
 {
    checkStoreName(storePathToName(info.path));
    if (info.ca != "" && !info.isContentAddressed(*this))
-        throw Error("cannot add path '%s' to the Nix store because it claims to be content-addressed but isn't", info.path);
+        throw Error("cannot add path '%s' to the Nix store because it claims to be content-addressed but isn't",
            printStorePath(info.path));
    state.stmtRegisterValidPath.use()
-        (info.path)
+        (printStorePath(info.path))
        (info.narHash.to_string(Base16))
        (info.registrationTime == 0 ? time(0) : info.registrationTime)
-        (info.deriver, info.deriver != "")
+        (info.deriver ? printStorePath(*info.deriver) : "", (bool) info.deriver)
        (info.narSize, info.narSize != 0)
        (info.ultimate ? 1 : 0, info.ultimate)
        (concatStringsSep(" ", info.sigs), !info.sigs.empty())
@ -606,8 +599,8 @@ uint64_t LocalStore::addValidPath(State & state,
       the database.  This is useful for the garbage collector: it can
       efficiently query whether a path is an output of some
       derivation. */
-    if (isDerivation(info.path)) {
+    if (info.path.isDerivation()) {
-        Derivation drv = readDerivation(realStoreDir + "/" + baseNameOf(info.path));
+        auto drv = readDerivation(*this, realStoreDir + "/" + std::string(info.path.to_string()));
        /* Verify that the output paths in the derivation are correct
           (i.e., follow the scheme for computing output paths from
@ -620,34 +613,31 @@ uint64_t LocalStore::addValidPath(State & state,
            state.stmtAddDerivationOutput.use()
                (id)
                (i.first)
-                (i.second.path)
+                (printStorePath(i.second.path))
                .exec();
        }
    }
    {
        auto state_(Store::state.lock());
-        state_->pathInfoCache.upsert(storePathToHash(info.path), std::make_shared<ValidPathInfo>(info));
+        state_->pathInfoCache.upsert(storePathToHash(printStorePath(info.path)), std::make_shared<ValidPathInfo>(info));
    }
    return id;
 }
-void LocalStore::queryPathInfoUncached(const Path & path,
+void LocalStore::queryPathInfoUncached(const StorePath & path,
    Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept
 {
    try {
-        auto info = std::make_shared<ValidPathInfo>();
+        auto info = std::make_shared<ValidPathInfo>(path.clone());
        info->path = path;
        assertStorePath(path);
        callback(retrySQLite<std::shared_ptr<ValidPathInfo>>([&]() {
            auto state(_state.lock());
            /* Get the path info. */
-            auto useQueryPathInfo(state->stmtQueryPathInfo.use()(path));
+            auto useQueryPathInfo(state->stmtQueryPathInfo.use()(printStorePath(info->path)));
            if (!useQueryPathInfo.next())
                return std::shared_ptr<ValidPathInfo>();
@ -657,13 +647,13 @@ void LocalStore::queryPathInfoUncached(const Path & path,
            try {
                info->narHash = Hash(useQueryPathInfo.getStr(1));
            } catch (BadHash & e) {
-                throw Error("in valid-path entry for '%s': %s", path, e.what());
+                throw Error("in valid-path entry for '%s': %s", printStorePath(path), e.what());
            }
            info->registrationTime = useQueryPathInfo.getInt(2);
            auto s = (const char *) sqlite3_column_text(state->stmtQueryPathInfo, 3);
-            if (s) info->deriver = s;
+            if (s) info->deriver = parseStorePath(s);
            /* Note that narSize = NULL yields 0. */
            info->narSize = useQueryPathInfo.getInt(4);
@ -680,7 +670,7 @@ void LocalStore::queryPathInfoUncached(const Path & path,
            auto useQueryReferences(state->stmtQueryReferences.use()(info->id));
            while (useQueryReferences.next())
-                info->references.insert(useQueryReferences.getStr(0));
+                info->references.insert(parseStorePath(useQueryReferences.getStr(0)));
            return info;
        }));
@ -698,27 +688,27 @@ void LocalStore::updatePathInfo(State & state, const ValidPathInfo & info)
        (info.ultimate ? 1 : 0, info.ultimate)
        (concatStringsSep(" ", info.sigs), !info.sigs.empty())
        (info.ca, !info.ca.empty())
-        (info.path)
+        (printStorePath(info.path))
        .exec();
 }
-uint64_t LocalStore::queryValidPathId(State & state, const Path & path)
+uint64_t LocalStore::queryValidPathId(State & state, const StorePath & path)
 {
-    auto use(state.stmtQueryPathInfo.use()(path));
+    auto use(state.stmtQueryPathInfo.use()(printStorePath(path)));
    if (!use.next())
-        throw Error(format("path '%1%' is not valid") % path);
+        throw Error("path '%s' is not valid", printStorePath(path));
    return use.getInt(0);
 }
-bool LocalStore::isValidPath_(State & state, const Path & path)
+bool LocalStore::isValidPath_(State & state, const StorePath & path)
 {
-    return state.stmtQueryPathInfo.use()(path).next();
+    return state.stmtQueryPathInfo.use()(printStorePath(path)).next();
 }
-bool LocalStore::isValidPathUncached(const Path & path)
+bool LocalStore::isValidPathUncached(const StorePath & path)
 {
    return retrySQLite<bool>([&]() {
        auto state(_state.lock());
@ -727,39 +717,38 @@ bool LocalStore::isValidPathUncached(const Path & path)
 }
-PathSet LocalStore::queryValidPaths(const PathSet & paths, SubstituteFlag maybeSubstitute)
+StorePathSet LocalStore::queryValidPaths(const StorePathSet & paths, SubstituteFlag maybeSubstitute)
 {
-    PathSet res;
+    StorePathSet res;
    for (auto & i : paths)
-        if (isValidPath(i)) res.insert(i);
+        if (isValidPath(i)) res.insert(i.clone());
    return res;
 }
-PathSet LocalStore::queryAllValidPaths()
+StorePathSet LocalStore::queryAllValidPaths()
 {
-    return retrySQLite<PathSet>([&]() {
+    return retrySQLite<StorePathSet>([&]() {
        auto state(_state.lock());
        auto use(state->stmtQueryValidPaths.use());
-        PathSet res;
+        StorePathSet res;
-        while (use.next()) res.insert(use.getStr(0));
+        while (use.next()) res.insert(parseStorePath(use.getStr(0)));
        return res;
    });
 }
-void LocalStore::queryReferrers(State & state, const Path & path, PathSet & referrers)
+void LocalStore::queryReferrers(State & state, const StorePath & path, StorePathSet & referrers)
 {
-    auto useQueryReferrers(state.stmtQueryReferrers.use()(path));
+    auto useQueryReferrers(state.stmtQueryReferrers.use()(printStorePath(path)));
    while (useQueryReferrers.next())
-        referrers.insert(useQueryReferrers.getStr(0));
+        referrers.insert(parseStorePath(useQueryReferrers.getStr(0)));
 }
-void LocalStore::queryReferrers(const Path & path, PathSet & referrers)
+void LocalStore::queryReferrers(const StorePath & path, StorePathSet & referrers)
 {
    assertStorePath(path);
    return retrySQLite<void>([&]() {
        auto state(_state.lock());
        queryReferrers(*state, path, referrers);
@ -767,42 +756,40 @@ void LocalStore::queryReferrers(const Path & path, PathSet & referrers)
 }
-PathSet LocalStore::queryValidDerivers(const Path & path)
+StorePathSet LocalStore::queryValidDerivers(const StorePath & path)
 {
-    assertStorePath(path);
+    return retrySQLite<StorePathSet>([&]() {
    return retrySQLite<PathSet>([&]() {
        auto state(_state.lock());
-        auto useQueryValidDerivers(state->stmtQueryValidDerivers.use()(path));
+        auto useQueryValidDerivers(state->stmtQueryValidDerivers.use()(printStorePath(path)));
-        PathSet derivers;
+        StorePathSet derivers;
        while (useQueryValidDerivers.next())
-            derivers.insert(useQueryValidDerivers.getStr(1));
+            derivers.insert(parseStorePath(useQueryValidDerivers.getStr(1)));
        return derivers;
    });
 }
-PathSet LocalStore::queryDerivationOutputs(const Path & path)
+StorePathSet LocalStore::queryDerivationOutputs(const StorePath & path)
 {
-    return retrySQLite<PathSet>([&]() {
+    return retrySQLite<StorePathSet>([&]() {
        auto state(_state.lock());
        auto useQueryDerivationOutputs(state->stmtQueryDerivationOutputs.use()
            (queryValidPathId(*state, path)));
-        PathSet outputs;
+        StorePathSet outputs;
        while (useQueryDerivationOutputs.next())
-            outputs.insert(useQueryDerivationOutputs.getStr(1));
+            outputs.insert(parseStorePath(useQueryDerivationOutputs.getStr(1)));
        return outputs;
    });
 }
-StringSet LocalStore::queryDerivationOutputNames(const Path & path)
+StringSet LocalStore::queryDerivationOutputNames(const StorePath & path)
 {
    return retrySQLite<StringSet>([&]() {
        auto state(_state.lock());
@ -819,31 +806,36 @@ StringSet LocalStore::queryDerivationOutputNames(const Path & path)
 }
-Path LocalStore::queryPathFromHashPart(const string & hashPart)
+std::optional<StorePath> LocalStore::queryPathFromHashPart(const std::string & hashPart)
 {
    if (hashPart.size() != storePathHashLen) throw Error("invalid hash part");
    Path prefix = storeDir + "/" + hashPart;
-    return retrySQLite<Path>([&]() -> std::string {
+    return retrySQLite<std::optional<StorePath>>([&]() -> std::optional<StorePath> {
        auto state(_state.lock());
        auto useQueryPathFromHashPart(state->stmtQueryPathFromHashPart.use()(prefix));
-        if (!useQueryPathFromHashPart.next()) return "";
+        if (!useQueryPathFromHashPart.next()) return {};
        const char * s = (const char *) sqlite3_column_text(state->stmtQueryPathFromHashPart, 0);
-        return s && prefix.compare(0, prefix.size(), s, prefix.size()) == 0 ? s : "";
+        if (s && prefix.compare(0, prefix.size(), s, prefix.size()) == 0)
            return parseStorePath(s);
        return {};
    });
 }
-PathSet LocalStore::querySubstitutablePaths(const PathSet & paths)
+StorePathSet LocalStore::querySubstitutablePaths(const StorePathSet & paths)
 {
-    if (!settings.useSubstitutes) return PathSet();
+    if (!settings.useSubstitutes) return StorePathSet();
-    auto remaining = paths;
+    StorePathSet remaining;
-    PathSet res;
+    for (auto & i : paths)
        remaining.insert(i.clone());
    StorePathSet res;
    for (auto & sub : getDefaultSubstituters()) {
        if (remaining.empty()) break;
@ -852,12 +844,12 @@ PathSet LocalStore::querySubstitutablePaths(const PathSet & paths)
        auto valid = sub->queryValidPaths(remaining);
-        PathSet remaining2;
+        StorePathSet remaining2;
        for (auto & path : remaining)
            if (valid.count(path))
-                res.insert(path);
+                res.insert(path.clone());
            else
-                remaining2.insert(path);
+                remaining2.insert(path.clone());
        std::swap(remaining, remaining2);
    }
@ -866,7 +858,7 @@ PathSet LocalStore::querySubstitutablePaths(const PathSet & paths)
 }
-void LocalStore::querySubstitutablePathInfos(const PathSet & paths,
+void LocalStore::querySubstitutablePathInfos(const StorePathSet & paths,
    SubstitutablePathInfos & infos)
 {
    if (!settings.useSubstitutes) return;
@ -874,17 +866,16 @@ void LocalStore::querySubstitutablePathInfos(const PathSet & paths,
        if (sub->storeDir != storeDir) continue;
        for (auto & path : paths) {
            if (infos.count(path)) continue;
-            debug(format("checking substituter '%s' for path '%s'")
+            debug("checking substituter '%s' for path '%s'", sub->getUri(), printStorePath(path));
                % sub->getUri() % path);
            try {
                auto info = sub->queryPathInfo(path);
                auto narInfo = std::dynamic_pointer_cast<const NarInfo>(
                    std::shared_ptr<const ValidPathInfo>(info));
-                infos[path] = SubstitutablePathInfo{
+                infos.insert_or_assign(path.clone(), SubstitutablePathInfo{
-                    info->deriver,
+                    info->deriver ? info->deriver->clone() : std::optional<StorePath>(),
-                    info->references,
+                    cloneStorePathSet(info->references),
                    narInfo ? narInfo->fileSize : 0,
-                    info->narSize};
+                    info->narSize});
            } catch (InvalidPath &) {
            } catch (SubstituterDisabled &) {
            } catch (Error & e) {
@ -918,7 +909,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
        auto state(_state.lock());
        SQLiteTxn txn(state->db);
-        PathSet paths;
+        StorePathSet paths;
        for (auto & i : infos) {
            assert(i.narHash.type == htSHA256);
@ -926,7 +917,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
                updatePathInfo(*state, i);
            else
                addValidPath(*state, i, false);
-            paths.insert(i.path);
+            paths.insert(i.path.clone());
        }
        for (auto & i : infos) {
@ -939,10 +930,10 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
           this in addValidPath() above, because the references might
           not be valid yet. */
        for (auto & i : infos)
-            if (isDerivation(i.path)) {
+            if (i.path.isDerivation()) {
                // FIXME: inefficient; we already loaded the derivation in addValidPath().
-                Derivation drv = readDerivation(realStoreDir + "/" + baseNameOf(i.path));
+                checkDerivationOutputs(i.path,
-                checkDerivationOutputs(i.path, drv);
+                    readDerivation(*this, realStoreDir + "/" + std::string(i.path.to_string())));
            }
        /* Do a topological sort of the paths.  This will throw an
@ -958,18 +949,18 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
 /* Invalidate a path.  The caller is responsible for checking that
   there are no referrers. */
-void LocalStore::invalidatePath(State & state, const Path & path)
+void LocalStore::invalidatePath(State & state, const StorePath & path)
 {
-    debug(format("invalidating path '%1%'") % path);
+    debug("invalidating path '%s'", printStorePath(path));
-    state.stmtInvalidatePath.use()(path).exec();
+    state.stmtInvalidatePath.use()(printStorePath(path)).exec();
    /* Note that the foreign key constraints on the Refs table take
       care of deleting the references entries for `path'. */
    {
        auto state_(Store::state.lock());
-        state_->pathInfoCache.erase(storePathToHash(path));
+        state_->pathInfoCache.erase(storePathToHash(printStorePath(path)));
    }
 }
@ -987,10 +978,10 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
    RepairFlag repair, CheckSigsFlag checkSigs, std::shared_ptr<FSAccessor> accessor)
 {
    if (!info.narHash)
-        throw Error("cannot add path '%s' because it lacks a hash", info.path);
+        throw Error("cannot add path '%s' because it lacks a hash", printStorePath(info.path));
    if (requireSigs && checkSigs && !info.checkSignatures(*this, getPublicKeys()))
-        throw Error("cannot add path '%s' because it lacks a valid signature", info.path);
+        throw Error("cannot add path '%s' because it lacks a valid signature", printStorePath(info.path));
    addTempRoot(info.path);
@ -998,12 +989,12 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
        PathLocks outputLock;
-        Path realPath = realStoreDir + "/" + baseNameOf(info.path);
+        Path realPath = realStoreDir + "/" + std::string(info.path.to_string());
        /* Lock the output path.  But don't lock if we're being called
           from a build hook (whose parent process already acquired a
           lock on this path). */
-        if (!locksHeld.count(info.path))
+        if (!locksHeld.count(printStorePath(info.path)))
            outputLock.lockPaths({realPath});
        if (repair || !isValidPath(info.path)) {
@ -1018,7 +1009,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
            else {
                if (!info.references.empty())
                    settings.requireExperimentalFeature("ca-references");
-                hashSink = std::make_unique<HashModuloSink>(htSHA256, storePathToHash(info.path));
+                hashSink = std::make_unique<HashModuloSink>(htSHA256, storePathToHash(printStorePath(info.path)));
            }
            LambdaSource wrapperSource([&](unsigned char * data, size_t len) -> size_t {
@ -1033,11 +1024,11 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
            if (hashResult.first != info.narHash)
                throw Error("hash mismatch importing path '%s';\n  wanted: %s\n  got:    %s",
-                    info.path, info.narHash.to_string(), hashResult.first.to_string());
+                    printStorePath(info.path), info.narHash.to_string(), hashResult.first.to_string());
            if (hashResult.second != info.narSize)
                throw Error("size mismatch importing path '%s';\n  wanted: %s\n  got:   %s",
-                    info.path, info.narSize, hashResult.second);
+                    printStorePath(info.path), info.narSize, hashResult.second);
            autoGC();
@ -1053,12 +1044,12 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 }
-Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
+StorePath LocalStore::addToStoreFromDump(const string & dump, const string & name,
    bool recursive, HashType hashAlgo, RepairFlag repair)
 {
    Hash h = hashString(hashAlgo, dump);
-    Path dstPath = makeFixedOutputPath(recursive, h, name);
+    auto dstPath = makeFixedOutputPath(recursive, h, name);
    addTempRoot(dstPath);
@ -1067,7 +1058,8 @@ Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
        /* The first check above is an optimisation to prevent
           unnecessary lock acquisition. */
-        Path realPath = realStoreDir + "/" + baseNameOf(dstPath);
+        Path realPath = realStoreDir + "/";
        realPath += dstPath.to_string();
        PathLocks outputLock({realPath});
@ -1098,8 +1090,7 @@ Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
            optimisePath(realPath); // FIXME: combine with hashPath()
-            ValidPathInfo info;
+            ValidPathInfo info(dstPath.clone());
            info.path = dstPath;
            info.narHash = hash.first;
            info.narSize = hash.second;
            info.ca = makeFixedOutputCA(recursive, h);
@ -1113,7 +1104,7 @@ Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
 }
-Path LocalStore::addToStore(const string & name, const Path & _srcPath,
+StorePath LocalStore::addToStore(const string & name, const Path & _srcPath,
    bool recursive, HashType hashAlgo, PathFilter & filter, RepairFlag repair)
 {
    Path srcPath(absPath(_srcPath));
@ -1131,8 +1122,8 @@ Path LocalStore::addToStore(const string & name, const Path & _srcPath,
 }
-Path LocalStore::addTextToStore(const string & name, const string & s,
+StorePath LocalStore::addTextToStore(const string & name, const string & s,
-    const PathSet & references, RepairFlag repair)
+    const StorePathSet & references, RepairFlag repair)
 {
    auto hash = hashString(htSHA256, s);
    auto dstPath = makeTextPath(name, hash, references);
@ -1141,7 +1132,8 @@ Path LocalStore::addTextToStore(const string & name, const string & s,
    if (repair || !isValidPath(dstPath)) {
-        Path realPath = realStoreDir + "/" + baseNameOf(dstPath);
+        Path realPath = realStoreDir + "/";
        realPath += dstPath.to_string();
        PathLocks outputLock({realPath});
@ -1161,11 +1153,10 @@ Path LocalStore::addTextToStore(const string & name, const string & s,
            optimisePath(realPath);
-            ValidPathInfo info;
+            ValidPathInfo info(dstPath.clone());
            info.path = dstPath;
            info.narHash = narHash;
            info.narSize = sink.s->size();
-            info.references = references;
+            info.references = cloneStorePathSet(references);
            info.ca = "text:" + hash.to_string();
            registerValidPath(info);
        }
@ -1187,27 +1178,25 @@ Path LocalStore::createTempDirInStore()
           the GC between createTempDir() and addTempRoot(), so repeat
           until `tmpDir' exists. */
        tmpDir = createTempDir(realStoreDir);
-        addTempRoot(tmpDir);
+        addTempRoot(parseStorePath(tmpDir));
    } while (!pathExists(tmpDir));
    return tmpDir;
 }
-void LocalStore::invalidatePathChecked(const Path & path)
+void LocalStore::invalidatePathChecked(const StorePath & path)
 {
    assertStorePath(path);
    retrySQLite<void>([&]() {
        auto state(_state.lock());
        SQLiteTxn txn(state->db);
        if (isValidPath_(*state, path)) {
-            PathSet referrers; queryReferrers(*state, path, referrers);
+            StorePathSet referrers; queryReferrers(*state, path, referrers);
            referrers.erase(path); /* ignore self-references */
            if (!referrers.empty())
-                throw PathInUse(format("cannot delete path '%1%' because it is in use by %2%")
+                throw PathInUse("cannot delete path '%s' because it is in use by %s",
-                    % path % showPaths(referrers));
+                    printStorePath(path), showPaths(referrers));
            invalidatePath(*state, path);
        }
@ -1226,18 +1215,19 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
       existing and valid paths. */
    AutoCloseFD fdGCLock = openGCLock(ltWrite);
-    PathSet store;
+    StringSet store;
    for (auto & i : readDirectory(realStoreDir)) store.insert(i.name);
    /* Check whether all valid paths actually exist. */
    printInfo("checking path existence...");
-    PathSet validPaths2 = queryAllValidPaths(), validPaths, done;
+    StorePathSet validPaths;
    PathSet done;
    fdGCLock = -1;
-    for (auto & i : validPaths2)
+    for (auto & i : queryAllValidPaths())
-        verifyPath(i, store, done, validPaths, repair, errors);
+        verifyPath(printStorePath(i), store, done, validPaths, repair, errors);
    /* Optionally, check the content hashes (slow). */
    if (checkContents) {
@ -1272,21 +1262,20 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                auto info = std::const_pointer_cast<ValidPathInfo>(std::shared_ptr<const ValidPathInfo>(queryPathInfo(i)));
                /* Check the content hash (optionally - slow). */
-                printMsg(lvlTalkative, format("checking contents of '%1%'") % i);
+                printMsg(lvlTalkative, "checking contents of '%s'", printStorePath(i));
                std::unique_ptr<AbstractHashSink> hashSink;
                if (info->ca == "")
                    hashSink = std::make_unique<HashSink>(info->narHash.type);
                else
-                    hashSink = std::make_unique<HashModuloSink>(info->narHash.type, storePathToHash(info->path));
+                    hashSink = std::make_unique<HashModuloSink>(info->narHash.type, storePathToHash(printStorePath(info->path)));
-                dumpPath(toRealPath(i), *hashSink);
+                dumpPath(toRealPath(printStorePath(i)), *hashSink);
                auto current = hashSink->finish();
                if (info->narHash != nullHash && info->narHash != current.first) {
-                    printError(format("path '%1%' was modified! "
+                    printError("path '%s' was modified! expected hash '%s', got '%s'",
-                            "expected hash '%2%', got '%3%'")
+                        printStorePath(i), info->narHash.to_string(), current.first.to_string());
                        % i % info->narHash.to_string() % current.first.to_string());
                    if (repair) repairPath(i); else errors = true;
                } else {
@ -1294,14 +1283,14 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                    /* Fill in missing hashes. */
                    if (info->narHash == nullHash) {
-                        printError(format("fixing missing hash on '%1%'") % i);
+                        printError("fixing missing hash on '%s'", printStorePath(i));
                        info->narHash = current.first;
                        update = true;
                    }
                    /* Fill in missing narSize fields (from old stores). */
                    if (info->narSize == 0) {
-                        printError(format("updating size field on '%1%' to %2%") % i % current.second);
+                        printError("updating size field on '%s' to %s", printStorePath(i), current.second);
                        info->narSize = current.second;
                        update = true;
                    }
@ -1317,9 +1306,9 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                /* It's possible that the path got GC'ed, so ignore
                   errors on invalid paths. */
                if (isValidPath(i))
-                    printError(format("error: %1%") % e.msg());
+                    printError("error: %s", e.msg());
                else
-                    printError(format("warning: %1%") % e.msg());
+                    warn(e.msg());
                errors = true;
            }
        }
@ -1329,43 +1318,43 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 }
-void LocalStore::verifyPath(const Path & path, const PathSet & store,
+void LocalStore::verifyPath(const Path & pathS, const StringSet & store,
-    PathSet & done, PathSet & validPaths, RepairFlag repair, bool & errors)
+    PathSet & done, StorePathSet & validPaths, RepairFlag repair, bool & errors)
 {
    checkInterrupt();
-    if (!done.insert(path).second) return;
+    if (!done.insert(pathS).second) return;
-    if (!isStorePath(path)) {
+    if (!isStorePath(pathS)) {
-        printError(format("path '%1%' is not in the Nix store") % path);
+        printError("path '%s' is not in the Nix store", pathS);
        auto state(_state.lock());
        invalidatePath(*state, path);
        return;
    }
-    if (store.find(baseNameOf(path)) == store.end()) {
+    auto path = parseStorePath(pathS);
    if (!store.count(std::string(path.to_string()))) {
        /* Check any referrers first.  If we can invalidate them
           first, then we can invalidate this path as well. */
        bool canInvalidate = true;
-        PathSet referrers; queryReferrers(path, referrers);
+        StorePathSet referrers; queryReferrers(path, referrers);
        for (auto & i : referrers)
            if (i != path) {
-                verifyPath(i, store, done, validPaths, repair, errors);
+                verifyPath(printStorePath(i), store, done, validPaths, repair, errors);
-                if (validPaths.find(i) != validPaths.end())
+                if (validPaths.count(i))
                    canInvalidate = false;
            }
        if (canInvalidate) {
-            printError(format("path '%1%' disappeared, removing from database...") % path);
+            printError("path '%s' disappeared, removing from database...", pathS);
            auto state(_state.lock());
            invalidatePath(*state, path);
        } else {
-            printError(format("path '%1%' disappeared, but it still has valid referrers!") % path);
+            printError("path '%s' disappeared, but it still has valid referrers!", pathS);
            if (repair)
                try {
                    repairPath(path);
                } catch (Error & e) {
-                    printError(format("warning: %1%") % e.msg());
+                    warn(e.msg());
                    errors = true;
                }
            else errors = true;
@ -1374,7 +1363,7 @@ void LocalStore::verifyPath(const Path & path, const PathSet & store,
        return;
    }
-    validPaths.insert(path);
+    validPaths.insert(std::move(path));
 }
@ -1443,7 +1432,7 @@ void LocalStore::vacuumDB()
 }
-void LocalStore::addSignatures(const Path & storePath, const StringSet & sigs)
+void LocalStore::addSignatures(const StorePath & storePath, const StringSet & sigs)
 {
    retrySQLite<void>([&]() {
        auto state(_state.lock());
@ -1469,7 +1458,7 @@ void LocalStore::signPathInfo(ValidPathInfo & info)
    for (auto & secretKeyFile : secretKeyFiles.get()) {
        SecretKey secretKey(readFile(secretKeyFile));
-        info.sign(secretKey);
+        info.sign(*this, secretKey);
    }
 }
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@ -119,36 +119,36 @@ public:
    std::string getUri() override;
-    bool isValidPathUncached(const Path & path) override;
+    bool isValidPathUncached(const StorePath & path) override;
-    PathSet queryValidPaths(const PathSet & paths,
+    StorePathSet queryValidPaths(const StorePathSet & paths,
        SubstituteFlag maybeSubstitute = NoSubstitute) override;
-    PathSet queryAllValidPaths() override;
+    StorePathSet queryAllValidPaths() override;
-    void queryPathInfoUncached(const Path & path,
+    void queryPathInfoUncached(const StorePath & path,
        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override;
-    void queryReferrers(const Path & path, PathSet & referrers) override;
+    void queryReferrers(const StorePath & path, StorePathSet & referrers) override;
-    PathSet queryValidDerivers(const Path & path) override;
+    StorePathSet queryValidDerivers(const StorePath & path) override;
-    PathSet queryDerivationOutputs(const Path & path) override;
+    StorePathSet queryDerivationOutputs(const StorePath & path) override;
-    StringSet queryDerivationOutputNames(const Path & path) override;
+    StringSet queryDerivationOutputNames(const StorePath & path) override;
-    Path queryPathFromHashPart(const string & hashPart) override;
+    std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
-    PathSet querySubstitutablePaths(const PathSet & paths) override;
+    StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
-    void querySubstitutablePathInfos(const PathSet & paths,
+    void querySubstitutablePathInfos(const StorePathSet & paths,
        SubstitutablePathInfos & infos) override;
    void addToStore(const ValidPathInfo & info, Source & source,
        RepairFlag repair, CheckSigsFlag checkSigs,
        std::shared_ptr<FSAccessor> accessor) override;
-    Path addToStore(const string & name, const Path & srcPath,
+    StorePath addToStore(const string & name, const Path & srcPath,
        bool recursive, HashType hashAlgo,
        PathFilter & filter, RepairFlag repair) override;
@ -156,20 +156,22 @@ public:
       in `dump', which is either a NAR serialisation (if recursive ==
       true) or simply the contents of a regular file (if recursive ==
       false). */
-    Path addToStoreFromDump(const string & dump, const string & name,
+    StorePath addToStoreFromDump(const string & dump, const string & name,
        bool recursive = true, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair) override;
-    Path addTextToStore(const string & name, const string & s,
+    StorePath addTextToStore(const string & name, const string & s,
-        const PathSet & references, RepairFlag repair) override;
+        const StorePathSet & references, RepairFlag repair) override;
-    void buildPaths(const PathSet & paths, BuildMode buildMode) override;
+    void buildPaths(
-
+        const std::vector<StorePathWithOutputs> & paths,
    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
        BuildMode buildMode) override;
-    void ensurePath(const Path & path) override;
+    BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
        BuildMode buildMode) override;
-    void addTempRoot(const Path & path) override;
+    void ensurePath(const StorePath & path) override;
    void addTempRoot(const StorePath & path) override;
    void addIndirectRoot(const Path & path) override;
@ -215,9 +217,9 @@ public:
    /* Repair the contents of the given path by redownloading it using
       a substituter (if available). */
-    void repairPath(const Path & path);
+    void repairPath(const StorePath & path);
-    void addSignatures(const Path & storePath, const StringSet & sigs) override;
+    void addSignatures(const StorePath & storePath, const StringSet & sigs) override;
    /* If free disk space in /nix/store if below minFree, delete
       garbage until it exceeds maxFree. */
@ -231,17 +233,17 @@ private:
    void makeStoreWritable();
-    uint64_t queryValidPathId(State & state, const Path & path);
+    uint64_t queryValidPathId(State & state, const StorePath & path);
    uint64_t addValidPath(State & state, const ValidPathInfo & info, bool checkOutputs = true);
-    void invalidatePath(State & state, const Path & path);
+    void invalidatePath(State & state, const StorePath & path);
    /* Delete a path from the Nix store. */
-    void invalidatePathChecked(const Path & path);
+    void invalidatePathChecked(const StorePath & path);
-    void verifyPath(const Path & path, const PathSet & store,
+    void verifyPath(const Path & path, const StringSet & store,
-        PathSet & done, PathSet & validPaths, RepairFlag repair, bool & errors);
+        PathSet & done, StorePathSet & validPaths, RepairFlag repair, bool & errors);
    void updatePathInfo(State & state, const ValidPathInfo & info);
@ -256,7 +258,7 @@ private:
    void tryToDelete(GCState & state, const Path & path);
-    bool canReachRoot(GCState & state, PathSet & visited, const Path & path);
+    bool canReachRoot(GCState & state, StorePathSet & visited, const StorePath & path);
    void deletePathRecursive(GCState & state, const Path & path);
@ -275,7 +277,7 @@ private:
    Path createTempDirInStore();
-    void checkDerivationOutputs(const Path & drvPath, const Derivation & drv);
+    void checkDerivationOutputs(const StorePath & drvPath, const Derivation & drv);
    typedef std::unordered_set<ino_t> InodeHash;
@ -284,8 +286,8 @@ private:
    void optimisePath_(Activity * act, OptimiseStats & stats, const Path & path, InodeHash & inodeHash);
    // Internal versions that are not wrapped in retry_sqlite.
-    bool isValidPath_(State & state, const Path & path);
+    bool isValidPath_(State & state, const StorePath & path);
-    void queryReferrers(State & state, const Path & path, PathSet & referrers);
+    void queryReferrers(State & state, const StorePath & path, StorePathSet & referrers);
    /* Add signatures to a ValidPathInfo using the secret keys
       specified by the ‘secret-key-files’ option. */
--- a/src/libstore/local.mk
+++ b/src/libstore/local.mk
@ -6,7 +6,7 @@ libstore_DIR := $(d)
 libstore_SOURCES := $(wildcard $(d)/*.cc $(d)/builtins/*.cc)
-libstore_LIBS = libutil
+libstore_LIBS = libutil libnixrust
 libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS) $(SODIUM_LIBS) -pthread
 ifneq ($(OS), FreeBSD)
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@ -9,13 +9,13 @@
 namespace nix {
-void Store::computeFSClosure(const PathSet & startPaths,
+void Store::computeFSClosure(const StorePathSet & startPaths,
-    PathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
+    StorePathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
 {
    struct State
    {
        size_t pending;
-        PathSet & paths;
+        StorePathSet & paths;
        std::exception_ptr exc;
    };
@ -29,45 +29,47 @@ void Store::computeFSClosure(const PathSet & startPaths,
        {
            auto state(state_.lock());
            if (state->exc) return;
-            if (!state->paths.insert(path).second) return;
+            if (!state->paths.insert(parseStorePath(path)).second) return;
            state->pending++;
        }
-        queryPathInfo(path, {[&, path](std::future<ref<const ValidPathInfo>> fut) {
+        queryPathInfo(parseStorePath(path), {[&, pathS(path)](std::future<ref<const ValidPathInfo>> fut) {
            // FIXME: calls to isValidPath() should be async
            try {
                auto info = fut.get();
                auto path = parseStorePath(pathS);
                if (flipDirection) {
-                    PathSet referrers;
+                    StorePathSet referrers;
                    queryReferrers(path, referrers);
                    for (auto & ref : referrers)
                        if (ref != path)
-                            enqueue(ref);
+                            enqueue(printStorePath(ref));
                    if (includeOutputs)
                        for (auto & i : queryValidDerivers(path))
-                            enqueue(i);
+                            enqueue(printStorePath(i));
-                    if (includeDerivers && isDerivation(path))
+                    if (includeDerivers && path.isDerivation())
                        for (auto & i : queryDerivationOutputs(path))
                            if (isValidPath(i) && queryPathInfo(i)->deriver == path)
-                                enqueue(i);
+                                enqueue(printStorePath(i));
                } else {
                    for (auto & ref : info->references)
                        if (ref != path)
-                            enqueue(ref);
+                            enqueue(printStorePath(ref));
-                    if (includeOutputs && isDerivation(path))
+                    if (includeOutputs && path.isDerivation())
                        for (auto & i : queryDerivationOutputs(path))
-                            if (isValidPath(i)) enqueue(i);
+                            if (isValidPath(i)) enqueue(printStorePath(i));
-                    if (includeDerivers && isValidPath(info->deriver))
+                    if (includeDerivers && info->deriver && isValidPath(*info->deriver))
-                        enqueue(info->deriver);
+                        enqueue(printStorePath(*info->deriver));
                }
@ -87,7 +89,7 @@ void Store::computeFSClosure(const PathSet & startPaths,
    };
    for (auto & startPath : startPaths)
-        enqueue(startPath);
+        enqueue(printStorePath(startPath));
    {
        auto state(state_.lock());
@ -97,15 +99,17 @@ void Store::computeFSClosure(const PathSet & startPaths,
 }
-void Store::computeFSClosure(const Path & startPath,
+void Store::computeFSClosure(const StorePath & startPath,
-    PathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
+    StorePathSet & paths_, bool flipDirection, bool includeOutputs, bool includeDerivers)
 {
-    computeFSClosure(PathSet{startPath}, paths_, flipDirection, includeOutputs, includeDerivers);
+    StorePathSet paths;
    paths.insert(startPath.clone());
    computeFSClosure(paths, paths_, flipDirection, includeOutputs, includeDerivers);
 }
-void Store::queryMissing(const PathSet & targets,
+void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
-    PathSet & willBuild_, PathSet & willSubstitute_, PathSet & unknown_,
+    StorePathSet & willBuild_, StorePathSet & willSubstitute_, StorePathSet & unknown_,
    unsigned long long & downloadSize_, unsigned long long & narSize_)
 {
    Activity act(*logger, lvlDebug, actUnknown, "querying info about missing paths");
@ -116,8 +120,8 @@ void Store::queryMissing(const PathSet & targets,
    struct State
    {
-        PathSet done;
+        std::unordered_set<std::string> done;
-        PathSet & unknown, & willSubstitute, & willBuild;
+        StorePathSet & unknown, & willSubstitute, & willBuild;
        unsigned long long & downloadSize;
        unsigned long long & narSize;
    };
@ -126,31 +130,36 @@ void Store::queryMissing(const PathSet & targets,
    {
        size_t left;
        bool done = false;
-        PathSet outPaths;
+        StorePathSet outPaths;
        DrvState(size_t left) : left(left) { }
    };
-    Sync<State> state_(State{PathSet(), unknown_, willSubstitute_, willBuild_, downloadSize_, narSize_});
+    Sync<State> state_(State{{}, unknown_, willSubstitute_, willBuild_, downloadSize_, narSize_});
-    std::function<void(Path)> doPath;
+    std::function<void(StorePathWithOutputs)> doPath;
-    auto mustBuildDrv = [&](const Path & drvPath, const Derivation & drv) {
+    auto mustBuildDrv = [&](const StorePath & drvPath, const Derivation & drv) {
        {
            auto state(state_.lock());
-            state->willBuild.insert(drvPath);
+            state->willBuild.insert(drvPath.clone());
        }
        for (auto & i : drv.inputDrvs)
-            pool.enqueue(std::bind(doPath, makeDrvPathWithOutputs(i.first, i.second)));
+            pool.enqueue(std::bind(doPath, StorePathWithOutputs(i.first, i.second)));
    };
    auto checkOutput = [&](
-        const Path & drvPath, ref<Derivation> drv, const Path & outPath, ref<Sync<DrvState>> drvState_)
+        const Path & drvPathS, ref<Derivation> drv, const Path & outPathS, ref<Sync<DrvState>> drvState_)
    {
        if (drvState_->lock()->done) return;
        auto drvPath = parseStorePath(drvPathS);
        auto outPath = parseStorePath(outPathS);
        SubstitutablePathInfos infos;
-        querySubstitutablePathInfos({outPath}, infos);
+        StorePathSet paths; // FIXME
        paths.insert(outPath.clone());
        querySubstitutablePathInfos(paths, infos);
        if (infos.empty()) {
            drvState_->lock()->done = true;
@ -161,74 +170,74 @@ void Store::queryMissing(const PathSet & targets,
                if (drvState->done) return;
                assert(drvState->left);
                drvState->left--;
-                drvState->outPaths.insert(outPath);
+                drvState->outPaths.insert(outPath.clone());
                if (!drvState->left) {
                    for (auto & path : drvState->outPaths)
-                        pool.enqueue(std::bind(doPath, path));
+                        pool.enqueue(std::bind(doPath, StorePathWithOutputs(path.clone())));
                }
            }
        }
    };
-    doPath = [&](const Path & path) {
+    doPath = [&](const StorePathWithOutputs & path) {
        {
            auto state(state_.lock());
-            if (!state->done.insert(path).second) return;
+            if (!state->done.insert(path.to_string(*this)).second) return;
        }
-        DrvPathWithOutputs i2 = parseDrvPathWithOutputs(path);
+        if (path.path.isDerivation()) {
-
+            if (!isValidPath(path.path)) {
        if (isDerivation(i2.first)) {
            if (!isValidPath(i2.first)) {
                // FIXME: we could try to substitute the derivation.
                auto state(state_.lock());
-                state->unknown.insert(path);
+                state->unknown.insert(path.path.clone());
                return;
            }
-            Derivation drv = derivationFromPath(i2.first);
+            auto drv = make_ref<Derivation>(derivationFromPath(path.path));
-            ParsedDerivation parsedDrv(i2.first, drv);
+            ParsedDerivation parsedDrv(path.path.clone(), *drv);
            PathSet invalid;
-            for (auto & j : drv.outputs)
+            for (auto & j : drv->outputs)
-                if (wantOutput(j.first, i2.second)
+                if (wantOutput(j.first, path.outputs)
                    && !isValidPath(j.second.path))
-                    invalid.insert(j.second.path);
+                    invalid.insert(printStorePath(j.second.path));
            if (invalid.empty()) return;
            if (settings.useSubstitutes && parsedDrv.substitutesAllowed()) {
                auto drvState = make_ref<Sync<DrvState>>(DrvState(invalid.size()));
                for (auto & output : invalid)
-                    pool.enqueue(std::bind(checkOutput, i2.first, make_ref<Derivation>(drv), output, drvState));
+                    pool.enqueue(std::bind(checkOutput, printStorePath(path.path), drv, output, drvState));
            } else
-                mustBuildDrv(i2.first, drv);
+                mustBuildDrv(path.path, *drv);
        } else {
-            if (isValidPath(path)) return;
+            if (isValidPath(path.path)) return;
            SubstitutablePathInfos infos;
-            querySubstitutablePathInfos({path}, infos);
+            StorePathSet paths; // FIXME
            paths.insert(path.path.clone());
            querySubstitutablePathInfos(paths, infos);
            if (infos.empty()) {
                auto state(state_.lock());
-                state->unknown.insert(path);
+                state->unknown.insert(path.path.clone());
                return;
            }
-            auto info = infos.find(path);
+            auto info = infos.find(path.path);
            assert(info != infos.end());
            {
                auto state(state_.lock());
-                state->willSubstitute.insert(path);
+                state->willSubstitute.insert(path.path.clone());
                state->downloadSize += info->second.downloadSize;
                state->narSize += info->second.narSize;
            }
            for (auto & ref : info->second.references)
-                pool.enqueue(std::bind(doPath, ref));
+                pool.enqueue(std::bind(doPath, StorePathWithOutputs(ref)));
        }
    };
@ -239,39 +248,42 @@ void Store::queryMissing(const PathSet & targets,
 }
-Paths Store::topoSortPaths(const PathSet & paths)
+StorePaths Store::topoSortPaths(const StorePathSet & paths)
 {
-    Paths sorted;
+    StorePaths sorted;
-    PathSet visited, parents;
+    StorePathSet visited, parents;
-    std::function<void(const Path & path, const Path * parent)> dfsVisit;
+    std::function<void(const StorePath & path, const StorePath * parent)> dfsVisit;
-    dfsVisit = [&](const Path & path, const Path * parent) {
+    dfsVisit = [&](const StorePath & path, const StorePath * parent) {
-        if (parents.find(path) != parents.end())
+        if (parents.count(path))
-            throw BuildError(format("cycle detected in the references of '%1%' from '%2%'") % path % *parent);
+            throw BuildError("cycle detected in the references of '%s' from '%s'",
                printStorePath(path), printStorePath(*parent));
-        if (!visited.insert(path).second) return;
+        if (!visited.insert(path.clone()).second) return;
-        parents.insert(path);
+        parents.insert(path.clone());
-        PathSet references;
+        StorePathSet references;
        try {
-            references = queryPathInfo(path)->references;
+            references = cloneStorePathSet(queryPathInfo(path)->references);
        } catch (InvalidPath &) {
        }
        for (auto & i : references)
            /* Don't traverse into paths that don't exist.  That can
               happen due to substitutes for non-existent paths. */
-            if (i != path && paths.find(i) != paths.end())
+            if (i != path && paths.count(i))
                dfsVisit(i, &path);
-        sorted.push_front(path);
+        sorted.push_back(path.clone());
        parents.erase(path);
    };
    for (auto & i : paths)
        dfsVisit(i, nullptr);
    std::reverse(sorted.begin(), sorted.end());
    return sorted;
 }
--- a/src/libstore/nar-info-disk-cache.cc
+++ b/src/libstore/nar-info-disk-cache.cc
@ -193,11 +193,8 @@ public:
            if (!queryNAR.getInt(0))
                return {oInvalid, 0};
            auto narInfo = make_ref<NarInfo>();
            auto namePart = queryNAR.getStr(1);
-            narInfo->path = cache.storeDir + "/" +
+            auto narInfo = make_ref<NarInfo>(StorePath::fromBaseName(hashPart + "-" + namePart));
                hashPart + (namePart.empty() ? "" : "-" + namePart);
            narInfo->url = queryNAR.getStr(2);
            narInfo->compression = queryNAR.getStr(3);
            if (!queryNAR.isNull(4))
@ -206,9 +203,9 @@ public:
            narInfo->narHash = Hash(queryNAR.getStr(6));
            narInfo->narSize = queryNAR.getInt(7);
            for (auto & r : tokenizeString<Strings>(queryNAR.getStr(8), " "))
-                narInfo->references.insert(cache.storeDir + "/" + r);
+                narInfo->references.insert(StorePath::fromBaseName(r));
            if (!queryNAR.isNull(9))
-                narInfo->deriver = cache.storeDir + "/" + queryNAR.getStr(9);
+                narInfo->deriver = StorePath::fromBaseName(queryNAR.getStr(9));
            for (auto & sig : tokenizeString<Strings>(queryNAR.getStr(10), " "))
                narInfo->sigs.insert(sig);
            narInfo->ca = queryNAR.getStr(11);
@ -230,12 +227,12 @@ public:
                auto narInfo = std::dynamic_pointer_cast<const NarInfo>(info);
-                assert(hashPart == storePathToHash(info->path));
+                //assert(hashPart == storePathToHash(info->path));
                state->insertNAR.use()
                    (cache.id)
                    (hashPart)
-                    (storePathToName(info->path))
+                    (std::string(info->path.name()))
                    (narInfo ? narInfo->url : "", narInfo != 0)
                    (narInfo ? narInfo->compression : "", narInfo != 0)
                    (narInfo && narInfo->fileHash ? narInfo->fileHash.to_string() : "", narInfo && narInfo->fileHash)
@ -243,7 +240,7 @@ public:
                    (info->narHash.to_string())
                    (info->narSize)
                    (concatStringsSep(" ", info->shortRefs()))
-                    (info->deriver != "" ? baseNameOf(info->deriver) : "", info->deriver != "")
+                    (info->deriver ? std::string(info->deriver->to_string()) : "", (bool) info->deriver)
                    (concatStringsSep(" ", info->sigs))
                    (info->ca)
                    (time(0)).exec();
--- a/src/libstore/nar-info.cc
+++ b/src/libstore/nar-info.cc
@ -4,6 +4,7 @@
 namespace nix {
 NarInfo::NarInfo(const Store & store, const std::string & s, const std::string & whence)
    : ValidPathInfo(StorePath::make((unsigned char *) "xxxxxxxxxxxxxxxxxxxx", "x")) // FIXME: hack
 {
    auto corrupt = [&]() {
        throw Error(format("NAR info file '%1%' is corrupt") % whence);
@ -18,6 +19,8 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
        }
    };
    bool havePath = false;
    size_t pos = 0;
    while (pos < s.size()) {
@ -32,8 +35,8 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
        std::string value(s, colon + 2, eol - colon - 2);
        if (name == "StorePath") {
-            if (!store.isStorePath(value)) corrupt();
+            path = store.parseStorePath(value);
-            path = value;
+            havePath = true;
        }
        else if (name == "URL")
            url = value;
@ -52,18 +55,12 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
        else if (name == "References") {
            auto refs = tokenizeString<Strings>(value, " ");
            if (!references.empty()) corrupt();
-            for (auto & r : refs) {
+            for (auto & r : refs)
-                auto r2 = store.storeDir + "/" + r;
+                references.insert(StorePath::fromBaseName(r));
                if (!store.isStorePath(r2)) corrupt();
                references.insert(r2);
            }
        }
        else if (name == "Deriver") {
-            if (value != "unknown-deriver") {
+            if (value != "unknown-deriver")
-                auto p = store.storeDir + "/" + value;
+                deriver = StorePath::fromBaseName(value);
                if (!store.isStorePath(p)) corrupt();
                deriver = p;
            }
        }
        else if (name == "System")
            system = value;
@ -79,13 +76,13 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
    if (compression == "") compression = "bzip2";
-    if (path.empty() || url.empty() || narSize == 0 || !narHash) corrupt();
+    if (!havePath || url.empty() || narSize == 0 || !narHash) corrupt();
 }
-std::string NarInfo::to_string() const
+std::string NarInfo::to_string(const Store & store) const
 {
    std::string res;
-    res += "StorePath: " + path + "\n";
+    res += "StorePath: " + store.printStorePath(path) + "\n";
    res += "URL: " + url + "\n";
    assert(compression != "");
    res += "Compression: " + compression + "\n";
@ -98,8 +95,8 @@ std::string NarInfo::to_string() const
    res += "References: " + concatStringsSep(" ", shortRefs()) + "\n";
-    if (!deriver.empty())
+    if (deriver)
-        res += "Deriver: " + baseNameOf(deriver) + "\n";
+        res += "Deriver: " + std::string(deriver->to_string()) + "\n";
    if (!system.empty())
        res += "System: " + system + "\n";
--- a/src/libstore/nar-info.hh
+++ b/src/libstore/nar-info.hh
@ -14,11 +14,12 @@ struct NarInfo : ValidPathInfo
    uint64_t fileSize = 0;
    std::string system;
-    NarInfo() { }
+    NarInfo() = delete;
    NarInfo(StorePath && path) : ValidPathInfo(std::move(path)) { }
    NarInfo(const ValidPathInfo & info) : ValidPathInfo(info) { }
    NarInfo(const Store & store, const std::string & s, const std::string & whence);
-    std::string to_string() const;
+    std::string to_string(const Store & store) const;
 };
 }
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@ -254,7 +254,7 @@ void LocalStore::optimiseStore(OptimiseStats & stats)
 {
    Activity act(*logger, actOptimiseStore);
-    PathSet paths = queryAllValidPaths();
+    auto paths = queryAllValidPaths();
    InodeHash inodeHash = loadInodeHash();
    act.progress(0, paths.size());
@ -265,8 +265,8 @@ void LocalStore::optimiseStore(OptimiseStats & stats)
        addTempRoot(i);
        if (!isValidPath(i)) continue; /* path was GC'ed, probably */
        {
-            Activity act(*logger, lvlTalkative, actUnknown, fmt("optimising path '%s'", i));
+            Activity act(*logger, lvlTalkative, actUnknown, fmt("optimising path '%s'", printStorePath(i)));
-            optimisePath_(&act, stats, realStoreDir + "/" + baseNameOf(i), inodeHash);
+            optimisePath_(&act, stats, realStoreDir + "/" + std::string(i.to_string()), inodeHash);
        }
        done++;
        act.progress(done, paths.size());
--- a/src/libstore/parsed-derivations.cc
+++ b/src/libstore/parsed-derivations.cc
@ -2,8 +2,8 @@
 namespace nix {
-ParsedDerivation::ParsedDerivation(const Path & drvPath, BasicDerivation & drv)
+ParsedDerivation::ParsedDerivation(StorePath && drvPath, BasicDerivation & drv)
-    : drvPath(drvPath), drv(drv)
+    : drvPath(std::move(drvPath)), drv(drv)
 {
    /* Parse the __json attribute, if any. */
    auto jsonAttr = drv.env.find("__json");
@ -11,7 +11,7 @@ ParsedDerivation::ParsedDerivation(const Path & drvPath, BasicDerivation & drv)
        try {
            structuredAttrs = nlohmann::json::parse(jsonAttr->second);
        } catch (std::exception & e) {
-            throw Error("cannot process __json attribute of '%s': %s", drvPath, e.what());
+            throw Error("cannot process __json attribute of '%s': %s", drvPath.to_string(), e.what());
        }
    }
 }
@ -24,7 +24,7 @@ std::optional<std::string> ParsedDerivation::getStringAttr(const std::string & n
            return {};
        else {
            if (!i->is_string())
-                throw Error("attribute '%s' of derivation '%s' must be a string", name, drvPath);
+                throw Error("attribute '%s' of derivation '%s' must be a string", name, drvPath.to_string());
            return i->get<std::string>();
        }
    } else {
@ -44,7 +44,7 @@ bool ParsedDerivation::getBoolAttr(const std::string & name, bool def) const
            return def;
        else {
            if (!i->is_boolean())
-                throw Error("attribute '%s' of derivation '%s' must be a Boolean", name, drvPath);
+                throw Error("attribute '%s' of derivation '%s' must be a Boolean", name, drvPath.to_string());
            return i->get<bool>();
        }
    } else {
@ -64,11 +64,11 @@ std::optional<Strings> ParsedDerivation::getStringsAttr(const std::string & name
            return {};
        else {
            if (!i->is_array())
-                throw Error("attribute '%s' of derivation '%s' must be a list of strings", name, drvPath);
+                throw Error("attribute '%s' of derivation '%s' must be a list of strings", name, drvPath.to_string());
            Strings res;
            for (auto j = i->begin(); j != i->end(); ++j) {
                if (!j->is_string())
-                    throw Error("attribute '%s' of derivation '%s' must be a list of strings", name, drvPath);
+                    throw Error("attribute '%s' of derivation '%s' must be a list of strings", name, drvPath.to_string());
                res.push_back(j->get<std::string>());
            }
            return res;
--- a/src/libstore/parsed-derivations.hh
+++ b/src/libstore/parsed-derivations.hh
@ -6,13 +6,13 @@ namespace nix {
 class ParsedDerivation
 {
-    Path drvPath;
+    StorePath drvPath;
    BasicDerivation & drv;
    std::optional<nlohmann::json> structuredAttrs;
 public:
-    ParsedDerivation(const Path & drvPath, BasicDerivation & drv);
+    ParsedDerivation(StorePath && drvPath, BasicDerivation & drv);
    const std::optional<nlohmann::json> & getStructuredAttrs() const
    {
--- a/src/libstore/path.cc
+++ b/src/libstore/path.cc
@ -0,0 +1,99 @@
 #include "store-api.hh"
 namespace nix {
 extern "C" {
    rust::Result<StorePath> ffi_StorePath_new(rust::StringSlice path, rust::StringSlice storeDir);
    rust::Result<StorePath> ffi_StorePath_new2(unsigned char hash[20], rust::StringSlice storeDir);
    rust::Result<StorePath> ffi_StorePath_fromBaseName(rust::StringSlice baseName);
    rust::String ffi_StorePath_to_string(const StorePath & _this);
    StorePath ffi_StorePath_clone(const StorePath & _this);
    rust::StringSlice ffi_StorePath_name(const StorePath & _this);
 }
 StorePath StorePath::make(std::string_view path, std::string_view storeDir)
 {
    return ffi_StorePath_new((rust::StringSlice) path, (rust::StringSlice) storeDir).unwrap();
 }
 StorePath StorePath::make(unsigned char hash[20], std::string_view name)
 {
    return ffi_StorePath_new2(hash, (rust::StringSlice) name).unwrap();
 }
 StorePath StorePath::fromBaseName(std::string_view baseName)
 {
    return ffi_StorePath_fromBaseName((rust::StringSlice) baseName).unwrap();
 }
 rust::String StorePath::to_string() const
 {
    return ffi_StorePath_to_string(*this);
 }
 StorePath StorePath::clone() const
 {
    return ffi_StorePath_clone(*this);
 }
 bool StorePath::isDerivation() const
 {
    return hasSuffix(name(), drvExtension);
 }
 std::string_view StorePath::name() const
 {
    return ffi_StorePath_name(*this);
 }
 StorePath Store::parseStorePath(std::string_view path) const
 {
    return StorePath::make(path, storeDir);
 }
 StorePathSet Store::parseStorePathSet(const PathSet & paths) const
 {
    StorePathSet res;
    for (auto & i : paths) res.insert(parseStorePath(i));
    return res;
 }
 std::string Store::printStorePath(const StorePath & path) const
 {
    auto s = storeDir + "/";
    s += (std::string_view) path.to_string();
    return s;
 }
 PathSet Store::printStorePathSet(const StorePathSet & paths) const
 {
    PathSet res;
    for (auto & i : paths) res.insert(printStorePath(i));
    return res;
 }
 StorePathSet cloneStorePathSet(const StorePathSet & paths)
 {
    StorePathSet res;
    for (auto & p : paths)
        res.insert(p.clone());
    return res;
 }
 StorePathSet storePathsToSet(const StorePaths & paths)
 {
    StorePathSet res;
    for (auto & p : paths)
        res.insert(p.clone());
    return res;
 }
 StorePathSet singleton(const StorePath & path)
 {
    StorePathSet res;
    res.insert(path.clone());
    return res;
 }
 }
--- a/src/libstore/path.hh
+++ b/src/libstore/path.hh
@ -0,0 +1,81 @@
 #pragma once
 #include "rust-ffi.hh"
 namespace nix {
 /* See path.rs. */
 struct StorePath;
 extern "C" {
    void ffi_StorePath_drop(void *);
    bool ffi_StorePath_less_than(const StorePath & a, const StorePath & b);
    bool ffi_StorePath_eq(const StorePath & a, const StorePath & b);
    unsigned char * ffi_StorePath_hash_data(const StorePath & p);
 }
 struct StorePath : rust::Value<3 * sizeof(void *) + 24, ffi_StorePath_drop>
 {
    static StorePath make(std::string_view path, std::string_view storeDir);
    static StorePath make(unsigned char hash[20], std::string_view name);
    static StorePath fromBaseName(std::string_view baseName);
    rust::String to_string() const;
    bool operator < (const StorePath & other) const
    {
        return ffi_StorePath_less_than(*this, other);
    }
    bool operator == (const StorePath & other) const
    {
        return ffi_StorePath_eq(*this, other);
    }
    bool operator != (const StorePath & other) const
    {
        return !(*this == other);
    }
    StorePath clone() const;
    /* Check whether a file name ends with the extension for
       derivations. */
    bool isDerivation() const;
    std::string_view name() const;
    unsigned char * hashData() const
    {
        return ffi_StorePath_hash_data(*this);
    }
 };
 typedef std::set<StorePath> StorePathSet;
 typedef std::vector<StorePath> StorePaths;
 StorePathSet cloneStorePathSet(const StorePathSet & paths);
 StorePathSet storePathsToSet(const StorePaths & paths);
 StorePathSet singleton(const StorePath & path);
 /* Size of the hash part of store paths, in base-32 characters. */
 const size_t storePathHashLen = 32; // i.e. 160 bits
 /* Extension of derivations in the Nix store. */
 const std::string drvExtension = ".drv";
 }
 namespace std {
 template<> struct hash<nix::StorePath> {
    std::size_t operator()(const nix::StorePath & path) const noexcept
    {
        return * (std::size_t *) path.hashData();
    }
 };
 }
--- a/src/libstore/profiles.cc
+++ b/src/libstore/profiles.cc
@ -40,7 +40,7 @@ Generations findGenerations(Path profile, int & curGen)
    Generations gens;
    Path profileDir = dirOf(profile);
-    string profileName = baseNameOf(profile);
+    auto profileName = std::string(baseNameOf(profile));
    for (auto & i : readDirectory(profileDir)) {
        int n;
@ -108,7 +108,7 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath)
       user environment etc. we've just built. */
    Path generation;
    makeName(profile, num + 1, generation);
-    store->addPermRoot(outPath, generation, false, true);
+    store->addPermRoot(store->parseStorePath(outPath), generation, false, true);
    return generation;
 }
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@ -89,7 +89,7 @@ PathSet scanForReferences(const string & path,
       hash part of the file name.  (This assumes that all references
       have the form `HASH-bla'). */
    for (auto & i : refs) {
-        string baseName = baseNameOf(i);
+        auto baseName = std::string(baseNameOf(i));
        string::size_type pos = baseName.find('-');
        if (pos == string::npos)
            throw Error(format("bad reference '%1%'") % i);
--- a/src/libstore/remote-fs-accessor.cc
+++ b/src/libstore/remote-fs-accessor.cc
@ -50,7 +50,7 @@ std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_)
    auto storePath = store->toStorePath(path);
    std::string restPath = std::string(path, storePath.size());
-    if (!store->isValidPath(storePath))
+    if (!store->isValidPath(store->parseStorePath(storePath)))
        throw InvalidPath(format("path '%1%' is not a valid store path") % storePath);
    auto i = nars.find(storePath);
@ -96,7 +96,7 @@ std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_)
        } catch (SysError &) { }
    }
-    store->narFromPath(storePath, sink);
+    store->narFromPath(store->parseStorePath(storePath), sink);
    auto narAccessor = makeNarAccessor(sink.s);
    addToCache(storePath, *sink.s, narAccessor);
    return {narAccessor, restPath};
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@ -22,23 +22,22 @@
 namespace nix {
-Path readStorePath(Store & store, Source & from)
+template<> StorePathSet readStorePaths(const Store & store, Source & from)
 {
-    Path path = readString(from);
+    StorePathSet paths;
-    store.assertStorePath(path);
+    for (auto & i : readStrings<Strings>(from))
-    return path;
+        paths.insert(store.parseStorePath(i));
 }
 template<class T> T readStorePaths(Store & store, Source & from)
 {
    T paths = readStrings<T>(from);
    for (auto & i : paths) store.assertStorePath(i);
    return paths;
 }
-template PathSet readStorePaths(Store & store, Source & from);
+
-template Paths readStorePaths(Store & store, Source & from);
+void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths)
 {
    out << paths.size();
    for (auto & i : paths)
        out << store.printStorePath(i);
 }
 /* TODO: Separate these store impls into different files, give them better names */
 RemoteStore::RemoteStore(const Params & params)
@ -254,60 +253,62 @@ ConnectionHandle RemoteStore::getConnection()
 }
-bool RemoteStore::isValidPathUncached(const Path & path)
+bool RemoteStore::isValidPathUncached(const StorePath & path)
 {
    auto conn(getConnection());
-    conn->to << wopIsValidPath << path;
+    conn->to << wopIsValidPath << printStorePath(path);
    conn.processStderr();
    return readInt(conn->from);
 }
-PathSet RemoteStore::queryValidPaths(const PathSet & paths, SubstituteFlag maybeSubstitute)
+StorePathSet RemoteStore::queryValidPaths(const StorePathSet & paths, SubstituteFlag maybeSubstitute)
 {
    auto conn(getConnection());
    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
-        PathSet res;
+        StorePathSet res;
        for (auto & i : paths)
-            if (isValidPath(i)) res.insert(i);
+            if (isValidPath(i)) res.insert(i.clone());
        return res;
    } else {
-        conn->to << wopQueryValidPaths << paths;
+        conn->to << wopQueryValidPaths;
        writeStorePaths(*this, conn->to, paths);
        conn.processStderr();
-        return readStorePaths<PathSet>(*this, conn->from);
+        return readStorePaths<StorePathSet>(*this, conn->from);
    }
 }
-PathSet RemoteStore::queryAllValidPaths()
+StorePathSet RemoteStore::queryAllValidPaths()
 {
    auto conn(getConnection());
    conn->to << wopQueryAllValidPaths;
    conn.processStderr();
-    return readStorePaths<PathSet>(*this, conn->from);
+    return readStorePaths<StorePathSet>(*this, conn->from);
 }
-PathSet RemoteStore::querySubstitutablePaths(const PathSet & paths)
+StorePathSet RemoteStore::querySubstitutablePaths(const StorePathSet & paths)
 {
    auto conn(getConnection());
    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
-        PathSet res;
+        StorePathSet res;
        for (auto & i : paths) {
-            conn->to << wopHasSubstitutes << i;
+            conn->to << wopHasSubstitutes << printStorePath(i);
            conn.processStderr();
-            if (readInt(conn->from)) res.insert(i);
+            if (readInt(conn->from)) res.insert(i.clone());
        }
        return res;
    } else {
-        conn->to << wopQuerySubstitutablePaths << paths;
+        conn->to << wopQuerySubstitutablePaths;
        writeStorePaths(*this, conn->to, paths);
        conn.processStderr();
-        return readStorePaths<PathSet>(*this, conn->from);
+        return readStorePaths<StorePathSet>(*this, conn->from);
    }
 }
-void RemoteStore::querySubstitutablePathInfos(const PathSet & paths,
+void RemoteStore::querySubstitutablePathInfos(const StorePathSet & paths,
    SubstitutablePathInfos & infos)
 {
    if (paths.empty()) return;
@ -318,29 +319,31 @@ void RemoteStore::querySubstitutablePathInfos(const PathSet & paths,
        for (auto & i : paths) {
            SubstitutablePathInfo info;
-            conn->to << wopQuerySubstitutablePathInfo << i;
+            conn->to << wopQuerySubstitutablePathInfo << printStorePath(i);
            conn.processStderr();
            unsigned int reply = readInt(conn->from);
            if (reply == 0) continue;
-            info.deriver = readString(conn->from);
+            auto deriver = readString(conn->from);
-            if (info.deriver != "") assertStorePath(info.deriver);
+            if (deriver != "")
-            info.references = readStorePaths<PathSet>(*this, conn->from);
+                info.deriver = parseStorePath(deriver);
            info.references = readStorePaths<StorePathSet>(*this, conn->from);
            info.downloadSize = readLongLong(conn->from);
            info.narSize = readLongLong(conn->from);
-            infos[i] = info;
+            infos.insert_or_assign(i.clone(), std::move(info));
        }
    } else {
-        conn->to << wopQuerySubstitutablePathInfos << paths;
+        conn->to << wopQuerySubstitutablePathInfos;
        writeStorePaths(*this, conn->to, paths);
        conn.processStderr();
        size_t count = readNum<size_t>(conn->from);
        for (size_t n = 0; n < count; n++) {
-            Path path = readStorePath(*this, conn->from);
+            SubstitutablePathInfo & info(infos[parseStorePath(readString(conn->from))]);
-            SubstitutablePathInfo & info(infos[path]);
+            auto deriver = readString(conn->from);
-            info.deriver = readString(conn->from);
+            if (deriver != "")
-            if (info.deriver != "") assertStorePath(info.deriver);
+                info.deriver = parseStorePath(deriver);
-            info.references = readStorePaths<PathSet>(*this, conn->from);
+            info.references = readStorePaths<StorePathSet>(*this, conn->from);
            info.downloadSize = readLongLong(conn->from);
            info.narSize = readLongLong(conn->from);
        }
@ -349,14 +352,14 @@ void RemoteStore::querySubstitutablePathInfos(const PathSet & paths,
 }
-void RemoteStore::queryPathInfoUncached(const Path & path,
+void RemoteStore::queryPathInfoUncached(const StorePath & path,
    Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept
 {
    try {
        std::shared_ptr<ValidPathInfo> info;
        {
            auto conn(getConnection());
-            conn->to << wopQueryPathInfo << path;
+            conn->to << wopQueryPathInfo << printStorePath(path);
            try {
                conn.processStderr();
            } catch (Error & e) {
@ -367,14 +370,13 @@ void RemoteStore::queryPathInfoUncached(const Path & path,
            }
            if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 17) {
                bool valid; conn->from >> valid;
-                if (!valid) throw InvalidPath(format("path '%s' is not valid") % path);
+                if (!valid) throw InvalidPath("path '%s' is not valid", printStorePath(path));
            }
-            info = std::make_shared<ValidPathInfo>();
+            info = std::make_shared<ValidPathInfo>(path.clone());
-            info->path = path;
+            auto deriver = readString(conn->from);
-            info->deriver = readString(conn->from);
+            if (deriver != "") info->deriver = parseStorePath(deriver);
            if (info->deriver != "") assertStorePath(info->deriver);
            info->narHash = Hash(readString(conn->from), htSHA256);
-            info->references = readStorePaths<PathSet>(*this, conn->from);
+            info->references = readStorePaths<StorePathSet>(*this, conn->from);
            conn->from >> info->registrationTime >> info->narSize;
            if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 16) {
                conn->from >> info->ultimate;
@ -387,52 +389,52 @@ void RemoteStore::queryPathInfoUncached(const Path & path,
 }
-void RemoteStore::queryReferrers(const Path & path,
+void RemoteStore::queryReferrers(const StorePath & path,
-    PathSet & referrers)
+    StorePathSet & referrers)
 {
    auto conn(getConnection());
-    conn->to << wopQueryReferrers << path;
+    conn->to << wopQueryReferrers << printStorePath(path);
    conn.processStderr();
-    PathSet referrers2 = readStorePaths<PathSet>(*this, conn->from);
+    for (auto & i : readStorePaths<StorePathSet>(*this, conn->from))
-    referrers.insert(referrers2.begin(), referrers2.end());
+        referrers.insert(i.clone());
 }
-PathSet RemoteStore::queryValidDerivers(const Path & path)
+StorePathSet RemoteStore::queryValidDerivers(const StorePath & path)
 {
    auto conn(getConnection());
-    conn->to << wopQueryValidDerivers << path;
+    conn->to << wopQueryValidDerivers << printStorePath(path);
    conn.processStderr();
-    return readStorePaths<PathSet>(*this, conn->from);
+    return readStorePaths<StorePathSet>(*this, conn->from);
 }
-PathSet RemoteStore::queryDerivationOutputs(const Path & path)
+StorePathSet RemoteStore::queryDerivationOutputs(const StorePath & path)
 {
    auto conn(getConnection());
-    conn->to << wopQueryDerivationOutputs << path;
+    conn->to << wopQueryDerivationOutputs << printStorePath(path);
    conn.processStderr();
-    return readStorePaths<PathSet>(*this, conn->from);
+    return readStorePaths<StorePathSet>(*this, conn->from);
 }
-PathSet RemoteStore::queryDerivationOutputNames(const Path & path)
+PathSet RemoteStore::queryDerivationOutputNames(const StorePath & path)
 {
    auto conn(getConnection());
-    conn->to << wopQueryDerivationOutputNames << path;
+    conn->to << wopQueryDerivationOutputNames << printStorePath(path);
    conn.processStderr();
    return readStrings<PathSet>(conn->from);
 }
-Path RemoteStore::queryPathFromHashPart(const string & hashPart)
+std::optional<StorePath> RemoteStore::queryPathFromHashPart(const std::string & hashPart)
 {
    auto conn(getConnection());
    conn->to << wopQueryPathFromHashPart << hashPart;
    conn.processStderr();
    Path path = readString(conn->from);
-    if (!path.empty()) assertStorePath(path);
+    if (path.empty()) return {};
-    return path;
+    return parseStorePath(path);
 }
@ -450,9 +452,10 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
            copyNAR(source, sink);
            sink
                << exportMagic
-                << info.path
+                << printStorePath(info.path);
-                << info.references
+            writeStorePaths(*this, sink, info.references);
-                << info.deriver
+            sink
                << (info.deriver ? printStorePath(*info.deriver) : "")
                << 0 // == no legacy signature
                << 0 // == no path follows
                ;
@ -460,14 +463,17 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
        conn.processStderr(0, source2.get());
-        auto importedPaths = readStorePaths<PathSet>(*this, conn->from);
+        auto importedPaths = readStorePaths<StorePathSet>(*this, conn->from);
        assert(importedPaths.size() <= 1);
    }
    else {
        conn->to << wopAddToStoreNar
-                 << info.path << info.deriver << info.narHash.to_string(Base16, false)
+                 << printStorePath(info.path)
-                 << info.references << info.registrationTime << info.narSize
+                 << (info.deriver ? printStorePath(*info.deriver) : "")
                 << info.narHash.to_string(Base16, false);
        writeStorePaths(*this, conn->to, info.references);
        conn->to << info.registrationTime << info.narSize
                 << info.ultimate << info.sigs << info.ca
                 << repair << !checkSigs;
        bool tunnel = GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21;
@ -477,7 +483,7 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
 }
-Path RemoteStore::addToStore(const string & name, const Path & _srcPath,
+StorePath RemoteStore::addToStore(const string & name, const Path & _srcPath,
    bool recursive, HashType hashAlgo, PathFilter & filter, RepairFlag repair)
 {
    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
@ -511,29 +517,33 @@ Path RemoteStore::addToStore(const string & name, const Path & _srcPath,
        throw;
    }
-    return readStorePath(*this, conn->from);
+    return parseStorePath(readString(conn->from));
 }
-Path RemoteStore::addTextToStore(const string & name, const string & s,
+StorePath RemoteStore::addTextToStore(const string & name, const string & s,
-    const PathSet & references, RepairFlag repair)
+    const StorePathSet & references, RepairFlag repair)
 {
    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
    auto conn(getConnection());
-    conn->to << wopAddTextToStore << name << s << references;
+    conn->to << wopAddTextToStore << name << s;
    writeStorePaths(*this, conn->to, references);
    conn.processStderr();
-    return readStorePath(*this, conn->from);
+    return parseStorePath(readString(conn->from));
 }
-void RemoteStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
+void RemoteStore::buildPaths(const std::vector<StorePathWithOutputs> & drvPaths, BuildMode buildMode)
 {
    auto conn(getConnection());
    conn->to << wopBuildPaths;
-    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 13) {
+    assert(GET_PROTOCOL_MINOR(conn->daemonVersion) >= 13);
-        conn->to << drvPaths;
+    Strings ss;
    for (auto & p : drvPaths)
        ss.push_back(p.to_string(*this));
    conn->to << ss;
    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 15)
        conn->to << buildMode;
    else
@ -541,24 +551,18 @@ void RemoteStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
           need to validate it here on the client side.  */
        if (buildMode != bmNormal)
            throw Error("repairing or checking is not supported when building through the Nix daemon");
    } else {
        /* For backwards compatibility with old daemons, strip output
           identifiers. */
        PathSet drvPaths2;
        for (auto & i : drvPaths)
            drvPaths2.insert(string(i, 0, i.find('!')));
        conn->to << drvPaths2;
    }
    conn.processStderr();
    readInt(conn->from);
 }
-BuildResult RemoteStore::buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
    BuildMode buildMode)
 {
    auto conn(getConnection());
-    conn->to << wopBuildDerivation << drvPath << drv << buildMode;
+    conn->to << wopBuildDerivation << printStorePath(drvPath);
    writeDerivation(conn->to, *this, drv);
    conn->to << buildMode;
    conn.processStderr();
    BuildResult res;
    unsigned int status;
@ -568,19 +572,19 @@ BuildResult RemoteStore::buildDerivation(const Path & drvPath, const BasicDeriva
 }
-void RemoteStore::ensurePath(const Path & path)
+void RemoteStore::ensurePath(const StorePath & path)
 {
    auto conn(getConnection());
-    conn->to << wopEnsurePath << path;
+    conn->to << wopEnsurePath << printStorePath(path);
    conn.processStderr();
    readInt(conn->from);
 }
-void RemoteStore::addTempRoot(const Path & path)
+void RemoteStore::addTempRoot(const StorePath & path)
 {
    auto conn(getConnection());
-    conn->to << wopAddTempRoot << path;
+    conn->to << wopAddTempRoot << printStorePath(path);
    conn.processStderr();
    readInt(conn->from);
 }
@ -613,8 +617,8 @@ Roots RemoteStore::findRoots(bool censor)
    Roots result;
    while (count--) {
        Path link = readString(conn->from);
-        Path target = readStorePath(*this, conn->from);
+        auto target = parseStorePath(readString(conn->from));
-        result[target].emplace(link);
+        result[std::move(target)].emplace(link);
    }
    return result;
 }
@ -625,7 +629,9 @@ void RemoteStore::collectGarbage(const GCOptions & options, GCResults & results)
    auto conn(getConnection());
    conn->to
-        << wopCollectGarbage << options.action << options.pathsToDelete << options.ignoreLiveness
+        << wopCollectGarbage << options.action;
    writeStorePaths(*this, conn->to, options.pathsToDelete);
    conn->to << options.ignoreLiveness
        << options.maxFreed
        /* removed options */
        << 0 << 0 << 0;
@ -661,17 +667,17 @@ bool RemoteStore::verifyStore(bool checkContents, RepairFlag repair)
 }
-void RemoteStore::addSignatures(const Path & storePath, const StringSet & sigs)
+void RemoteStore::addSignatures(const StorePath & storePath, const StringSet & sigs)
 {
    auto conn(getConnection());
-    conn->to << wopAddSignatures << storePath << sigs;
+    conn->to << wopAddSignatures << printStorePath(storePath) << sigs;
    conn.processStderr();
    readInt(conn->from);
 }
-void RemoteStore::queryMissing(const PathSet & targets,
+void RemoteStore::queryMissing(const std::vector<StorePathWithOutputs> & targets,
-    PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
+    StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
    unsigned long long & downloadSize, unsigned long long & narSize)
 {
    {
@ -680,11 +686,15 @@ void RemoteStore::queryMissing(const PathSet & targets,
            // Don't hold the connection handle in the fallback case
            // to prevent a deadlock.
            goto fallback;
-        conn->to << wopQueryMissing << targets;
+        conn->to << wopQueryMissing;
        Strings ss;
        for (auto & p : targets)
            ss.push_back(p.to_string(*this));
        conn->to << ss;
        conn.processStderr();
-        willBuild = readStorePaths<PathSet>(*this, conn->from);
+        willBuild = readStorePaths<StorePathSet>(*this, conn->from);
-        willSubstitute = readStorePaths<PathSet>(*this, conn->from);
+        willSubstitute = readStorePaths<StorePathSet>(*this, conn->from);
-        unknown = readStorePaths<PathSet>(*this, conn->from);
+        unknown = readStorePaths<StorePathSet>(*this, conn->from);
        conn->from >> downloadSize >> narSize;
        return;
    }
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@ -35,50 +35,50 @@ public:
    /* Implementations of abstract store API methods. */
-    bool isValidPathUncached(const Path & path) override;
+    bool isValidPathUncached(const StorePath & path) override;
-    PathSet queryValidPaths(const PathSet & paths,
+    StorePathSet queryValidPaths(const StorePathSet & paths,
        SubstituteFlag maybeSubstitute = NoSubstitute) override;
-    PathSet queryAllValidPaths() override;
+    StorePathSet queryAllValidPaths() override;
-    void queryPathInfoUncached(const Path & path,
+    void queryPathInfoUncached(const StorePath & path,
        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept override;
-    void queryReferrers(const Path & path, PathSet & referrers) override;
+    void queryReferrers(const StorePath & path, StorePathSet & referrers) override;
-    PathSet queryValidDerivers(const Path & path) override;
+    StorePathSet queryValidDerivers(const StorePath & path) override;
-    PathSet queryDerivationOutputs(const Path & path) override;
+    StorePathSet queryDerivationOutputs(const StorePath & path) override;
-    StringSet queryDerivationOutputNames(const Path & path) override;
+    StringSet queryDerivationOutputNames(const StorePath & path) override;
-    Path queryPathFromHashPart(const string & hashPart) override;
+    std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) override;
-    PathSet querySubstitutablePaths(const PathSet & paths) override;
+    StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
-    void querySubstitutablePathInfos(const PathSet & paths,
+    void querySubstitutablePathInfos(const StorePathSet & paths,
        SubstitutablePathInfos & infos) override;
    void addToStore(const ValidPathInfo & info, Source & nar,
        RepairFlag repair, CheckSigsFlag checkSigs,
        std::shared_ptr<FSAccessor> accessor) override;
-    Path addToStore(const string & name, const Path & srcPath,
+    StorePath addToStore(const string & name, const Path & srcPath,
        bool recursive = true, HashType hashAlgo = htSHA256,
        PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair) override;
-    Path addTextToStore(const string & name, const string & s,
+    StorePath addTextToStore(const string & name, const string & s,
-        const PathSet & references, RepairFlag repair) override;
+        const StorePathSet & references, RepairFlag repair) override;
-    void buildPaths(const PathSet & paths, BuildMode buildMode) override;
+    void buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode) override;
-    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+    BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
        BuildMode buildMode) override;
-    void ensurePath(const Path & path) override;
+    void ensurePath(const StorePath & path) override;
-    void addTempRoot(const Path & path) override;
+    void addTempRoot(const StorePath & path) override;
    void addIndirectRoot(const Path & path) override;
@ -92,10 +92,10 @@ public:
    bool verifyStore(bool checkContents, RepairFlag repair) override;
-    void addSignatures(const Path & storePath, const StringSet & sigs) override;
+    void addSignatures(const StorePath & storePath, const StringSet & sigs) override;
-    void queryMissing(const PathSet & targets,
+    void queryMissing(const std::vector<StorePathWithOutputs> & targets,
-        PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
+        StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
        unsigned long long & downloadSize, unsigned long long & narSize) override;
    void connect() override;
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@ -222,7 +222,7 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
       fetches the .narinfo file, rather than first checking for its
       existence via a HEAD request. Since .narinfos are small, doing
       a GET is unlikely to be slower than HEAD. */
-    bool isValidPathUncached(const Path & storePath) override
+    bool isValidPathUncached(const StorePath & storePath) override
    {
        try {
            queryPathInfo(storePath);
@ -382,9 +382,9 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
            throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache '%s'", path, getUri());
    }
-    PathSet queryAllValidPaths() override
+    StorePathSet queryAllValidPaths() override
    {
-        PathSet paths;
+        StorePathSet paths;
        std::string marker;
        do {
@ -405,7 +405,7 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
            for (auto object : contents) {
                auto & key = object.GetKey();
                if (key.size() != 40 || !hasSuffix(key, ".narinfo")) continue;
-                paths.insert(storeDir + "/" + key.substr(0, key.size() - 8));
+                paths.insert(parseStorePath(storeDir + "/" + key.substr(0, key.size() - 8) + "-unknown"));
            }
            marker = res.GetNextMarker();
--- a/src/libstore/ssh-store.cc
+++ b/src/libstore/ssh-store.cc
@ -38,7 +38,7 @@ public:
    bool sameMachine() override
    { return false; }
-    void narFromPath(const Path & path, Sink & sink) override;
+    void narFromPath(const StorePath & path, Sink & sink) override;
    ref<FSAccessor> getFSAccessor() override;
@ -66,10 +66,10 @@ private:
    };
 };
-void SSHStore::narFromPath(const Path & path, Sink & sink)
+void SSHStore::narFromPath(const StorePath & path, Sink & sink)
 {
    auto conn(connections->get());
-    conn->to << wopNarFromPath << path;
+    conn->to << wopNarFromPath << printStorePath(path);
    conn->processStderr();
    copyNAR(conn->from, sink);
 }
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@ -27,13 +27,6 @@ bool Store::isStorePath(const Path & path) const
 }
 void Store::assertStorePath(const Path & path) const
 {
    if (!isStorePath(path))
        throw Error(format("path '%1%' is not in the Nix store") % path);
 }
 Path Store::toStorePath(const Path & path) const
 {
    if (!isInStore(path))
@ -60,17 +53,9 @@ Path Store::followLinksToStore(const Path & _path) const
 }
-Path Store::followLinksToStorePath(const Path & path) const
+StorePath Store::followLinksToStorePath(const Path & path) const
 {
-    return toStorePath(followLinksToStore(path));
+    return parseStorePath(toStorePath(followLinksToStore(path)));
 }
 string storePathToName(const Path & path)
 {
    auto base = baseNameOf(path);
    assert(base.size() == storePathHashLen || (base.size() > storePathHashLen && base[storePathHashLen] == '-'));
    return base.size() == storePathHashLen ? "" : string(base, storePathHashLen + 1);
 }
@ -82,41 +67,6 @@ string storePathToHash(const Path & path)
 }
 void checkStoreName(const string & name)
 {
    string validChars = "+-._?=";
    auto baseError = format("The path name '%2%' is invalid: %3%. "
        "Path names are alphanumeric and can include the symbols %1% "
        "and must not begin with a period. "
        "Note: If '%2%' is a source file and you cannot rename it on "
        "disk, 'builtins.path { name = ... }' can be used to give it an "
        "alternative name.") % validChars % name;
    if (name.empty())
        throw Error(baseError % "it is an empty string");
    /* Disallow names starting with a dot for possible security
       reasons (e.g., "." and ".."). */
    if (name[0] == '.')
        throw Error(baseError % "it is illegal to start the name with a period");
    /* Disallow names longer than 211 characters. ext4’s max is 256,
       but we need extra space for the hash and .chroot extensions. */
    if (name.length() > 211)
        throw Error(baseError % "name must be less than 212 characters");
    for (auto & i : name)
        if (!((i >= 'A' && i <= 'Z') ||
              (i >= 'a' && i <= 'z') ||
              (i >= '0' && i <= '9') ||
              validChars.find(i) != string::npos))
        {
            throw Error(baseError % (format("the '%1%' character is invalid") % i));
        }
 }
 /* Store paths have the following form:
   <store>/<h>-<name>
@ -188,43 +138,48 @@ void checkStoreName(const string & name)
 */
-Path Store::makeStorePath(const string & type,
+StorePath Store::makeStorePath(const string & type,
-    const Hash & hash, const string & name) const
+    const Hash & hash, std::string_view name) const
 {
    /* e.g., "source:sha256:1abc...:/nix/store:foo.tar.gz" */
-    string s = type + ":" + hash.to_string(Base16) + ":" + storeDir + ":" + name;
+    string s = type + ":" + hash.to_string(Base16) + ":" + storeDir + ":" + std::string(name);
-
+    auto h = compressHash(hashString(htSHA256, s), 20);
-    checkStoreName(name);
+    return StorePath::make(h.hash, name);
    return storeDir + "/"
        + compressHash(hashString(htSHA256, s), 20).to_string(Base32, false)
        + "-" + name;
 }
-Path Store::makeOutputPath(const string & id,
+StorePath Store::makeOutputPath(const string & id,
-    const Hash & hash, const string & name) const
+    const Hash & hash, std::string_view name) const
 {
    return makeStorePath("output:" + id, hash,
-        name + (id == "out" ? "" : "-" + id));
+        std::string(name) + (id == "out" ? "" : "-" + id));
 }
-static std::string makeType(string && type, const PathSet & references)
+static std::string makeType(
    const Store & store,
    string && type,
    const StorePathSet & references,
    bool hasSelfReference = false)
 {
    for (auto & i : references) {
        type += ":";
-        type += i;
+        type += store.printStorePath(i);
    }
    if (hasSelfReference) type += ":self";
    return std::move(type);
 }
-Path Store::makeFixedOutputPath(bool recursive,
+StorePath Store::makeFixedOutputPath(
-    const Hash & hash, const string & name, const PathSet & references) const
+    bool recursive,
    const Hash & hash,
    std::string_view name,
    const StorePathSet & references,
    bool hasSelfReference) const
 {
    if (hash.type == htSHA256 && recursive) {
-        return makeStorePath(makeType("source", references), hash, name);
+        return makeStorePath(makeType(*this, "source", references, hasSelfReference), hash, name);
    } else {
        assert(references.empty());
        return makeStorePath("output:out", hashString(htSHA256,
@ -234,28 +189,27 @@ Path Store::makeFixedOutputPath(bool recursive,
 }
-Path Store::makeTextPath(const string & name, const Hash & hash,
+StorePath Store::makeTextPath(std::string_view name, const Hash & hash,
-    const PathSet & references) const
+    const StorePathSet & references) const
 {
    assert(hash.type == htSHA256);
    /* Stuff the references (if any) into the type.  This is a bit
       hacky, but we can't put them in `s' since that would be
       ambiguous. */
-    return makeStorePath(makeType("text", references), hash, name);
+    return makeStorePath(makeType(*this, "text", references), hash, name);
 }
-std::pair<Path, Hash> Store::computeStorePathForPath(const string & name,
+std::pair<StorePath, Hash> Store::computeStorePathForPath(std::string_view name,
    const Path & srcPath, bool recursive, HashType hashAlgo, PathFilter & filter) const
 {
    Hash h = recursive ? hashPath(hashAlgo, srcPath, filter).first : hashFile(hashAlgo, srcPath);
-    Path dstPath = makeFixedOutputPath(recursive, h, name);
+    return std::make_pair(makeFixedOutputPath(recursive, h, name), h);
    return std::pair<Path, Hash>(dstPath, h);
 }
-Path Store::computeStorePathForText(const string & name, const string & s,
+StorePath Store::computeStorePathForText(const string & name, const string & s,
-    const PathSet & references) const
+    const StorePathSet & references) const
 {
    return makeTextPath(name, hashString(htSHA256, s), references);
 }
@ -274,11 +228,9 @@ std::string Store::getUri()
 }
-bool Store::isValidPath(const Path & storePath)
+bool Store::isValidPath(const StorePath & storePath)
 {
-    assertStorePath(storePath);
+    auto hashPart = storePathToHash(printStorePath(storePath));
    auto hashPart = storePathToHash(storePath);
    {
        auto state_(state.lock());
@ -312,7 +264,7 @@ bool Store::isValidPath(const Path & storePath)
 /* Default implementation for stores that only implement
   queryPathInfoUncached(). */
-bool Store::isValidPathUncached(const Path & path)
+bool Store::isValidPathUncached(const StorePath & path)
 {
    try {
        queryPathInfo(path);
@ -323,7 +275,7 @@ bool Store::isValidPathUncached(const Path & path)
 }
-ref<const ValidPathInfo> Store::queryPathInfo(const Path & storePath)
+ref<const ValidPathInfo> Store::queryPathInfo(const StorePath & storePath)
 {
    std::promise<ref<const ValidPathInfo>> promise;
@ -340,22 +292,20 @@ ref<const ValidPathInfo> Store::queryPathInfo(const Path & storePath)
 }
-void Store::queryPathInfo(const Path & storePath,
+void Store::queryPathInfo(const StorePath & storePath,
    Callback<ref<const ValidPathInfo>> callback) noexcept
 {
    std::string hashPart;
    try {
-        assertStorePath(storePath);
+        hashPart = storePathToHash(printStorePath(storePath));
        hashPart = storePathToHash(storePath);
        {
            auto res = state.lock()->pathInfoCache.get(hashPart);
            if (res) {
                stats.narInfoReadAverted++;
                if (!*res)
-                    throw InvalidPath(format("path '%s' is not valid") % storePath);
+                    throw InvalidPath("path '%s' is not valid", printStorePath(storePath));
                return callback(ref<const ValidPathInfo>(*res));
            }
        }
@ -369,8 +319,8 @@ void Store::queryPathInfo(const Path & storePath,
                    state_->pathInfoCache.upsert(hashPart,
                        res.first == NarInfoDiskCache::oInvalid ? 0 : res.second);
                    if (res.first == NarInfoDiskCache::oInvalid ||
-                        (res.second->path != storePath && storePathToName(storePath) != ""))
+                        res.second->path != storePath)
-                        throw InvalidPath(format("path '%s' is not valid") % storePath);
+                        throw InvalidPath("path '%s' is not valid", printStorePath(storePath));
                }
                return callback(ref<const ValidPathInfo>(res.second));
            }
@ -381,7 +331,7 @@ void Store::queryPathInfo(const Path & storePath,
    auto callbackPtr = std::make_shared<decltype(callback)>(std::move(callback));
    queryPathInfoUncached(storePath,
-        {[this, storePath, hashPart, callbackPtr](std::future<std::shared_ptr<const ValidPathInfo>> fut) {
+        {[this, storePath{printStorePath(storePath)}, hashPart, callbackPtr](std::future<std::shared_ptr<const ValidPathInfo>> fut) {
            try {
                auto info = fut.get();
@ -394,9 +344,7 @@ void Store::queryPathInfo(const Path & storePath,
                    state_->pathInfoCache.upsert(hashPart, info);
                }
-                if (!info
+                if (!info || info->path != parseStorePath(storePath)) {
                    || (info->path != storePath && storePathToName(storePath) != ""))
                {
                    stats.narInfoMissing++;
                    throw InvalidPath("path '%s' is not valid", storePath);
                }
@ -407,27 +355,27 @@ void Store::queryPathInfo(const Path & storePath,
 }
-PathSet Store::queryValidPaths(const PathSet & paths, SubstituteFlag maybeSubstitute)
+StorePathSet Store::queryValidPaths(const StorePathSet & paths, SubstituteFlag maybeSubstitute)
 {
    struct State
    {
        size_t left;
-        PathSet valid;
+        StorePathSet valid;
        std::exception_ptr exc;
    };
-    Sync<State> state_(State{paths.size(), PathSet()});
+    Sync<State> state_(State{paths.size(), StorePathSet()});
    std::condition_variable wakeup;
    ThreadPool pool;
-    auto doQuery = [&](const Path & path ) {
+    auto doQuery = [&](const Path & path) {
        checkInterrupt();
-        queryPathInfo(path, {[path, &state_, &wakeup](std::future<ref<const ValidPathInfo>> fut) {
+        queryPathInfo(parseStorePath(path), {[path, this, &state_, &wakeup](std::future<ref<const ValidPathInfo>> fut) {
            auto state(state_.lock());
            try {
                auto info = fut.get();
-                state->valid.insert(path);
+                state->valid.insert(parseStorePath(path));
            } catch (InvalidPath &) {
            } catch (...) {
                state->exc = std::current_exception();
@ -439,7 +387,7 @@ PathSet Store::queryValidPaths(const PathSet & paths, SubstituteFlag maybeSubsti
    };
    for (auto & path : paths)
-        pool.enqueue(std::bind(doQuery, path));
+        pool.enqueue(std::bind(doQuery, printStorePath(path))); // FIXME
    pool.process();
@ -447,7 +395,7 @@ PathSet Store::queryValidPaths(const PathSet & paths, SubstituteFlag maybeSubsti
        auto state(state_.lock());
        if (!state->left) {
            if (state->exc) std::rethrow_exception(state->exc);
-            return state->valid;
+            return std::move(state->valid);
        }
        state.wait(wakeup);
    }
@ -457,13 +405,13 @@ PathSet Store::queryValidPaths(const PathSet & paths, SubstituteFlag maybeSubsti
 /* Return a string accepted by decodeValidPathInfo() that
   registers the specified paths as valid.  Note: it's the
   responsibility of the caller to provide a closure. */
-string Store::makeValidityRegistration(const PathSet & paths,
+string Store::makeValidityRegistration(const StorePathSet & paths,
    bool showDerivers, bool showHash)
 {
    string s = "";
    for (auto & i : paths) {
-        s += i + "\n";
+        s += printStorePath(i) + "\n";
        auto info = queryPathInfo(i);
@ -472,31 +420,30 @@ string Store::makeValidityRegistration(const PathSet & paths,
            s += (format("%1%\n") % info->narSize).str();
        }
-        Path deriver = showDerivers ? info->deriver : "";
+        auto deriver = showDerivers && info->deriver ? printStorePath(*info->deriver) : "";
        s += deriver + "\n";
        s += (format("%1%\n") % info->references.size()).str();
        for (auto & j : info->references)
-            s += j + "\n";
+            s += printStorePath(j) + "\n";
    }
    return s;
 }
-void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths,
+void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & storePaths,
    bool includeImpureInfo, bool showClosureSize, AllowInvalidFlag allowInvalid)
 {
    auto jsonList = jsonOut.list();
-    for (auto storePath : storePaths) {
+    for (auto & storePath : storePaths) {
        auto jsonPath = jsonList.object();
-        jsonPath.attr("path", storePath);
+        jsonPath.attr("path", printStorePath(storePath));
        try {
            auto info = queryPathInfo(storePath);
            storePath = info->path;
            jsonPath
                .attr("narHash", info->narHash.to_string())
@ -505,7 +452,7 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths
            {
                auto jsonRefs = jsonPath.list("references");
                for (auto & ref : info->references)
-                    jsonRefs.elem(ref);
+                    jsonRefs.elem(printStorePath(ref));
            }
            if (info->ca != "")
@ -514,14 +461,14 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths
            std::pair<uint64_t, uint64_t> closureSizes;
            if (showClosureSize) {
-                closureSizes = getClosureSize(storePath);
+                closureSizes = getClosureSize(info->path);
                jsonPath.attr("closureSize", closureSizes.first);
            }
            if (includeImpureInfo) {
-                if (info->deriver != "")
+                if (info->deriver)
-                    jsonPath.attr("deriver", info->deriver);
+                    jsonPath.attr("deriver", printStorePath(*info->deriver));
                if (info->registrationTime)
                    jsonPath.attr("registrationTime", info->registrationTime);
@ -557,10 +504,10 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths
 }
-std::pair<uint64_t, uint64_t> Store::getClosureSize(const Path & storePath)
+std::pair<uint64_t, uint64_t> Store::getClosureSize(const StorePath & storePath)
 {
    uint64_t totalNarSize = 0, totalDownloadSize = 0;
-    PathSet closure;
+    StorePathSet closure;
    computeFSClosure(storePath, closure, false, false);
    for (auto & p : closure) {
        auto info = queryPathInfo(p);
@ -584,30 +531,34 @@ const Store::Stats & Store::getStats()
 }
-void Store::buildPaths(const PathSet & paths, BuildMode buildMode)
+void Store::buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode)
 {
-    for (auto & path : paths)
+    StorePathSet paths2;
        if (isDerivation(path))
            unsupported("buildPaths");
-    if (queryValidPaths(paths).size() != paths.size())
+    for (auto & path : paths) {
        if (path.path.isDerivation())
            unsupported("buildPaths");
        paths2.insert(path.path.clone());
    }
    if (queryValidPaths(paths2).size() != paths2.size())
        unsupported("buildPaths");
 }
 void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
-    const Path & storePath, RepairFlag repair, CheckSigsFlag checkSigs)
+    const StorePath & storePath, RepairFlag repair, CheckSigsFlag checkSigs)
 {
    auto srcUri = srcStore->getUri();
    auto dstUri = dstStore->getUri();
    Activity act(*logger, lvlInfo, actCopyPath,
        srcUri == "local" || srcUri == "daemon"
-          ? fmt("copying path '%s' to '%s'", storePath, dstUri)
+        ? fmt("copying path '%s' to '%s'", srcStore->printStorePath(storePath), dstUri)
          : dstUri == "local" || dstUri == "daemon"
-            ? fmt("copying path '%s' from '%s'", storePath, srcUri)
+        ? fmt("copying path '%s' from '%s'", srcStore->printStorePath(storePath), srcUri)
-            : fmt("copying path '%s' from '%s' to '%s'", storePath, srcUri, dstUri),
+          : fmt("copying path '%s' from '%s' to '%s'", srcStore->printStorePath(storePath), srcUri, dstUri),
-        {storePath, srcUri, dstUri});
+        {srcStore->printStorePath(storePath), srcUri, dstUri});
    PushActivity pact(act.id);
    auto info = srcStore->queryPathInfo(storePath);
@ -640,23 +591,23 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
            total += len;
            act.progress(total, info->narSize);
        });
-        srcStore->narFromPath({storePath}, wrapperSink);
+        srcStore->narFromPath(storePath, wrapperSink);
    }, [&]() {
-        throw EndOfFile("NAR for '%s' fetched from '%s' is incomplete", storePath, srcStore->getUri());
+           throw EndOfFile("NAR for '%s' fetched from '%s' is incomplete", srcStore->printStorePath(storePath), srcStore->getUri());
    });
    dstStore->addToStore(*info, *source, repair, checkSigs);
 }
-void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const PathSet & storePaths,
+void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
    RepairFlag repair, CheckSigsFlag checkSigs, SubstituteFlag substitute)
 {
-    PathSet valid = dstStore->queryValidPaths(storePaths, substitute);
+    auto valid = dstStore->queryValidPaths(storePaths, substitute);
    PathSet missing;
    for (auto & path : storePaths)
-        if (!valid.count(path)) missing.insert(path);
+        if (!valid.count(path)) missing.insert(srcStore->printStorePath(path));
    if (missing.empty()) return;
@ -677,23 +628,25 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const PathSet & storePa
        PathSet(missing.begin(), missing.end()),
        [&](const Path & storePath) {
-            if (dstStore->isValidPath(storePath)) {
+            if (dstStore->isValidPath(dstStore->parseStorePath(storePath))) {
                nrDone++;
                showProgress();
                return PathSet();
            }
-            auto info = srcStore->queryPathInfo(storePath);
+            auto info = srcStore->queryPathInfo(srcStore->parseStorePath(storePath));
            bytesExpected += info->narSize;
            act.setExpected(actCopyPath, bytesExpected);
-            return info->references;
+            return srcStore->printStorePathSet(info->references);
        },
-        [&](const Path & storePath) {
+        [&](const Path & storePathS) {
            checkInterrupt();
            auto storePath = dstStore->parseStorePath(storePathS);
            if (!dstStore->isValidPath(storePath)) {
                MaintainCount<decltype(nrRunning)> mc(nrRunning);
                showProgress();
@ -703,7 +656,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const PathSet & storePa
                    nrFailed++;
                    if (!settings.keepGoing)
                        throw e;
-                    logger->log(lvlError, format("could not copy %s: %s") % storePath % e.what());
+                    logger->log(lvlError, fmt("could not copy %s: %s", storePathS, e.what()));
                    showProgress();
                    return;
                }
@ -716,20 +669,36 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const PathSet & storePa
 void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
-    const PathSet & storePaths, RepairFlag repair, CheckSigsFlag checkSigs,
+    const StorePathSet & storePaths, RepairFlag repair, CheckSigsFlag checkSigs,
    SubstituteFlag substitute)
 {
-    PathSet closure;
+    StorePathSet closure;
-    srcStore->computeFSClosure({storePaths}, closure);
+    srcStore->computeFSClosure(storePaths, closure);
    copyPaths(srcStore, dstStore, closure, repair, checkSigs, substitute);
 }
-ValidPathInfo decodeValidPathInfo(std::istream & str, bool hashGiven)
+ValidPathInfo::ValidPathInfo(const ValidPathInfo & other)
    : path(other.path.clone())
    , deriver(other.deriver ? other.deriver->clone(): std::optional<StorePath>{})
    , narHash(other.narHash)
    , references(cloneStorePathSet(other.references))
    , registrationTime(other.registrationTime)
    , narSize(other.narSize)
    , id(other.id)
    , ultimate(other.ultimate)
    , sigs(other.sigs)
    , ca(other.ca)
 {
-    ValidPathInfo info;
+}
-    getline(str, info.path);
+
-    if (str.eof()) { info.path = ""; return info; }
+
 std::optional<ValidPathInfo> decodeValidPathInfo(const Store & store, std::istream & str, bool hashGiven)
 {
    std::string path;
    getline(str, path);
    if (str.eof()) { return {}; }
    ValidPathInfo info(store.parseStorePath(path));
    if (hashGiven) {
        string s;
        getline(str, s);
@ -737,16 +706,29 @@ ValidPathInfo decodeValidPathInfo(std::istream & str, bool hashGiven)
        getline(str, s);
        if (!string2Int(s, info.narSize)) throw Error("number expected");
    }
-    getline(str, info.deriver);
+    std::string deriver;
    getline(str, deriver);
    if (deriver != "") info.deriver = store.parseStorePath(deriver);
    string s; int n;
    getline(str, s);
    if (!string2Int(s, n)) throw Error("number expected");
    while (n--) {
        getline(str, s);
-        info.references.insert(s);
+        info.references.insert(store.parseStorePath(s));
    }
    if (!str || str.eof()) throw Error("missing input");
-    return info;
+    return std::optional<ValidPathInfo>(std::move(info));
 }
 std::string Store::showPaths(const StorePathSet & paths)
 {
    std::string s;
    for (auto & i : paths) {
        if (s.size() != 0) s += ", ";
        s += "'" + printStorePath(i) + "'";
    }
    return s;
 }
@ -761,34 +743,34 @@ string showPaths(const PathSet & paths)
 }
-std::string ValidPathInfo::fingerprint() const
+std::string ValidPathInfo::fingerprint(const Store & store) const
 {
    if (narSize == 0 || !narHash)
-        throw Error(format("cannot calculate fingerprint of path '%s' because its size/hash is not known")
+        throw Error("cannot calculate fingerprint of path '%s' because its size/hash is not known",
-            % path);
+            store.printStorePath(path));
    return
-        "1;" + path + ";"
+        "1;" + store.printStorePath(path) + ";"
        + narHash.to_string(Base32) + ";"
        + std::to_string(narSize) + ";"
-        + concatStringsSep(",", references);
+        + concatStringsSep(",", store.printStorePathSet(references));
 }
-void ValidPathInfo::sign(const SecretKey & secretKey)
+void ValidPathInfo::sign(const Store & store, const SecretKey & secretKey)
 {
-    sigs.insert(secretKey.signDetached(fingerprint()));
+    sigs.insert(secretKey.signDetached(fingerprint(store)));
 }
 bool ValidPathInfo::isContentAddressed(const Store & store) const
 {
    auto warn = [&]() {
-        printError(format("warning: path '%s' claims to be content-addressed but isn't") % path);
+        printError("warning: path '%s' claims to be content-addressed but isn't", store.printStorePath(path));
    };
    if (hasPrefix(ca, "text:")) {
        Hash hash(std::string(ca, 5));
-        if (store.makeTextPath(storePathToName(path), hash, references) == path)
+        if (store.makeTextPath(path.name(), hash, references) == path)
            return true;
        else
            warn();
@ -797,9 +779,13 @@ bool ValidPathInfo::isContentAddressed(const Store & store) const
    else if (hasPrefix(ca, "fixed:")) {
        bool recursive = ca.compare(6, 2, "r:") == 0;
        Hash hash(std::string(ca, recursive ? 8 : 6));
-        auto refs = references;
+        auto refs = cloneStorePathSet(references);
-        replaceInSet(refs, path, std::string("self"));
+        bool hasSelfReference = false;
-        if (store.makeFixedOutputPath(recursive, hash, storePathToName(path), refs) == path)
+        if (refs.count(path)) {
            hasSelfReference = true;
            refs.erase(path);
        }
        if (store.makeFixedOutputPath(recursive, hash, path.name(), refs, hasSelfReference) == path)
            return true;
        else
            warn();
@ -815,15 +801,15 @@ size_t ValidPathInfo::checkSignatures(const Store & store, const PublicKeys & pu
    size_t good = 0;
    for (auto & sig : sigs)
-        if (checkSignature(publicKeys, sig))
+        if (checkSignature(store, publicKeys, sig))
            good++;
    return good;
 }
-bool ValidPathInfo::checkSignature(const PublicKeys & publicKeys, const std::string & sig) const
+bool ValidPathInfo::checkSignature(const Store & store, const PublicKeys & publicKeys, const std::string & sig) const
 {
-    return verifyDetached(fingerprint(), sig, publicKeys);
+    return verifyDetached(fingerprint(store), sig, publicKeys);
 }
@ -831,7 +817,7 @@ Strings ValidPathInfo::shortRefs() const
 {
    Strings refs;
    for (auto & r : references)
-        refs.push_back(baseNameOf(r));
+        refs.push_back(std::string(r.to_string()));
    return refs;
 }
@ -944,7 +930,7 @@ static RegisterStoreImplementation regStore([](
    const std::string & uri, const Store::Params & params)
    -> std::shared_ptr<Store>
 {
-    switch (getStoreType(uri, get(params, "state", settings.nixStateDir))) {
+    switch (getStoreType(uri, get(params, "state").value_or(settings.nixStateDir))) {
        case tDaemon:
            return std::shared_ptr<Store>(std::make_shared<UDSRemoteStore>(params));
        case tLocal: {
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@ -1,5 +1,6 @@
 #pragma once
 #include "path.hh"
 #include "hash.hh"
 #include "serialise.hh"
 #include "crypto.hh"
@ -42,14 +43,11 @@ enum SubstituteFlag : bool { NoSubstitute = false, Substitute = true };
 enum AllowInvalidFlag : bool { DisallowInvalid = false, AllowInvalid = true };
 /* Size of the hash part of store paths, in base-32 characters. */
 const size_t storePathHashLen = 32; // i.e. 160 bits
 /* Magic header of exportPath() output (obsolete). */
 const uint32_t exportMagic = 0x4558494e;
-typedef std::unordered_map<Path, std::unordered_set<std::string>> Roots;
+typedef std::unordered_map<StorePath, std::unordered_set<std::string>> Roots;
 struct GCOptions
@ -83,7 +81,7 @@ struct GCOptions
    bool ignoreLiveness{false};
    /* For `gcDeleteSpecific', the paths to delete. */
-    PathSet pathsToDelete;
+    StorePathSet pathsToDelete;
    /* Stop after at least `maxFreed' bytes have been freed. */
    unsigned long long maxFreed{std::numeric_limits<unsigned long long>::max()};
@ -104,21 +102,21 @@ struct GCResults
 struct SubstitutablePathInfo
 {
-    Path deriver;
+    std::optional<StorePath> deriver;
-    PathSet references;
+    StorePathSet references;
    unsigned long long downloadSize; /* 0 = unknown or inapplicable */
    unsigned long long narSize; /* 0 = unknown */
 };
-typedef std::map<Path, SubstitutablePathInfo> SubstitutablePathInfos;
+typedef std::map<StorePath, SubstitutablePathInfo> SubstitutablePathInfos;
 struct ValidPathInfo
 {
-    Path path;
+    StorePath path;
-    Path deriver;
+    std::optional<StorePath> deriver;
    Hash narHash;
-    PathSet references;
+    StorePathSet references;
    time_t registrationTime = 0;
    uint64_t narSize = 0; // 0 = unknown
    uint64_t id; // internal use only
@ -143,7 +141,7 @@ struct ValidPathInfo
       Ideally, the content-addressability assertion would just be a
       Boolean, and the store path would be computed from
-       ‘storePathToName(path)’, ‘narHash’ and ‘references’. However,
+       the name component, ‘narHash’ and ‘references’. However,
       1) we've accumulated several types of content-addressed paths
       over the years; and 2) fixed-output derivations support
       multiple hash algorithms and serialisation methods (flat file
@ -171,9 +169,9 @@ struct ValidPathInfo
       the NAR, and the sorted references. The size field is strictly
       speaking superfluous, but might prevent endless/excessive data
       attacks. */
-    std::string fingerprint() const;
+    std::string fingerprint(const Store & store) const;
-    void sign(const SecretKey & secretKey);
+    void sign(const Store & store, const SecretKey & secretKey);
    /* Return true iff the path is verifiably content-addressed. */
    bool isContentAddressed(const Store & store) const;
@ -186,10 +184,13 @@ struct ValidPathInfo
    size_t checkSignatures(const Store & store, const PublicKeys & publicKeys) const;
    /* Verify a single signature. */
-    bool checkSignature(const PublicKeys & publicKeys, const std::string & sig) const;
+    bool checkSignature(const Store & store, const PublicKeys & publicKeys, const std::string & sig) const;
    Strings shortRefs() const;
    ValidPathInfo(StorePath && path) : path(std::move(path)) { }
    explicit ValidPathInfo(const ValidPathInfo & other);
    virtual ~ValidPathInfo() { }
 };
@ -240,6 +241,23 @@ struct BuildResult
 };
 struct StorePathWithOutputs
 {
    StorePath path;
    std::set<std::string> outputs;
    StorePathWithOutputs(const StorePath & path, const std::set<std::string> & outputs = {})
        : path(path.clone()), outputs(outputs)
    { }
    StorePathWithOutputs(const StorePathWithOutputs & other)
        : path(other.path.clone()), outputs(other.outputs)
    { }
    std::string to_string(const Store & store) const;
 };
 class Store : public std::enable_shared_from_this<Store>, public Config
 {
 public:
@ -258,6 +276,7 @@ protected:
    struct State
    {
        // FIXME: fix key
        LRUCache<std::string, std::shared_ptr<const ValidPathInfo>> pathInfoCache;
    };
@ -273,6 +292,24 @@ public:
    virtual std::string getUri() = 0;
    StorePath parseStorePath(std::string_view path) const;
    std::string printStorePath(const StorePath & path) const;
    // FIXME: remove
    StorePathSet parseStorePathSet(const PathSet & paths) const;
    PathSet printStorePathSet(const StorePathSet & path) const;
    /* Split a string specifying a derivation and a set of outputs
       (/nix/store/hash-foo!out1,out2,...) into the derivation path
       and the outputs. */
    StorePathWithOutputs parseDrvPathWithOutputs(const string & s);
    /* Display a set of paths in human-readable form (i.e., between quotes
       and separated by commas). */
    std::string showPaths(const StorePathSet & paths);
    /* Return true if ‘path’ is in the Nix store (but not the Nix
       store itself). */
    bool isInStore(const Path & path) const;
@ -281,9 +318,6 @@ public:
       the Nix store. */
    bool isStorePath(const Path & path) const;
    /* Throw an exception if ‘path’ is not a store path. */
    void assertStorePath(const Path & path) const;
    /* Chop off the parts after the top-level store name, e.g.,
       /nix/store/abcd-foo/bar => /nix/store/abcd-foo. */
    Path toStorePath(const Path & path) const;
@ -293,26 +327,27 @@ public:
    /* Same as followLinksToStore(), but apply toStorePath() to the
       result. */
-    Path followLinksToStorePath(const Path & path) const;
+    StorePath followLinksToStorePath(const Path & path) const;
    /* Constructs a unique store path name. */
-    Path makeStorePath(const string & type,
+    StorePath makeStorePath(const string & type,
-        const Hash & hash, const string & name) const;
+        const Hash & hash, std::string_view name) const;
-    Path makeOutputPath(const string & id,
+    StorePath makeOutputPath(const string & id,
-        const Hash & hash, const string & name) const;
+        const Hash & hash, std::string_view name) const;
-    Path makeFixedOutputPath(bool recursive,
+    StorePath makeFixedOutputPath(bool recursive,
-        const Hash & hash, const string & name,
+        const Hash & hash, std::string_view name,
-        const PathSet & references = {}) const;
+        const StorePathSet & references = {},
        bool hasSelfReference = false) const;
-    Path makeTextPath(const string & name, const Hash & hash,
+    StorePath makeTextPath(std::string_view name, const Hash & hash,
-        const PathSet & references) const;
+        const StorePathSet & references) const;
    /* This is the preparatory part of addToStore(); it computes the
       store path to which srcPath is to be copied.  Returns the store
       path and the cryptographic hash of the contents of srcPath. */
-    std::pair<Path, Hash> computeStorePathForPath(const string & name,
+    std::pair<StorePath, Hash> computeStorePathForPath(std::string_view name,
        const Path & srcPath, bool recursive = true,
        HashType hashAlgo = htSHA256, PathFilter & filter = defaultPathFilter) const;
@ -330,21 +365,21 @@ public:
       simply yield a different store path, so other users wouldn't be
       affected), but it has some backwards compatibility issues (the
       hashing scheme changes), so I'm not doing that for now. */
-    Path computeStorePathForText(const string & name, const string & s,
+    StorePath computeStorePathForText(const string & name, const string & s,
-        const PathSet & references) const;
+        const StorePathSet & references) const;
    /* Check whether a path is valid. */
-    bool isValidPath(const Path & path);
+    bool isValidPath(const StorePath & path);
 protected:
-    virtual bool isValidPathUncached(const Path & path);
+    virtual bool isValidPathUncached(const StorePath & path);
 public:
    /* Query which of the given paths is valid. Optionally, try to
       substitute missing paths. */
-    virtual PathSet queryValidPaths(const PathSet & paths,
+    virtual StorePathSet queryValidPaths(const StorePathSet & paths,
        SubstituteFlag maybeSubstitute = NoSubstitute);
    /* Query the set of all valid paths. Note that for some store
@ -352,54 +387,54 @@ public:
       (i.e. you'll get /nix/store/<hash> rather than
       /nix/store/<hash>-<name>). Use queryPathInfo() to obtain the
       full store path. */
-    virtual PathSet queryAllValidPaths()
+    virtual StorePathSet queryAllValidPaths()
    { unsupported("queryAllValidPaths"); }
    /* Query information about a valid path. It is permitted to omit
       the name part of the store path. */
-    ref<const ValidPathInfo> queryPathInfo(const Path & path);
+    ref<const ValidPathInfo> queryPathInfo(const StorePath & path);
    /* Asynchronous version of queryPathInfo(). */
-    void queryPathInfo(const Path & path,
+    void queryPathInfo(const StorePath & path,
        Callback<ref<const ValidPathInfo>> callback) noexcept;
 protected:
-    virtual void queryPathInfoUncached(const Path & path,
+    virtual void queryPathInfoUncached(const StorePath & path,
        Callback<std::shared_ptr<const ValidPathInfo>> callback) noexcept = 0;
 public:
    /* Queries the set of incoming FS references for a store path.
       The result is not cleared. */
-    virtual void queryReferrers(const Path & path, PathSet & referrers)
+    virtual void queryReferrers(const StorePath & path, StorePathSet & referrers)
    { unsupported("queryReferrers"); }
    /* Return all currently valid derivations that have `path' as an
       output.  (Note that the result of `queryDeriver()' is the
       derivation that was actually used to produce `path', which may
       not exist anymore.) */
-    virtual PathSet queryValidDerivers(const Path & path) { return {}; };
+    virtual StorePathSet queryValidDerivers(const StorePath & path) { return {}; };
    /* Query the outputs of the derivation denoted by `path'. */
-    virtual PathSet queryDerivationOutputs(const Path & path)
+    virtual StorePathSet queryDerivationOutputs(const StorePath & path)
    { unsupported("queryDerivationOutputs"); }
    /* Query the output names of the derivation denoted by `path'. */
-    virtual StringSet queryDerivationOutputNames(const Path & path)
+    virtual StringSet queryDerivationOutputNames(const StorePath & path)
    { unsupported("queryDerivationOutputNames"); }
    /* Query the full store path given the hash part of a valid store
-       path, or "" if the path doesn't exist. */
+       path, or empty if the path doesn't exist. */
-    virtual Path queryPathFromHashPart(const string & hashPart) = 0;
+    virtual std::optional<StorePath> queryPathFromHashPart(const std::string & hashPart) = 0;
    /* Query which of the given paths have substitutes. */
-    virtual PathSet querySubstitutablePaths(const PathSet & paths) { return {}; };
+    virtual StorePathSet querySubstitutablePaths(const StorePathSet & paths) { return {}; };
    /* Query substitute info (i.e. references, derivers and download
       sizes) of a set of paths.  If a path does not have substitute
       info, it's omitted from the resulting ‘infos’ map. */
-    virtual void querySubstitutablePathInfos(const PathSet & paths,
+    virtual void querySubstitutablePathInfos(const StorePathSet & paths,
        SubstitutablePathInfos & infos) { return; };
    virtual bool wantMassQuery() { return false; }
@ -418,12 +453,12 @@ public:
       validity the resulting path.  The resulting path is returned.
       The function object `filter' can be used to exclude files (see
       libutil/archive.hh). */
-    virtual Path addToStore(const string & name, const Path & srcPath,
+    virtual StorePath addToStore(const string & name, const Path & srcPath,
        bool recursive = true, HashType hashAlgo = htSHA256,
        PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair) = 0;
    // FIXME: remove?
-    virtual Path addToStoreFromDump(const string & dump, const string & name,
+    virtual StorePath addToStoreFromDump(const string & dump, const string & name,
        bool recursive = true, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair)
    {
        throw Error("addToStoreFromDump() is not supported by this store");
@ -431,11 +466,11 @@ public:
    /* Like addToStore, but the contents written to the output path is
       a regular file containing the given string. */
-    virtual Path addTextToStore(const string & name, const string & s,
+    virtual StorePath addTextToStore(const string & name, const string & s,
-        const PathSet & references, RepairFlag repair = NoRepair) = 0;
+        const StorePathSet & references, RepairFlag repair = NoRepair) = 0;
    /* Write a NAR dump of a store path. */
-    virtual void narFromPath(const Path & path, Sink & sink) = 0;
+    virtual void narFromPath(const StorePath & path, Sink & sink) = 0;
    /* For each path, if it's a derivation, build it.  Building a
       derivation means ensuring that the output paths are valid.  If
@ -445,22 +480,24 @@ public:
       output paths can be created by running the builder, after
       recursively building any sub-derivations. For inputs that are
       not derivations, substitute them. */
-    virtual void buildPaths(const PathSet & paths, BuildMode buildMode = bmNormal);
+    virtual void buildPaths(
        const std::vector<StorePathWithOutputs> & paths,
        BuildMode buildMode = bmNormal);
    /* Build a single non-materialized derivation (i.e. not from an
       on-disk .drv file). Note that ‘drvPath’ is only used for
       informational purposes. */
-    virtual BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+    virtual BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
        BuildMode buildMode = bmNormal) = 0;
    /* Ensure that a path is valid.  If it is not currently valid, it
       may be made valid by running a substitute (if defined for the
       path). */
-    virtual void ensurePath(const Path & path) = 0;
+    virtual void ensurePath(const StorePath & path) = 0;
    /* Add a store path as a temporary root of the garbage collector.
       The root disappears as soon as we exit. */
-    virtual void addTempRoot(const Path & path)
+    virtual void addTempRoot(const StorePath & path)
    { unsupported("addTempRoot"); }
    /* Add an indirect root, which is merely a symlink to `path' from
@ -506,7 +543,7 @@ public:
    /* Return a string representing information about the path that
       can be loaded into the database using `nix-store --load-db' or
       `nix-store --register-validity'. */
-    string makeValidityRegistration(const PathSet & paths,
+    string makeValidityRegistration(const StorePathSet & paths,
        bool showDerivers, bool showHash);
    /* Write a JSON representation of store path metadata, such as the
@ -514,14 +551,14 @@ public:
       variable elements such as the registration time are
       included. If ‘showClosureSize’ is true, the closure size of
       each path is included. */
-    void pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths,
+    void pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & storePaths,
        bool includeImpureInfo, bool showClosureSize,
        AllowInvalidFlag allowInvalid = DisallowInvalid);
    /* Return the size of the closure of the specified path, that is,
       the sum of the size of the NAR serialisation of each path in
       the closure. */
-    std::pair<uint64_t, uint64_t> getClosureSize(const Path & storePath);
+    std::pair<uint64_t, uint64_t> getClosureSize(const StorePath & storePath);
    /* Optimise the disk space usage of the Nix store by hard-linking files
       with the same contents. */
@ -537,14 +574,14 @@ public:
    /* Add signatures to the specified store path. The signatures are
       not verified. */
-    virtual void addSignatures(const Path & storePath, const StringSet & sigs)
+    virtual void addSignatures(const StorePath & storePath, const StringSet & sigs)
    { unsupported("addSignatures"); }
    /* Utility functions. */
    /* Read a derivation, after ensuring its existence through
       ensurePath(). */
-    Derivation derivationFromPath(const Path & drvPath);
+    Derivation derivationFromPath(const StorePath & drvPath);
    /* Place in `out' the set of all store paths in the file system
       closure of `storePath'; that is, all paths than can be directly
@ -553,36 +590,36 @@ public:
       `storePath' is returned; that is, the closures under the
       `referrers' relation instead of the `references' relation is
       returned. */
-    virtual void computeFSClosure(const PathSet & paths,
+    virtual void computeFSClosure(const StorePathSet & paths,
-        PathSet & out, bool flipDirection = false,
+        StorePathSet & out, bool flipDirection = false,
        bool includeOutputs = false, bool includeDerivers = false);
-    void computeFSClosure(const Path & path,
+    void computeFSClosure(const StorePath & path,
-        PathSet & out, bool flipDirection = false,
+        StorePathSet & out, bool flipDirection = false,
        bool includeOutputs = false, bool includeDerivers = false);
    /* Given a set of paths that are to be built, return the set of
       derivations that will be built, and the set of output paths
       that will be substituted. */
-    virtual void queryMissing(const PathSet & targets,
+    virtual void queryMissing(const std::vector<StorePathWithOutputs> & targets,
-        PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
+        StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
        unsigned long long & downloadSize, unsigned long long & narSize);
    /* Sort a set of paths topologically under the references
       relation.  If p refers to q, then p precedes q in this list. */
-    Paths topoSortPaths(const PathSet & paths);
+    StorePaths topoSortPaths(const StorePathSet & paths);
    /* Export multiple paths in the format expected by ‘nix-store
       --import’. */
-    void exportPaths(const Paths & paths, Sink & sink);
+    void exportPaths(const StorePathSet & paths, Sink & sink);
-    void exportPath(const Path & path, Sink & sink);
+    void exportPath(const StorePath & path, Sink & sink);
    /* Import a sequence of NAR dumps created by exportPaths() into
       the Nix store. Optionally, the contents of the NARs are
       preloaded into the specified FS accessor to speed up subsequent
       access. */
-    Paths importPaths(Source & source, std::shared_ptr<FSAccessor> accessor,
+    StorePaths importPaths(Source & source, std::shared_ptr<FSAccessor> accessor,
        CheckSigsFlag checkSigs = CheckSigs);
    struct Stats
@ -606,7 +643,7 @@ public:
    /* Return the build log of the specified store path, if available,
       or null otherwise. */
-    virtual std::shared_ptr<std::string> getBuildLog(const Path & path)
+    virtual std::shared_ptr<std::string> getBuildLog(const StorePath & path)
    { return nullptr; }
    /* Hack to allow long-running processes like hydra-queue-runner to
@ -672,11 +709,11 @@ public:
    LocalFSStore(const Params & params);
-    void narFromPath(const Path & path, Sink & sink) override;
+    void narFromPath(const StorePath & path, Sink & sink) override;
    ref<FSAccessor> getFSAccessor() override;
    /* Register a permanent GC root. */
-    Path addPermRoot(const Path & storePath,
+    Path addPermRoot(const StorePath & storePath,
        const Path & gcRoot, bool indirect, bool allowOutsideRootsDir = false);
    virtual Path getRealStoreDir() { return storeDir; }
@ -687,25 +724,17 @@ public:
        return getRealStoreDir() + "/" + std::string(storePath, storeDir.size() + 1);
    }
-    std::shared_ptr<std::string> getBuildLog(const Path & path) override;
+    std::shared_ptr<std::string> getBuildLog(const StorePath & path) override;
 };
 /* Extract the name part of the given store path. */
 string storePathToName(const Path & path);
 /* Extract the hash part of the given store path. */
 string storePathToHash(const Path & path);
 /* Check whether ‘name’ is a valid store path name part, i.e. contains
   only the characters [a-zA-Z0-9\+\-\.\_\?\=] and doesn't start with
   a dot. */
 void checkStoreName(const string & name);
 /* Copy a path from one store to another. */
 void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
-    const Path & storePath, RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs);
+    const StorePath & storePath, RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs);
 /* Copy store paths from one store to another. The paths may be copied
@ -713,7 +742,7 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
   (i.e. if A is a reference of B, then A is copied before B), but
   the set of store paths is not automatically closed; use
   copyClosure() for that. */
-void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const PathSet & storePaths,
+void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
    RepairFlag repair = NoRepair,
    CheckSigsFlag checkSigs = CheckSigs,
    SubstituteFlag substitute = NoSubstitute);
@ -721,7 +750,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const PathSet & storePa
 /* Copy the closure of the specified paths from one store to another. */
 void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
-    const PathSet & storePaths,
+    const StorePathSet & storePaths,
    RepairFlag repair = NoRepair,
    CheckSigsFlag checkSigs = CheckSigs,
    SubstituteFlag substitute = NoSubstitute);
@ -804,7 +833,9 @@ struct RegisterStoreImplementation
 string showPaths(const PathSet & paths);
-ValidPathInfo decodeValidPathInfo(std::istream & str,
+std::optional<ValidPathInfo> decodeValidPathInfo(
    const Store & store,
    std::istream & str,
    bool hashGiven = false);
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@ -65,8 +65,9 @@ typedef enum {
 class Store;
 struct Source;
-Path readStorePath(Store & store, Source & from);
+template<class T> T readStorePaths(const Store & store, Source & from);
-template<class T> T readStorePaths(Store & store, Source & from);
+
 void writeStorePaths(const Store & store, Sink & out, const StorePathSet & paths);
 }
--- a/src/libutil/rust-ffi.cc
+++ b/src/libutil/rust-ffi.cc
@ -1,12 +1,18 @@
 #include "logging.hh"
 #include "rust-ffi.hh"
 namespace nix {
 extern "C" std::exception_ptr * make_error(rust::StringSlice s)
 {
    // FIXME: leak
-    return new std::exception_ptr(std::make_exception_ptr(Error(std::string(s.ptr, s.size))));
+    return new std::exception_ptr(std::make_exception_ptr(nix::Error(std::string(s.ptr, s.size))));
 }
 namespace rust {
 std::ostream & operator << (std::ostream & str, const String & s)
 {
    str << (std::string_view) s;
    return str;
 }
 }
--- a/src/libutil/rust-ffi.hh
+++ b/src/libutil/rust-ffi.hh
@ -1,16 +1,91 @@
 #pragma once
 #include "serialise.hh"
 #include <string_view>
 #include <cstring>
 #include <array>
 namespace rust {
-// Depending on the internal representation of Rust slices is slightly
+typedef void (*DropFun)(void *);
-// evil...
+
 /* A Rust value of N bytes. It can be moved but not copied. When it
   goes out of scope, the C++ destructor will run the drop
   function. */
 template<std::size_t N, DropFun drop>
 struct Value
 {
 protected:
    std::array<char, N> raw;
    ~Value()
    {
        if (!isEvacuated()) {
            drop(this);
            evacuate();
        }
    }
    // Must not be called directly.
    Value()
    { }
    Value(Value && other)
        : raw(other.raw)
    {
        other.evacuate();
    }
    void operator =(Value && other)
    {
        if (!isEvacuated())
            drop(this);
        raw = other.raw;
        other.evacuate();
    }
 private:
    /* FIXME: optimize these (ideally in such a way that the compiler
       can elide most calls to evacuate() / isEvacuated(). */
    inline void evacuate()
    {
        for (auto & i : raw) i = 0;
    }
    inline bool isEvacuated()
    {
        for (auto & i : raw)
            if (i != 0) return false;
        return true;
    }
 };
 /* A Rust vector. */
 template<typename T, DropFun drop>
 struct Vec : Value<3 * sizeof(void *), drop>
 {
    inline size_t size() const
    {
        return ((const size_t *) &this->raw)[2];
    }
    const T * data() const
    {
        return ((const T * *) &this->raw)[0];
    }
 };
 /* A Rust slice. */
 template<typename T>
 struct Slice
 {
-    T * ptr;
+    const T * ptr;
    size_t size;
-    Slice(T * ptr, size_t size) : ptr(ptr), size(size)
+    Slice(const T * ptr, size_t size) : ptr(ptr), size(size)
    {
        assert(ptr);
    }
@ -18,9 +93,44 @@ struct Slice
 struct StringSlice : Slice<char>
 {
-    StringSlice(const std::string & s): Slice((char *) s.data(), s.size()) {}
+    StringSlice(const std::string & s): Slice(s.data(), s.size()) {}
    explicit StringSlice(std::string_view s): Slice(s.data(), s.size()) {}
    StringSlice(const char * s): Slice(s, strlen(s)) {}
    operator std::string_view() const
    {
        return std::string_view(ptr, size);
    }
 };
 /* A Rust string. */
 struct String;
 extern "C" {
    void ffi_String_new(StringSlice s, String * out);
    void ffi_String_drop(void * s);
 }
 struct String : Vec<char, ffi_String_drop>
 {
    String(std::string_view s)
    {
        ffi_String_new(StringSlice(s), this);
    }
    String(const char * s)
        : String({s, std::strlen(s)})
    {
    }
    operator std::string_view() const
    {
        return std::string_view(data(), size());
    }
 };
 std::ostream & operator << (std::ostream & str, const String & s);
 struct Source
 {
    size_t (*fun)(void * source_this, rust::Slice<uint8_t> data);
@ -33,7 +143,7 @@ struct Source
    // FIXME: how to propagate exceptions?
    static size_t sourceWrapper(void * _this, rust::Slice<uint8_t> data)
    {
-        auto n = ((nix::Source *) _this)->read(data.ptr, data.size);
+        auto n = ((nix::Source *) _this)->read((unsigned char *) data.ptr, data.size);
        return n;
    }
 };
@ -49,11 +159,20 @@ struct Result
        std::exception_ptr * exc;
    };
    ~Result()
    {
        if (tag == 0)
            data.~T();
        else if (tag == 1)
            // FIXME: don't leak exc
            ;
    }
    /* Rethrow the wrapped exception or return the wrapped value. */
    T unwrap()
    {
        if (tag == 0)
-            return data;
+            return std::move(data);
        else if (tag == 1)
            std::rethrow_exception(*exc);
        else
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@ -17,7 +17,7 @@ void unpackTarfile(Source & source, const Path & destDir)
 void unpackTarfile(const Path & tarFile, const Path & destDir,
    std::optional<std::string> baseName)
 {
-    if (!baseName) baseName = baseNameOf(tarFile);
+    if (!baseName) baseName = std::string(baseNameOf(tarFile));
    auto source = sinkToSource([&](Sink & sink) {
        // FIXME: look at first few bytes to determine compression type.
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@ -188,22 +188,22 @@ Path dirOf(const Path & path)
 }
-string baseNameOf(const Path & path)
+std::string_view baseNameOf(std::string_view path)
 {
    if (path.empty())
        return "";
-    Path::size_type last = path.length() - 1;
+    auto last = path.size() - 1;
    if (path[last] == '/' && last > 0)
        last -= 1;
-    Path::size_type pos = path.rfind('/', last);
+    auto pos = path.rfind('/', last);
    if (pos == string::npos)
        pos = 0;
    else
        pos += 1;
-    return string(path, pos, last - pos + 1);
+    return path.substr(pos, last - pos + 1);
 }
@ -1302,9 +1302,10 @@ bool hasPrefix(const string & s, const string & prefix)
 }
-bool hasSuffix(const string & s, const string & suffix)
+bool hasSuffix(std::string_view s, std::string_view suffix)
 {
-    return s.size() >= suffix.size() && string(s, s.size() - suffix.size()) == suffix;
+    return s.size() >= suffix.size()
        && s.substr(s.size() - suffix.size()) == suffix;
 }
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@ -62,7 +62,7 @@ Path dirOf(const Path & path);
 /* Return the base name of the given canonical path, i.e., everything
   following the final `/'. */
-string baseNameOf(const Path & path);
+std::string_view baseNameOf(std::string_view path);
 /* Check whether 'path' is a descendant of 'dir'. */
 bool isInDir(const Path & path, const Path & dir);
@ -407,7 +407,7 @@ bool hasPrefix(const string & s, const string & prefix);
 /* Return true iff `s' ends in `suffix'. */
-bool hasSuffix(const string & s, const string & suffix);
+bool hasSuffix(std::string_view s, std::string_view suffix);
 /* Convert a string to lower case. */
@ -450,10 +450,10 @@ string base64Decode(const string & s);
 /* Get a value for the specified key from an associate container, or a
   default value if the key doesn't exist. */
 template <class T>
-string get(const T & map, const string & key, const string & def = "")
+std::optional<std::string> get(const T & map, const std::string & key)
 {
    auto i = map.find(key);
-    return i == map.end() ? def : i->second;
+    return i == map.end() ? std::optional<std::string>() : i->second;
 }
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@ -317,11 +317,11 @@ static void _main(int argc, char * * argv)
    state->printStats();
-    auto buildPaths = [&](const PathSet & paths) {
+    auto buildPaths = [&](const std::vector<StorePathWithOutputs> & paths) {
        /* Note: we do this even when !printMissing to efficiently
           fetch binary cache data. */
        unsigned long long downloadSize, narSize;
-        PathSet willBuild, willSubstitute, unknown;
+        StorePathSet willBuild, willSubstitute, unknown;
        store->queryMissing(paths,
            willBuild, willSubstitute, unknown, downloadSize, narSize);
@ -337,9 +337,9 @@ static void _main(int argc, char * * argv)
            throw UsageError("nix-shell requires a single derivation");
        auto & drvInfo = drvs.front();
-        auto drv = store->derivationFromPath(drvInfo.queryDrvPath());
+        auto drv = store->derivationFromPath(store->parseStorePath(drvInfo.queryDrvPath()));
-        PathSet pathsToBuild;
+        std::vector<StorePathWithOutputs> pathsToBuild;
        /* Figure out what bash shell to use. If $NIX_BUILD_SHELL
           is not set, then build bashInteractive from
@ -358,7 +358,7 @@ static void _main(int argc, char * * argv)
                if (!drv)
                    throw Error("the 'bashInteractive' attribute in <nixpkgs> did not evaluate to a derivation");
-                pathsToBuild.insert(drv->queryDrvPath());
+                pathsToBuild.emplace_back(store->parseStorePath(drv->queryDrvPath()));
                shell = drv->queryOutPath() + "/bin/bash";
@ -370,10 +370,11 @@ static void _main(int argc, char * * argv)
        // Build or fetch all dependencies of the derivation.
        for (const auto & input : drv.inputDrvs)
-            if (std::all_of(envExclude.cbegin(), envExclude.cend(), [&](const string & exclude) { return !std::regex_search(input.first, std::regex(exclude)); }))
+            if (std::all_of(envExclude.cbegin(), envExclude.cend(),
-                pathsToBuild.insert(makeDrvPathWithOutputs(input.first, input.second));
+                    [&](const string & exclude) { return !std::regex_search(store->printStorePath(input.first), std::regex(exclude)); }))
                pathsToBuild.emplace_back(input.first, input.second);
        for (const auto & src : drv.inputSrcs)
-            pathsToBuild.insert(src);
+            pathsToBuild.emplace_back(src);
        buildPaths(pathsToBuild);
@ -399,7 +400,7 @@ static void _main(int argc, char * * argv)
        env["NIX_STORE"] = store->storeDir;
        env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
-        auto passAsFile = tokenizeString<StringSet>(get(drv.env, "passAsFile", ""));
+        auto passAsFile = tokenizeString<StringSet>(get(drv.env, "passAsFile").value_or(""));
        bool keepTmp = false;
        int fileNr = 0;
@ -468,7 +469,7 @@ static void _main(int argc, char * * argv)
    else {
-        PathSet pathsToBuild;
+        std::vector<StorePathWithOutputs> pathsToBuild;
        std::map<Path, Path> drvPrefixes;
        std::map<Path, Path> resultSymlinks;
@ -482,7 +483,7 @@ static void _main(int argc, char * * argv)
            if (outputName == "")
                throw Error("derivation '%s' lacks an 'outputName' attribute", drvPath);
-            pathsToBuild.insert(drvPath + "!" + outputName);
+            pathsToBuild.emplace_back(store->parseStorePath(drvPath), StringSet{outputName});
            std::string drvPrefix;
            auto i = drvPrefixes.find(drvPath);
@ -508,7 +509,7 @@ static void _main(int argc, char * * argv)
        for (auto & symlink : resultSymlinks)
            if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
-                store2->addPermRoot(symlink.second, absPath(symlink.first), true);
+                store2->addPermRoot(store->parseStorePath(symlink.second), absPath(symlink.first), true);
        for (auto & path : outPaths)
            std::cout << path << '\n';
--- a/src/nix-channel/nix-channel.cc
+++ b/src/nix-channel/nix-channel.cc
@ -27,7 +27,7 @@ static void readChannels()
            continue;
        auto split = tokenizeString<std::vector<string>>(line, " ");
        auto url = std::regex_replace(split[0], std::regex("/*$"), "");
-        auto name = split.size() > 1 ? split[1] : baseNameOf(url);
+        auto name = split.size() > 1 ? split[1] : std::string(baseNameOf(url));
        channels[name] = url;
    }
 }
@ -98,10 +98,9 @@ static void update(const StringSet & channelNames)
        // shows something useful).
        auto cname = name;
        std::smatch match;
-        auto urlBase = baseNameOf(url);
+        auto urlBase = std::string(baseNameOf(url));
-        if (std::regex_search(urlBase, match, std::regex("(-\\d.*)$"))) {
+        if (std::regex_search(urlBase, match, std::regex("(-\\d.*)$")))
            cname = cname + (string) match[1];
        }
        std::string extraAttrs;
--- a/src/nix-copy-closure/nix-copy-closure.cc
+++ b/src/nix-copy-closure/nix-copy-closure.cc
@ -52,11 +52,11 @@ static int _main(int argc, char ** argv)
        auto to = toMode ? openStore(remoteUri) : openStore();
        auto from = toMode ? openStore() : openStore(remoteUri);
-        PathSet storePaths2;
+        StorePathSet storePaths2;
        for (auto & path : storePaths)
            storePaths2.insert(from->followLinksToStorePath(path));
-        PathSet closure;
+        StorePathSet closure;
        from->computeFSClosure(storePaths2, closure, false, includeOutputs);
        copyPaths(from, to, closure, NoRepair, NoCheckSigs, useSubstitutes);
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@ -286,7 +286,7 @@ static int _main(int argc, char * * argv)
                if (chdir(socketDir.c_str()) == -1)
                    throw SysError(format("changing to socket directory '%1%'") % socketDir);
-                auto socketName = baseNameOf(socketPath);
+                auto socketName = std::string(baseNameOf(socketPath));
                auto addr = sockaddr_un{};
                addr.sun_family = AF_UNIX;
                if (socketName.size() + 1 >= sizeof(addr.sun_path))
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@ -208,10 +208,11 @@ static long comparePriorities(EvalState & state, DrvInfo & drv1, DrvInfo & drv2)
 // at a time.
 static bool isPrebuilt(EvalState & state, DrvInfo & elem)
 {
-    Path path = elem.queryOutPath();
+    auto path = state.store->parseStorePath(elem.queryOutPath());
    if (state.store->isValidPath(path)) return true;
-    PathSet ps = state.store->querySubstitutablePaths({path});
+    StorePathSet paths;
-    return ps.find(path) != ps.end();
+    paths.insert(path.clone()); // FIXME: why doesn't StorePathSet{path.clone()} work?
    return state.store->querySubstitutablePaths(paths).count(path);
 }
@ -371,24 +372,21 @@ static void queryInstSources(EvalState & state,
        case srcStorePaths: {
            for (auto & i : args) {
-                Path path = state.store->followLinksToStorePath(i);
+                auto path = state.store->followLinksToStorePath(i);
-                string name = baseNameOf(path);
+                std::string name(path.name());
                string::size_type dash = name.find('-');
                if (dash != string::npos)
                    name = string(name, dash + 1);
                DrvInfo elem(state, "", nullptr);
                elem.setName(name);
-                if (isDerivation(path)) {
+                if (path.isDerivation()) {
-                    elem.setDrvPath(path);
+                    elem.setDrvPath(state.store->printStorePath(path));
-                    elem.setOutPath(state.store->derivationFromPath(path).findOutput("out"));
+                    elem.setOutPath(state.store->printStorePath(state.store->derivationFromPath(path).findOutput("out")));
                    if (name.size() >= drvExtension.size() &&
                        string(name, name.size() - drvExtension.size()) == drvExtension)
                        name = string(name, 0, name.size() - drvExtension.size());
                }
-                else elem.setOutPath(path);
+                else elem.setOutPath(state.store->printStorePath(path));
                elems.push_back(elem);
            }
@ -421,13 +419,13 @@ static void queryInstSources(EvalState & state,
 static void printMissing(EvalState & state, DrvInfos & elems)
 {
-    PathSet targets;
+    std::vector<StorePathWithOutputs> targets;
    for (auto & i : elems) {
        Path drvPath = i.queryDrvPath();
        if (drvPath != "")
-            targets.insert(drvPath);
+            targets.emplace_back(state.store->parseStorePath(drvPath));
        else
-            targets.insert(i.queryOutPath());
+            targets.emplace_back(state.store->parseStorePath(i.queryOutPath()));
    }
    printMissing(state.store, targets);
@ -697,15 +695,15 @@ static void opSet(Globals & globals, Strings opFlags, Strings opArgs)
        drv.setName(globals.forceName);
    if (drv.queryDrvPath() != "") {
-        PathSet paths = {drv.queryDrvPath()};
+        std::vector<StorePathWithOutputs> paths{globals.state->store->parseStorePath(drv.queryDrvPath())};
        printMissing(globals.state->store, paths);
        if (globals.dryRun) return;
        globals.state->store->buildPaths(paths, globals.state->repair ? bmRepair : bmNormal);
-    }
+    } else {
-    else {
+        printMissing(globals.state->store,
-        printMissing(globals.state->store, {drv.queryOutPath()});
+            {globals.state->store->parseStorePath(drv.queryOutPath())});
        if (globals.dryRun) return;
-        globals.state->store->ensurePath(drv.queryOutPath());
+        globals.state->store->ensurePath(globals.state->store->parseStorePath(drv.queryOutPath()));
    }
    debug(format("switching to new user environment"));
@ -729,7 +727,7 @@ static void uninstallDerivations(Globals & globals, Strings & selectors,
            for (auto & j : selectors)
                /* !!! the repeated calls to followLinksToStorePath()
                   are expensive, should pre-compute them. */
-                if ((isPath(j) && i.queryOutPath() == globals.state->store->followLinksToStorePath(j))
+                if ((isPath(j) && globals.state->store->parseStorePath(i.queryOutPath()) == globals.state->store->followLinksToStorePath(j))
                    || DrvName(j).matches(drvName))
                {
                    printInfo("uninstalling '%s'", i.queryName());
@ -953,12 +951,13 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
    /* Query which paths have substitutes. */
-    PathSet validPaths, substitutablePaths;
+    StorePathSet validPaths;
    StorePathSet substitutablePaths;
    if (printStatus || globals.prebuiltOnly) {
-        PathSet paths;
+        StorePathSet paths;
        for (auto & i : elems)
            try {
-                paths.insert(i.queryOutPath());
+                paths.insert(globals.state->store->parseStorePath(i.queryOutPath()));
            } catch (AssertionError & e) {
                printMsg(lvlTalkative, "skipping derivation named '%s' which gives an assertion failure", i.queryName());
                i.setFailed();
@ -989,8 +988,8 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
            //Activity act(*logger, lvlDebug, format("outputting query result '%1%'") % i.attrPath);
            if (globals.prebuiltOnly &&
-                validPaths.find(i.queryOutPath()) == validPaths.end() &&
+                !validPaths.count(globals.state->store->parseStorePath(i.queryOutPath())) &&
-                substitutablePaths.find(i.queryOutPath()) == substitutablePaths.end())
+                !substitutablePaths.count(globals.state->store->parseStorePath(i.queryOutPath())))
                continue;
            /* For table output. */
@ -1001,9 +1000,9 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
            if (printStatus) {
                Path outPath = i.queryOutPath();
-                bool hasSubs = substitutablePaths.find(outPath) != substitutablePaths.end();
+                bool hasSubs = substitutablePaths.count(globals.state->store->parseStorePath(outPath));
                bool isInstalled = installed.find(outPath) != installed.end();
-                bool isValid = validPaths.find(outPath) != validPaths.end();
+                bool isValid = validPaths.count(globals.state->store->parseStorePath(outPath));
                if (xmlOutput) {
                    attrs["installed"] = isInstalled ? "1" : "0";
                    attrs["valid"] = isValid ? "1" : "0";
@ -1347,7 +1346,7 @@ static int _main(int argc, char * * argv)
            using LegacyArgs::LegacyArgs;
        };
-        MyArgs myArgs(baseNameOf(argv[0]), [&](Strings::iterator & arg, const Strings::iterator & end) {
+        MyArgs myArgs(std::string(baseNameOf(argv[0])), [&](Strings::iterator & arg, const Strings::iterator & end) {
            Operation oldOp = op;
            if (*arg == "--help")
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@ -32,16 +32,16 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
 {
    /* Build the components in the user environment, if they don't
       exist already. */
-    PathSet drvsToBuild;
+    std::vector<StorePathWithOutputs> drvsToBuild;
    for (auto & i : elems)
        if (i.queryDrvPath() != "")
-            drvsToBuild.insert(i.queryDrvPath());
+            drvsToBuild.push_back({state.store->parseStorePath(i.queryDrvPath())});
    debug(format("building user environment dependencies"));
    state.store->buildPaths(drvsToBuild, state.repair ? bmRepair : bmNormal);
    /* Construct the whole top level derivation. */
-    PathSet references;
+    StorePathSet references;
    Value manifest;
    state.mkList(manifest, elems.size());
    unsigned int n = 0;
@ -77,10 +77,10 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
            /* This is only necessary when installing store paths, e.g.,
               `nix-env -i /nix/store/abcd...-foo'. */
-            state.store->addTempRoot(j.second);
+            state.store->addTempRoot(state.store->parseStorePath(j.second));
-            state.store->ensurePath(j.second);
+            state.store->ensurePath(state.store->parseStorePath(j.second));
-            references.insert(j.second);
+            references.insert(state.store->parseStorePath(j.second));
        }
        // Copy the meta attributes.
@ -95,14 +95,14 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
        vMeta.attrs->sort();
        v.attrs->sort();
-        if (drvPath != "") references.insert(drvPath);
+        if (drvPath != "") references.insert(state.store->parseStorePath(drvPath));
    }
    /* Also write a copy of the list of user environment elements to
       the store; we need it for future modifications of the
       environment. */
-    Path manifestFile = state.store->addTextToStore("env-manifest.nix",
+    auto manifestFile = state.store->addTextToStore("env-manifest.nix",
-        (format("%1%") % manifest).str(), references);
+        fmt("%s", manifest), references);
    /* Get the environment builder expression. */
    Value envBuilder;
@ -113,7 +113,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
    Value args, topLevel;
    state.mkAttrs(args, 3);
    mkString(*state.allocAttr(args, state.symbols.create("manifest")),
-        manifestFile, {manifestFile});
+        state.store->printStorePath(manifestFile), {state.store->printStorePath(manifestFile)});
    args.attrs->push_back(Attr(state.symbols.create("derivations"), &manifest));
    args.attrs->sort();
    mkApp(topLevel, envBuilder, args);
@ -123,13 +123,15 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
    state.forceValue(topLevel);
    PathSet context;
    Attr & aDrvPath(*topLevel.attrs->find(state.sDrvPath));
-    Path topLevelDrv = state.coerceToPath(aDrvPath.pos ? *(aDrvPath.pos) : noPos, *(aDrvPath.value), context);
+    auto topLevelDrv = state.store->parseStorePath(state.coerceToPath(aDrvPath.pos ? *(aDrvPath.pos) : noPos, *(aDrvPath.value), context));
    Attr & aOutPath(*topLevel.attrs->find(state.sOutPath));
    Path topLevelOut = state.coerceToPath(aOutPath.pos ? *(aOutPath.pos) : noPos, *(aOutPath.value), context);
    /* Realise the resulting store expression. */
    debug("building user environment");
-    state.store->buildPaths({topLevelDrv}, state.repair ? bmRepair : bmNormal);
+    std::vector<StorePathWithOutputs> topLevelDrvs;
    topLevelDrvs.push_back(StorePathWithOutputs{topLevelDrv.clone()});
    state.store->buildPaths(topLevelDrvs, state.repair ? bmRepair : bmNormal);
    /* Switch the current user environment to the output path. */
    auto store2 = state.store.dynamic_pointer_cast<LocalFSStore>();
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@ -75,9 +75,9 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                    if (++rootNr > 1) rootName += "-" + std::to_string(rootNr);
                    auto store2 = state.store.dynamic_pointer_cast<LocalFSStore>();
                    if (store2)
-                        drvPath = store2->addPermRoot(drvPath, rootName, indirectRoot);
+                        drvPath = store2->addPermRoot(store2->parseStorePath(drvPath), rootName, indirectRoot);
                }
-                std::cout << format("%1%%2%\n") % drvPath % (outputName != "out" ? "!" + outputName : "");
+                std::cout << fmt("%s%s\n", drvPath, (outputName != "out" ? "!" + outputName : ""));
            }
        }
    }
@ -105,7 +105,7 @@ static int _main(int argc, char * * argv)
            using LegacyArgs::LegacyArgs;
        };
-        MyArgs myArgs(baseNameOf(argv[0]), [&](Strings::iterator & arg, const Strings::iterator & end) {
+        MyArgs myArgs(std::string(baseNameOf(argv[0])), [&](Strings::iterator & arg, const Strings::iterator & end) {
            if (*arg == "--help")
                showManPage("nix-instantiate");
            else if (*arg == "--version")
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@ -64,7 +64,7 @@ static int _main(int argc, char * * argv)
            using LegacyArgs::LegacyArgs;
        };
-        MyArgs myArgs(baseNameOf(argv[0]), [&](Strings::iterator & arg, const Strings::iterator & end) {
+        MyArgs myArgs(std::string(baseNameOf(argv[0])), [&](Strings::iterator & arg, const Strings::iterator & end) {
            if (*arg == "--help")
                showManPage("nix-prefetch-url");
            else if (*arg == "--version")
@ -156,17 +156,17 @@ static int _main(int argc, char * * argv)
        /* If an expected hash is given, the file may already exist in
           the store. */
        Hash hash, expectedHash(ht);
-        Path storePath;
+        std::optional<StorePath> storePath;
        if (args.size() == 2) {
            expectedHash = Hash(args[1], ht);
            storePath = store->makeFixedOutputPath(unpack, expectedHash, name);
-            if (store->isValidPath(storePath))
+            if (store->isValidPath(*storePath))
                hash = expectedHash;
            else
-                storePath.clear();
+                storePath.reset();
        }
-        if (storePath.empty()) {
+        if (!storePath) {
            auto actualUri = resolveMirrorUri(*state, uri);
@ -193,7 +193,7 @@ static int _main(int argc, char * * argv)
                if (hasSuffix(baseNameOf(uri), ".zip"))
                    runProgram("unzip", true, {"-qq", tmpFile, "-d", unpacked});
                else
-                    unpackTarfile(tmpFile, unpacked, baseNameOf(uri));
+                    unpackTarfile(tmpFile, unpacked, std::string(baseNameOf(uri)));
                /* If the archive unpacks to a single file/directory, then use
                   that as the top-level. */
@ -217,17 +217,17 @@ static int _main(int argc, char * * argv)
               into the Nix store. */
            storePath = store->addToStore(name, tmpFile, unpack, ht);
-            assert(storePath == store->makeFixedOutputPath(unpack, hash, name));
+            assert(*storePath == store->makeFixedOutputPath(unpack, hash, name));
        }
        stopProgressBar();
        if (!printPath)
-            printInfo(format("path is '%1%'") % storePath);
+            printInfo("path is '%s'", store->printStorePath(*storePath));
        std::cout << printHash16or32(hash) << std::endl;
        if (printPath)
-            std::cout << storePath << std::endl;
+            std::cout << store->printStorePath(*storePath) << std::endl;
        return 0;
    }
--- a/src/nix-store/dotgraph.cc
+++ b/src/nix-store/dotgraph.cc
@ -10,9 +10,9 @@ using std::cout;
 namespace nix {
-static string dotQuote(const string & s)
+static string dotQuote(std::string_view s)
 {
-    return "\"" + s + "\"";
+    return "\"" + std::string(s) + "\"";
 }
@ -34,7 +34,7 @@ static string makeEdge(const string & src, const string & dst)
 }
-static string makeNode(const string & id, const string & label,
+static string makeNode(const string & id, std::string_view label,
    const string & colour)
 {
    format f = format("%1% [label = %2%, shape = box, "
@ -44,13 +44,6 @@ static string makeNode(const string & id, const string & label,
 }
 static string symbolicName(const string & path)
 {
    string p = baseNameOf(path);
    return string(p, p.find('-') + 1);
 }
 #if 0
 string pathLabel(const Path & nePath, const string & elemPath)
 {
@ -91,25 +84,24 @@ void printClosure(const Path & nePath, const StoreExpr & fs)
 #endif
-void printDotGraph(ref<Store> store, const PathSet & roots)
+void printDotGraph(ref<Store> store, StorePathSet && roots)
 {
-    PathSet workList(roots);
+    StorePathSet workList(std::move(roots));
-    PathSet doneSet;
+    StorePathSet doneSet;
    cout << "digraph G {\n";
    while (!workList.empty()) {
-        Path path = *(workList.begin());
+        auto path = std::move(workList.extract(workList.begin()).value());
        workList.erase(path);
-        if (!doneSet.insert(path).second) continue;
+        if (!doneSet.insert(path.clone()).second) continue;
-        cout << makeNode(path, symbolicName(path), "#ff0000");
+        cout << makeNode(std::string(path.to_string()), path.name(), "#ff0000");
        for (auto & p : store->queryPathInfo(path)->references) {
            if (p != path) {
-                workList.insert(p);
+                workList.insert(p.clone());
-                cout << makeEdge(p, path);
+                cout << makeEdge(std::string(p.to_string()), std::string(p.to_string()));
            }
        }
--- a/src/nix-store/dotgraph.hh
+++ b/src/nix-store/dotgraph.hh
@ -1,11 +1,9 @@
 #pragma once
-#include "types.hh"
+#include "store-api.hh"
 namespace nix {
-class Store;
+void printDotGraph(ref<Store> store, StorePathSet && roots);
 void printDotGraph(ref<Store> store, const PathSet & roots);
 }
--- a/src/nix-store/graphml.cc
+++ b/src/nix-store/graphml.cc
@ -11,7 +11,7 @@ using std::cout;
 namespace nix {
-static inline const string & xmlQuote(const string & s)
+static inline std::string_view xmlQuote(std::string_view s)
 {
    // Luckily, store paths shouldn't contain any character that needs to be
    // quoted.
@ -19,14 +19,13 @@ static inline const string & xmlQuote(const string & s)
 }
-static string symbolicName(const string & path)
+static string symbolicName(const std::string & p)
 {
    string p = baseNameOf(path);
    return string(p, p.find('-') + 1);
 }
-static string makeEdge(const string & src, const string & dst)
+static string makeEdge(std::string_view src, std::string_view dst)
 {
    return fmt("  <edge source=\"%1%\" target=\"%2%\"/>\n",
        xmlQuote(src), xmlQuote(dst));
@ -41,18 +40,18 @@ static string makeNode(const ValidPathInfo & info)
        "    <data key=\"name\">%3%</data>\n"
        "    <data key=\"type\">%4%</data>\n"
        "  </node>\n",
-        info.path,
+        info.path.to_string(),
        info.narSize,
-        symbolicName(info.path),
+        symbolicName(std::string(info.path.name())),
-        (isDerivation(info.path) ? "derivation" : "output-path"));
+        (info.path.isDerivation() ? "derivation" : "output-path"));
 }
-void printGraphML(ref<Store> store, const PathSet & roots)
+void printGraphML(ref<Store> store, StorePathSet && roots)
 {
-    PathSet workList(roots);
+    StorePathSet workList(std::move(roots));
-    PathSet doneSet;
+    StorePathSet doneSet;
-    std::pair<PathSet::iterator,bool> ret;
+    std::pair<StorePathSet::iterator, bool> ret;
    cout << "<?xml version='1.0' encoding='utf-8'?>\n"
         << "<graphml xmlns='http://graphml.graphdrawing.org/xmlns'\n"
@ -64,19 +63,18 @@ void printGraphML(ref<Store> store, const PathSet & roots)
         << "<graph id='G' edgedefault='directed'>\n";
    while (!workList.empty()) {
-        Path path = *(workList.begin());
+        auto path = std::move(workList.extract(workList.begin()).value());
        workList.erase(path);
-        ret = doneSet.insert(path);
+        ret = doneSet.insert(path.clone());
        if (ret.second == false) continue;
-        ValidPathInfo info = *(store->queryPathInfo(path));
+        auto info = store->queryPathInfo(path);
-        cout << makeNode(info);
+        cout << makeNode(*info);
-        for (auto & p : store->queryPathInfo(path)->references) {
+        for (auto & p : info->references) {
            if (p != path) {
-                workList.insert(p);
+                workList.insert(p.clone());
-                cout << makeEdge(path, p);
+                cout << makeEdge(path.to_string(), p.to_string());
            }
        }
--- a/src/nix-store/graphml.hh
+++ b/src/nix-store/graphml.hh
@ -1,11 +1,9 @@
 #pragma once
-#include "types.hh"
+#include "store-api.hh"
 namespace nix {
-class Store;
+void printGraphML(ref<Store> store, StorePathSet && roots);
 void printGraphML(ref<Store> store, const PathSet & roots);
 }
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@ -47,38 +47,37 @@ ref<LocalStore> ensureLocalStore()
 }
-static Path useDeriver(Path path)
+static StorePath useDeriver(const StorePath & path)
 {
-    if (isDerivation(path)) return path;
+    if (path.isDerivation()) return path.clone();
-    Path drvPath = store->queryPathInfo(path)->deriver;
+    auto info = store->queryPathInfo(path);
-    if (drvPath == "")
+    if (!info->deriver)
-        throw Error(format("deriver of path '%1%' is not known") % path);
+        throw Error("deriver of path '%s' is not known", store->printStorePath(path));
-    return drvPath;
+    return info->deriver->clone();
 }
 /* Realise the given path.  For a derivation that means build it; for
   other paths it means ensure their validity. */
-static PathSet realisePath(Path path, bool build = true)
+static PathSet realisePath(StorePathWithOutputs path, bool build = true)
 {
    DrvPathWithOutputs p = parseDrvPathWithOutputs(path);
    auto store2 = std::dynamic_pointer_cast<LocalFSStore>(store);
-    if (isDerivation(p.first)) {
+    if (path.path.isDerivation()) {
        if (build) store->buildPaths({path});
-        Derivation drv = store->derivationFromPath(p.first);
+        Derivation drv = store->derivationFromPath(path.path);
        rootNr++;
-        if (p.second.empty())
+        if (path.outputs.empty())
-            for (auto & i : drv.outputs) p.second.insert(i.first);
+            for (auto & i : drv.outputs) path.outputs.insert(i.first);
        PathSet outputs;
-        for (auto & j : p.second) {
+        for (auto & j : path.outputs) {
            DerivationOutputs::iterator i = drv.outputs.find(j);
            if (i == drv.outputs.end())
-                throw Error(format("derivation '%1%' does not have an output named '%2%'") % p.first % j);
+                throw Error("derivation '%s' does not have an output named '%s'",
-            Path outPath = i->second.path;
+                    store2->printStorePath(path.path), j);
            auto outPath = store2->printStorePath(i->second.path);
            if (store2) {
                if (gcRoot == "")
                    printGCWarning();
@ -86,7 +85,7 @@ static PathSet realisePath(Path path, bool build = true)
                    Path rootName = gcRoot;
                    if (rootNr > 1) rootName += "-" + std::to_string(rootNr);
                    if (i->first != "out") rootName += "-" + i->first;
-                    outPath = store2->addPermRoot(outPath, rootName, indirectRoot);
+                    outPath = store2->addPermRoot(store->parseStorePath(outPath), rootName, indirectRoot);
                }
            }
            outputs.insert(outPath);
@ -95,8 +94,9 @@ static PathSet realisePath(Path path, bool build = true)
    }
    else {
-        if (build) store->ensurePath(path);
+        if (build) store->ensurePath(path.path);
-        else if (!store->isValidPath(path)) throw Error(format("path '%1%' does not exist and cannot be created") % path);
+        else if (!store->isValidPath(path.path))
            throw Error("path '%s' does not exist and cannot be created", store->printStorePath(path.path));
        if (store2) {
            if (gcRoot == "")
                printGCWarning();
@ -104,10 +104,10 @@ static PathSet realisePath(Path path, bool build = true)
                Path rootName = gcRoot;
                rootNr++;
                if (rootNr > 1) rootName += "-" + std::to_string(rootNr);
-                path = store2->addPermRoot(path, rootName, indirectRoot);
+                return {store2->addPermRoot(path.path, rootName, indirectRoot)};
            }
        }
-        return {path};
+        return {store->printStorePath(path.path)};
    }
 }
@ -126,23 +126,20 @@ static void opRealise(Strings opFlags, Strings opArgs)
        else if (i == "--ignore-unknown") ignoreUnknown = true;
        else throw UsageError(format("unknown flag '%1%'") % i);
-    Paths paths;
+    std::vector<StorePathWithOutputs> paths;
-    for (auto & i : opArgs) {
+    for (auto & i : opArgs)
-        DrvPathWithOutputs p = parseDrvPathWithOutputs(i);
+        paths.push_back(store->parseDrvPathWithOutputs(i));
        paths.push_back(makeDrvPathWithOutputs(store->followLinksToStorePath(p.first), p.second));
    }
    unsigned long long downloadSize, narSize;
-    PathSet willBuild, willSubstitute, unknown;
+    StorePathSet willBuild, willSubstitute, unknown;
-    store->queryMissing(PathSet(paths.begin(), paths.end()),
+    store->queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize, narSize);
        willBuild, willSubstitute, unknown, downloadSize, narSize);
    if (ignoreUnknown) {
-        Paths paths2;
+        std::vector<StorePathWithOutputs> paths2;
        for (auto & i : paths)
-            if (unknown.find(i) == unknown.end()) paths2.push_back(i);
+            if (!unknown.count(i.path)) paths2.push_back(i);
-        paths = paths2;
+        paths = std::move(paths2);
-        unknown = PathSet();
+        unknown = StorePathSet();
    }
    if (settings.printMissing)
@ -151,14 +148,14 @@ static void opRealise(Strings opFlags, Strings opArgs)
    if (dryRun) return;
    /* Build all paths at the same time to exploit parallelism. */
-    store->buildPaths(PathSet(paths.begin(), paths.end()), buildMode);
+    store->buildPaths(paths, buildMode);
    if (!ignoreUnknown)
        for (auto & i : paths) {
-            PathSet paths = realisePath(i, false);
+            auto paths2 = realisePath(i, false);
            if (!noOutput)
-                for (auto & j : paths)
+                for (auto & j : paths2)
-                    cout << format("%1%\n") % j;
+                    cout << fmt("%1%\n", j);
        }
 }
@ -169,7 +166,7 @@ static void opAdd(Strings opFlags, Strings opArgs)
    if (!opFlags.empty()) throw UsageError("unknown flag");
    for (auto & i : opArgs)
-        cout << format("%1%\n") % store->addToStore(baseNameOf(i), i);
+        cout << fmt("%s\n", store->printStorePath(store->addToStore(std::string(baseNameOf(i)), i)));
 }
@ -190,7 +187,7 @@ static void opAddFixed(Strings opFlags, Strings opArgs)
    opArgs.pop_front();
    for (auto & i : opArgs)
-        cout << format("%1%\n") % store->addToStore(baseNameOf(i), i, recursive, hashAlgo);
+        cout << fmt("%s\n", store->printStorePath(store->addToStore(std::string(baseNameOf(i)), i, recursive, hashAlgo)));
 }
@ -211,22 +208,21 @@ static void opPrintFixedPath(Strings opFlags, Strings opArgs)
    string hash = *i++;
    string name = *i++;
-    cout << format("%1%\n") %
+    cout << fmt("%s\n", store->printStorePath(store->makeFixedOutputPath(recursive, Hash(hash, hashAlgo), name)));
        store->makeFixedOutputPath(recursive, Hash(hash, hashAlgo), name);
 }
-static PathSet maybeUseOutputs(const Path & storePath, bool useOutput, bool forceRealise)
+static StorePathSet maybeUseOutputs(const StorePath & storePath, bool useOutput, bool forceRealise)
 {
    if (forceRealise) realisePath(storePath);
-    if (useOutput && isDerivation(storePath)) {
+    if (useOutput && storePath.isDerivation()) {
-        Derivation drv = store->derivationFromPath(storePath);
+        auto drv = store->derivationFromPath(storePath);
-        PathSet outputs;
+        StorePathSet outputs;
        for (auto & i : drv.outputs)
-            outputs.insert(i.second.path);
+            outputs.insert(i.second.path.clone());
        return outputs;
    }
-    else return {storePath};
+    else return singleton(storePath.clone());
 }
@ -239,23 +235,23 @@ const string treeLine = "|   ";
 const string treeNull = "    ";
-static void printTree(const Path & path,
+static void printTree(const StorePath & path,
-    const string & firstPad, const string & tailPad, PathSet & done)
+    const string & firstPad, const string & tailPad, StorePathSet & done)
 {
-    if (!done.insert(path).second) {
+    if (!done.insert(path.clone()).second) {
-        cout << format("%1%%2% [...]\n") % firstPad % path;
+        cout << fmt("%s%s [...]\n", firstPad, store->printStorePath(path));
        return;
    }
-    cout << format("%1%%2%\n") % firstPad % path;
+    cout << fmt("%s%s\n", firstPad, store->printStorePath(path));
-    auto references = store->queryPathInfo(path)->references;
+    auto info = store->queryPathInfo(path);
    /* Topologically sort under the relation A < B iff A \in
       closure(B).  That is, if derivation A is an (possibly indirect)
       input of B, then A is printed first.  This has the effect of
       flattening the tree, preventing deeply nested structures.  */
-    Paths sorted = store->topoSortPaths(references);
+    auto sorted = store->topoSortPaths(info->references);
    reverse(sorted.begin(), sorted.end());
    for (auto i = sorted.begin(); i != sorted.end(); ++i) {
@ -318,11 +314,11 @@ static void opQuery(Strings opFlags, Strings opArgs)
        case qOutputs: {
            for (auto & i : opArgs) {
-                i = store->followLinksToStorePath(i);
+                auto i2 = store->followLinksToStorePath(i);
-                if (forceRealise) realisePath(i);
+                if (forceRealise) realisePath(i2);
-                Derivation drv = store->derivationFromPath(i);
+                Derivation drv = store->derivationFromPath(i2);
                for (auto & j : drv.outputs)
-                    cout << format("%1%\n") % j.second.path;
+                    cout << fmt("%1%\n", store->printStorePath(j.second.path));
            }
            break;
        }
@ -331,51 +327,54 @@ static void opQuery(Strings opFlags, Strings opArgs)
        case qReferences:
        case qReferrers:
        case qReferrersClosure: {
-            PathSet paths;
+            StorePathSet paths;
            for (auto & i : opArgs) {
-                PathSet ps = maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise);
+                auto ps = maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise);
                for (auto & j : ps) {
                    if (query == qRequisites) store->computeFSClosure(j, paths, false, includeOutputs);
                    else if (query == qReferences) {
                        for (auto & p : store->queryPathInfo(j)->references)
-                            paths.insert(p);
+                            paths.insert(p.clone());
                    }
                    else if (query == qReferrers) {
                        StorePathSet tmp;
                        store->queryReferrers(j, tmp);
                        for (auto & i : tmp)
                            paths.insert(i.clone());
                    }
                    else if (query == qReferrers) store->queryReferrers(j, paths);
                    else if (query == qReferrersClosure) store->computeFSClosure(j, paths, true);
                }
            }
-            Paths sorted = store->topoSortPaths(paths);
+            auto sorted = store->topoSortPaths(paths);
-            for (Paths::reverse_iterator i = sorted.rbegin();
+            for (StorePaths::reverse_iterator i = sorted.rbegin();
                 i != sorted.rend(); ++i)
-                cout << format("%s\n") % *i;
+                cout << fmt("%s\n", store->printStorePath(*i));
            break;
        }
        case qDeriver:
            for (auto & i : opArgs) {
-                Path deriver = store->queryPathInfo(store->followLinksToStorePath(i))->deriver;
+                auto info = store->queryPathInfo(store->followLinksToStorePath(i));
-                cout << format("%1%\n") %
+                cout << fmt("%s\n", info->deriver ? store->printStorePath(*info->deriver) : "unknown-deriver");
                    (deriver == "" ? "unknown-deriver" : deriver);
            }
            break;
        case qBinding:
            for (auto & i : opArgs) {
-                Path path = useDeriver(store->followLinksToStorePath(i));
+                auto path = useDeriver(store->followLinksToStorePath(i));
                Derivation drv = store->derivationFromPath(path);
                StringPairs::iterator j = drv.env.find(bindingName);
                if (j == drv.env.end())
-                    throw Error(format("derivation '%1%' has no environment binding named '%2%'")
+                    throw Error("derivation '%s' has no environment binding named '%s'",
-                        % path % bindingName);
+                        store->printStorePath(path), bindingName);
-                cout << format("%1%\n") % j->second;
+                cout << fmt("%s\n", j->second);
            }
            break;
        case qHash:
        case qSize:
            for (auto & i : opArgs) {
-                PathSet paths = maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise);
+                for (auto & j : maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise)) {
                for (auto & j : paths) {
                    auto info = store->queryPathInfo(j);
                    if (query == qHash) {
                        assert(info->narHash.type == htSHA256);
@ -387,50 +386,51 @@ static void opQuery(Strings opFlags, Strings opArgs)
            break;
        case qTree: {
-            PathSet done;
+            StorePathSet done;
            for (auto & i : opArgs)
                printTree(store->followLinksToStorePath(i), "", "", done);
            break;
        }
        case qGraph: {
-            PathSet roots;
+            StorePathSet roots;
-            for (auto & i : opArgs) {
+            for (auto & i : opArgs)
-                PathSet paths = maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise);
+                for (auto & j : maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise))
-                roots.insert(paths.begin(), paths.end());
+                    roots.insert(j.clone());
-            }
+            printDotGraph(ref<Store>(store), std::move(roots));
            printDotGraph(ref<Store>(store), roots);
            break;
        }
        case qGraphML: {
-            PathSet roots;
+            StorePathSet roots;
-            for (auto & i : opArgs) {
+            for (auto & i : opArgs)
-                PathSet paths = maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise);
+                for (auto & j : maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise))
-                roots.insert(paths.begin(), paths.end());
+                    roots.insert(j.clone());
-            }
+            printGraphML(ref<Store>(store), std::move(roots));
            printGraphML(ref<Store>(store), roots);
            break;
        }
        case qResolve: {
            for (auto & i : opArgs)
-                cout << format("%1%\n") % store->followLinksToStorePath(i);
+                cout << fmt("%s\n", store->printStorePath(store->followLinksToStorePath(i)));
            break;
        }
        case qRoots: {
-            PathSet referrers;
+            StorePathSet args;
-            for (auto & i : opArgs) {
+            for (auto & i : opArgs)
                for (auto & p : maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise))
                    args.insert(p.clone());
            StorePathSet referrers;
            store->computeFSClosure(
-                    maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise),
+                args, referrers, true, settings.gcKeepOutputs, settings.gcKeepDerivations);
-                    referrers, true, settings.gcKeepOutputs, settings.gcKeepDerivations);
+
            }
            Roots roots = store->findRoots(false);
            for (auto & [target, links] : roots)
                if (referrers.find(target) != referrers.end())
                    for (auto & link : links)
-                        cout << format("%1% -> %2%\n") % link % target;
+                        cout << fmt("%1% -> %2%\n", link, store->printStorePath(target));
            break;
        }
@ -446,7 +446,7 @@ static void opPrintEnv(Strings opFlags, Strings opArgs)
    if (opArgs.size() != 1) throw UsageError("'--print-env' requires one derivation store path");
    Path drvPath = opArgs.front();
-    Derivation drv = store->derivationFromPath(drvPath);
+    Derivation drv = store->derivationFromPath(store->parseStorePath(drvPath));
    /* Print each environment variable in the derivation in a format
       that can be sourced by the shell. */
@ -476,7 +476,7 @@ static void opReadLog(Strings opFlags, Strings opArgs)
        auto path = store->followLinksToStorePath(i);
        auto log = store->getBuildLog(path);
        if (!log)
-            throw Error("build log of derivation '%s' is not available", path);
+            throw Error("build log of derivation '%s' is not available", store->printStorePath(path));
        std::cout << *log;
    }
 }
@ -487,13 +487,10 @@ static void opDumpDB(Strings opFlags, Strings opArgs)
    if (!opFlags.empty()) throw UsageError("unknown flag");
    if (!opArgs.empty()) {
        for (auto & i : opArgs)
-            i = store->followLinksToStorePath(i);
+            cout << store->makeValidityRegistration(singleton(store->followLinksToStorePath(i)), true, true);
        for (auto & i : opArgs)
            cout << store->makeValidityRegistration({i}, true, true);
    } else {
-        PathSet validPaths = store->queryAllValidPaths();
+        for (auto & i : store->queryAllValidPaths())
-        for (auto & i : validPaths)
+            cout << store->makeValidityRegistration(singleton(i), true, true);
            cout << store->makeValidityRegistration({i}, true, true);
    }
 }
@ -503,18 +500,18 @@ static void registerValidity(bool reregister, bool hashGiven, bool canonicalise)
    ValidPathInfos infos;
    while (1) {
-        ValidPathInfo info = decodeValidPathInfo(cin, hashGiven);
+        auto info = decodeValidPathInfo(*store, cin, hashGiven);
-        if (info.path == "") break;
+        if (!info) break;
-        if (!store->isValidPath(info.path) || reregister) {
+        if (!store->isValidPath(info->path) || reregister) {
            /* !!! races */
            if (canonicalise)
-                canonicalisePathMetaData(info.path, -1);
+                canonicalisePathMetaData(store->printStorePath(info->path), -1);
            if (!hashGiven) {
-                HashResult hash = hashPath(htSHA256, info.path);
+                HashResult hash = hashPath(htSHA256, store->printStorePath(info->path));
-                info.narHash = hash.first;
+                info->narHash = hash.first;
-                info.narSize = hash.second;
+                info->narSize = hash.second;
            }
-            infos.push_back(info);
+            infos.push_back(std::move(*info));
        }
    }
@ -556,12 +553,12 @@ static void opCheckValidity(Strings opFlags, Strings opArgs)
        else throw UsageError(format("unknown flag '%1%'") % i);
    for (auto & i : opArgs) {
-        Path path = store->followLinksToStorePath(i);
+        auto path = store->followLinksToStorePath(i);
        if (!store->isValidPath(path)) {
            if (printInvalid)
-                cout << format("%1%\n") % path;
+                cout << fmt("%s\n", store->printStorePath(path));
            else
-                throw Error(format("path '%1%' is not valid") % path);
+                throw Error("path '%s' is not valid", store->printStorePath(path));
        }
    }
 }
@ -591,13 +588,13 @@ static void opGC(Strings opFlags, Strings opArgs)
    if (printRoots) {
        Roots roots = store->findRoots(false);
-        std::set<std::pair<Path, Path>> roots2;
+        std::set<std::pair<Path, StorePath>> roots2;
        // Transpose and sort the roots.
        for (auto & [target, links] : roots)
            for (auto & link : links)
-                roots2.emplace(link, target);
+                roots2.emplace(link, target.clone());
        for (auto & [link, target] : roots2)
-            std::cout << link << " -> " << target << "\n";
+            std::cout << link << " -> " << store->printStorePath(target) << "\n";
    }
    else {
@ -661,11 +658,13 @@ static void opExport(Strings opFlags, Strings opArgs)
    for (auto & i : opFlags)
        throw UsageError(format("unknown flag '%1%'") % i);
    StorePathSet paths;
    for (auto & i : opArgs)
-        i = store->followLinksToStorePath(i);
+        paths.insert(store->followLinksToStorePath(i));
    FdSink sink(STDOUT_FILENO);
-    store->exportPaths(opArgs, sink);
+    store->exportPaths(paths, sink);
    sink.flush();
 }
@ -678,10 +677,10 @@ static void opImport(Strings opFlags, Strings opArgs)
    if (!opArgs.empty()) throw UsageError("no arguments expected");
    FdSource source(STDIN_FILENO);
-    Paths paths = store->importPaths(source, nullptr, NoCheckSigs);
+    auto paths = store->importPaths(source, nullptr, NoCheckSigs);
    for (auto & i : paths)
-        cout << format("%1%\n") % i << std::flush;
+        cout << fmt("%s\n", store->printStorePath(i)) << std::flush;
 }
@ -726,16 +725,16 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
    int status = 0;
    for (auto & i : opArgs) {
-        Path path = store->followLinksToStorePath(i);
+        auto path = store->followLinksToStorePath(i);
-        printMsg(lvlTalkative, format("checking path '%1%'...") % path);
+        printMsg(lvlTalkative, "checking path '%s'...", store->printStorePath(path));
        auto info = store->queryPathInfo(path);
        HashSink sink(info->narHash.type);
        store->narFromPath(path, sink);
        auto current = sink.finish();
        if (current.first != info->narHash) {
            printError(
-                format("path '%1%' was modified! expected hash '%2%', got '%3%'")
+                "path '%s' was modified! expected hash '%s', got '%s'",
-                % path % info->narHash.to_string() % current.first.to_string());
+                store->printStorePath(path), info->narHash.to_string(), current.first.to_string());
            status = 1;
        }
    }
@ -751,10 +750,8 @@ static void opRepairPath(Strings opFlags, Strings opArgs)
    if (!opFlags.empty())
        throw UsageError("no flags expected");
-    for (auto & i : opArgs) {
+    for (auto & i : opArgs)
-        Path path = store->followLinksToStorePath(i);
+        ensureLocalStore()->repairPath(store->followLinksToStorePath(i));
        ensureLocalStore()->repairPath(path);
    }
 }
 /* Optimise the disk space usage of the Nix store by hard-linking
@ -818,7 +815,7 @@ static void opServe(Strings opFlags, Strings opArgs)
            case cmdQueryValidPaths: {
                bool lock = readInt(in);
                bool substitute = readInt(in);
-                PathSet paths = readStorePaths<PathSet>(*store, in);
+                auto paths = readStorePaths<StorePathSet>(*store, in);
                if (lock && writeAllowed)
                    for (auto & path : paths)
                        store->addTempRoot(path);
@ -828,34 +825,39 @@ static void opServe(Strings opFlags, Strings opArgs)
                   flag. */
                if (substitute && writeAllowed) {
                    /* Filter out .drv files (we don't want to build anything). */
-                    PathSet paths2;
+                    std::vector<StorePathWithOutputs> paths2;
                    for (auto & path : paths)
-                        if (!isDerivation(path)) paths2.insert(path);
+                        if (!path.isDerivation())
                            paths2.emplace_back(path.clone());
                    unsigned long long downloadSize, narSize;
-                    PathSet willBuild, willSubstitute, unknown;
+                    StorePathSet willBuild, willSubstitute, unknown;
-                    store->queryMissing(PathSet(paths2.begin(), paths2.end()),
+                    store->queryMissing(paths2,
                        willBuild, willSubstitute, unknown, downloadSize, narSize);
                    /* FIXME: should use ensurePath(), but it only
                       does one path at a time. */
                    if (!willSubstitute.empty())
                        try {
-                            store->buildPaths(willSubstitute);
+                            std::vector<StorePathWithOutputs> subs;
                            for (auto & p : willSubstitute) subs.emplace_back(p.clone());
                            store->buildPaths(subs);
                        } catch (Error & e) {
                            printError(format("warning: %1%") % e.msg());
                        }
                }
-                out << store->queryValidPaths(paths);
+                writeStorePaths(*store, out, store->queryValidPaths(paths));
                break;
            }
            case cmdQueryPathInfos: {
-                PathSet paths = readStorePaths<PathSet>(*store, in);
+                auto paths = readStorePaths<StorePathSet>(*store, in);
                // !!! Maybe we want a queryPathInfos?
                for (auto & i : paths) {
                    try {
                        auto info = store->queryPathInfo(i);
-                        out << info->path << info->deriver << info->references;
+                        out << store->printStorePath(info->path)
                            << (info->deriver ? store->printStorePath(*info->deriver) : "");
                        writeStorePaths(*store, out, info->references);
                        // !!! Maybe we want compression?
                        out << info->narSize // downloadSize
                            << info->narSize;
@ -869,7 +871,7 @@ static void opServe(Strings opFlags, Strings opArgs)
            }
            case cmdDumpStorePath:
-                store->narFromPath(readStorePath(*store, in), out);
+                store->narFromPath(store->parseStorePath(readString(in)), out);
                break;
            case cmdImportPaths: {
@ -881,14 +883,17 @@ static void opServe(Strings opFlags, Strings opArgs)
            case cmdExportPaths: {
                readInt(in); // obsolete
-                store->exportPaths(readStorePaths<Paths>(*store, in), out);
+                store->exportPaths(readStorePaths<StorePathSet>(*store, in), out);
                break;
            }
            case cmdBuildPaths: {
                if (!writeAllowed) throw Error("building paths is not allowed");
-                PathSet paths = readStorePaths<PathSet>(*store, in);
+
                std::vector<StorePathWithOutputs> paths;
                for (auto & s : readStrings<Strings>(in))
                    paths.emplace_back(store->parseDrvPathWithOutputs(s));
                getBuildSettings();
@ -907,7 +912,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                if (!writeAllowed) throw Error("building paths is not allowed");
-                Path drvPath = readStorePath(*store, in); // informational only
+                auto drvPath = store->parseStorePath(readString(in)); // informational only
                BasicDerivation drv;
                readDerivation(in, *store, drv);
@ -926,30 +931,29 @@ static void opServe(Strings opFlags, Strings opArgs)
            case cmdQueryClosure: {
                bool includeOutputs = readInt(in);
-                PathSet closure;
+                StorePathSet closure;
-                store->computeFSClosure(readStorePaths<PathSet>(*store, in),
+                store->computeFSClosure(readStorePaths<StorePathSet>(*store, in),
                    closure, false, includeOutputs);
-                out << closure;
+                writeStorePaths(*store, out, closure);
                break;
            }
            case cmdAddToStoreNar: {
                if (!writeAllowed) throw Error("importing paths is not allowed");
-                ValidPathInfo info;
+                auto path = readString(in);
-                info.path = readStorePath(*store, in);
+                ValidPathInfo info(store->parseStorePath(path));
-                in >> info.deriver;
+                auto deriver = readString(in);
-                if (!info.deriver.empty())
+                if (deriver != "")
-                    store->assertStorePath(info.deriver);
+                    info.deriver = store->parseStorePath(deriver);
                info.narHash = Hash(readString(in), htSHA256);
-                info.references = readStorePaths<PathSet>(*store, in);
+                info.references = readStorePaths<StorePathSet>(*store, in);
                in >> info.registrationTime >> info.narSize >> info.ultimate;
                info.sigs = readStrings<StringSet>(in);
                in >> info.ca;
-                if (info.narSize == 0) {
+                if (info.narSize == 0)
                    throw Error("narInfo is too old and missing the narSize field");
                }
                SizedSource sizedSource(in, info.narSize);
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@ -40,16 +40,17 @@ struct CmdAddToStore : MixDryRun, StoreCommand
        StringSink sink;
        dumpPath(path, sink);
-        ValidPathInfo info;
+        auto narHash = hashString(htSHA256, *sink.s);
-        info.narHash = hashString(htSHA256, *sink.s);
+
        ValidPathInfo info(store->makeFixedOutputPath(true, narHash, *namePart));
        info.narHash = narHash;
        info.narSize = sink.s->size();
        info.path = store->makeFixedOutputPath(true, info.narHash, *namePart);
        info.ca = makeFixedOutputCA(true, info.narHash);
        if (!dryRun)
            store->addToStore(info, sink.s);
-        std::cout << fmt("%s\n", info.path);
+        std::cout << fmt("%s\n", store->printStorePath(info.path));
    }
 };
--- a/src/nix/command.cc
+++ b/src/nix/command.cc
@ -48,28 +48,29 @@ StorePathsCommand::StorePathsCommand(bool recursive)
 void StorePathsCommand::run(ref<Store> store)
 {
-    Paths storePaths;
+    StorePaths storePaths;
    if (all) {
        if (installables.size())
            throw UsageError("'--all' does not expect arguments");
        for (auto & p : store->queryAllValidPaths())
-            storePaths.push_back(p);
+            storePaths.push_back(p.clone());
    }
    else {
        for (auto & p : toStorePaths(store, realiseMode, installables))
-            storePaths.push_back(p);
+            storePaths.push_back(p.clone());
        if (recursive) {
-            PathSet closure;
+            StorePathSet closure;
-            store->computeFSClosure(PathSet(storePaths.begin(), storePaths.end()),
+            store->computeFSClosure(storePathsToSet(storePaths), closure, false, false);
-                closure, false, false);
+            storePaths.clear();
-            storePaths = Paths(closure.begin(), closure.end());
+            for (auto & p : closure)
                storePaths.push_back(p.clone());
        }
    }
-    run(store, storePaths);
+    run(store, std::move(storePaths));
 }
 void StorePathCommand::run(ref<Store> store)
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@ -2,6 +2,7 @@
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
 namespace nix {
@ -28,8 +29,8 @@ private:
 struct Buildable
 {
-    Path drvPath; // may be empty
+    std::optional<StorePath> drvPath;
-    std::map<std::string, Path> outputs;
+    std::map<std::string, StorePath> outputs;
 };
 typedef std::vector<Buildable> Buildables;
@ -131,7 +132,7 @@ public:
    using StoreCommand::run;
-    virtual void run(ref<Store> store, Paths storePaths) = 0;
+    virtual void run(ref<Store> store, std::vector<StorePath> storePaths) = 0;
    void run(ref<Store> store) override;
@ -143,7 +144,7 @@ struct StorePathCommand : public InstallablesCommand
 {
    using StoreCommand::run;
-    virtual void run(ref<Store> store, const Path & storePath) = 0;
+    virtual void run(ref<Store> store, const StorePath & storePath) = 0;
    void run(ref<Store> store) override;
 };
@ -174,13 +175,13 @@ std::shared_ptr<Installable> parseInstallable(
 Buildables build(ref<Store> store, RealiseMode mode,
    std::vector<std::shared_ptr<Installable>> installables);
-PathSet toStorePaths(ref<Store> store, RealiseMode mode,
+std::set<StorePath> toStorePaths(ref<Store> store, RealiseMode mode,
    std::vector<std::shared_ptr<Installable>> installables);
-Path toStorePath(ref<Store> store, RealiseMode mode,
+StorePath toStorePath(ref<Store> store, RealiseMode mode,
    std::shared_ptr<Installable> installable);
-PathSet toDerivations(ref<Store> store,
+std::set<StorePath> toDerivations(ref<Store> store,
    std::vector<std::shared_ptr<Installable>> installables,
    bool useDeriver = false);
--- a/src/nix/copy.cc
+++ b/src/nix/copy.cc
@ -80,14 +80,14 @@ struct CmdCopy : StorePathsCommand
        return srcUri.empty() ? StoreCommand::createStore() : openStore(srcUri);
    }
-    void run(ref<Store> srcStore, Paths storePaths) override
+    void run(ref<Store> srcStore, StorePaths storePaths) override
    {
        if (srcUri.empty() && dstUri.empty())
            throw UsageError("you must pass '--from' and/or '--to'");
        ref<Store> dstStore = dstUri.empty() ? openStore() : openStore(dstUri);
-        copyPaths(srcStore, dstStore, PathSet(storePaths.begin(), storePaths.end()),
+        copyPaths(srcStore, dstStore, storePathsToSet(storePaths),
            NoRepair, checkSigs, substitute);
    }
 };
--- a/src/nix/dump-path.cc
+++ b/src/nix/dump-path.cc
@ -20,7 +20,7 @@ struct CmdDumpPath : StorePathCommand
        };
    }
-    void run(ref<Store> store, const Path & storePath) override
+    void run(ref<Store> store, const StorePath & storePath) override
    {
        FdSink sink(STDOUT_FILENO);
        store->narFromPath(storePath, sink);
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@ -89,15 +89,25 @@ Buildable Installable::toBuildable()
 struct InstallableStorePath : Installable
 {
-    Path storePath;
+    ref<Store> store;
    StorePath storePath;
-    InstallableStorePath(const Path & storePath) : storePath(storePath) { }
+    InstallableStorePath(ref<Store> store, const Path & storePath)
        : store(store), storePath(store->parseStorePath(storePath)) { }
-    std::string what() override { return storePath; }
+    std::string what() override { return store->printStorePath(storePath); }
    Buildables toBuildables() override
    {
-        return {{isDerivation(storePath) ? storePath : "", {{"out", storePath}}}};
+        std::map<std::string, StorePath> outputs;
        outputs.insert_or_assign("out", storePath.clone());
        Buildable b{
            .drvPath = storePath.isDerivation() ? storePath.clone() : std::optional<StorePath>(),
            .outputs = std::move(outputs)
        };
        Buildables bs;
        bs.push_back(std::move(b));
        return bs;
    }
 };
@ -120,17 +130,17 @@ struct InstallableValue : Installable
        Buildables res;
-        PathSet drvPaths;
+        StorePathSet drvPaths;
        for (auto & drv : drvs) {
-            Buildable b{drv.queryDrvPath()};
+            Buildable b{.drvPath = state->store->parseStorePath(drv.queryDrvPath())};
-            drvPaths.insert(b.drvPath);
+            drvPaths.insert(b.drvPath->clone());
            auto outputName = drv.queryOutputName();
            if (outputName == "")
-                throw Error("derivation '%s' lacks an 'outputName' attribute", b.drvPath);
+                throw Error("derivation '%s' lacks an 'outputName' attribute", state->store->printStorePath(*b.drvPath));
-            b.outputs.emplace(outputName, drv.queryOutPath());
+            b.outputs.emplace(outputName, state->store->parseStorePath(drv.queryOutPath()));
            res.push_back(std::move(b));
        }
@ -138,10 +148,13 @@ struct InstallableValue : Installable
        // Hack to recognize .all: if all drvs have the same drvPath,
        // merge the buildables.
        if (drvPaths.size() == 1) {
-            Buildable b{*drvPaths.begin()};
+            Buildable b{.drvPath = drvPaths.begin()->clone()};
            for (auto & b2 : res)
-                b.outputs.insert(b2.outputs.begin(), b2.outputs.end());
+                for (auto & output : b2.outputs)
-            return {b};
+                    b.outputs.insert_or_assign(output.first, output.second.clone());
            Buildables bs;
            bs.push_back(std::move(b));
            return bs;
        } else
            return res;
    }
@ -212,7 +225,7 @@ static std::vector<std::shared_ptr<Installable>> parseInstallables(
            auto path = store->toStorePath(store->followLinksToStore(s));
            if (store->isStorePath(path))
-                result.push_back(std::make_shared<InstallableStorePath>(path));
+                result.push_back(std::make_shared<InstallableStorePath>(store, path));
        }
        else if (s == "" || std::regex_match(s, attrPathRegex))
@ -242,19 +255,18 @@ Buildables build(ref<Store> store, RealiseMode mode,
    Buildables buildables;
-    PathSet pathsToBuild;
+    std::vector<StorePathWithOutputs> pathsToBuild;
    for (auto & i : installables) {
        for (auto & b : i->toBuildables()) {
-            if (b.drvPath != "") {
+            if (b.drvPath) {
                StringSet outputNames;
                for (auto & output : b.outputs)
                    outputNames.insert(output.first);
-                pathsToBuild.insert(
+                pathsToBuild.push_back({*b.drvPath, outputNames});
                    b.drvPath + "!" + concatStringsSep(",", outputNames));
            } else
                for (auto & output : b.outputs)
-                    pathsToBuild.insert(output.second);
+                    pathsToBuild.push_back({output.second.clone()});
            buildables.push_back(std::move(b));
        }
    }
@ -267,19 +279,19 @@ Buildables build(ref<Store> store, RealiseMode mode,
    return buildables;
 }
-PathSet toStorePaths(ref<Store> store, RealiseMode mode,
+StorePathSet toStorePaths(ref<Store> store, RealiseMode mode,
    std::vector<std::shared_ptr<Installable>> installables)
 {
-    PathSet outPaths;
+    StorePathSet outPaths;
    for (auto & b : build(store, mode, installables))
        for (auto & output : b.outputs)
-            outPaths.insert(output.second);
+            outPaths.insert(output.second.clone());
    return outPaths;
 }
-Path toStorePath(ref<Store> store, RealiseMode mode,
+StorePath toStorePath(ref<Store> store, RealiseMode mode,
    std::shared_ptr<Installable> installable)
 {
    auto paths = toStorePaths(store, mode, {installable});
@ -287,17 +299,17 @@ Path toStorePath(ref<Store> store, RealiseMode mode,
    if (paths.size() != 1)
            throw Error("argument '%s' should evaluate to one store path", installable->what());
-    return *paths.begin();
+    return paths.begin()->clone();
 }
-PathSet toDerivations(ref<Store> store,
+StorePathSet toDerivations(ref<Store> store,
    std::vector<std::shared_ptr<Installable>> installables, bool useDeriver)
 {
-    PathSet drvPaths;
+    StorePathSet drvPaths;
    for (auto & i : installables)
        for (auto & b : i->toBuildables()) {
-            if (b.drvPath.empty()) {
+            if (!b.drvPath) {
                if (!useDeriver)
                    throw Error("argument '%s' did not evaluate to a derivation", i->what());
                for (auto & output : b.outputs) {
@ -305,10 +317,10 @@ PathSet toDerivations(ref<Store> store,
                    if (derivers.empty())
                        throw Error("'%s' does not have a known deriver", i->what());
                    // FIXME: use all derivers?
-                    drvPaths.insert(*derivers.begin());
+                    drvPaths.insert(derivers.begin()->clone());
                }
            } else
-                drvPaths.insert(b.drvPath);
+                drvPaths.insert(b.drvPath->clone());
        }
    return drvPaths;
--- a/src/nix/local.mk
+++ b/src/nix/local.mk
@ -15,7 +15,7 @@ nix_SOURCES := \
  $(wildcard src/nix-prefetch-url/*.cc) \
  $(wildcard src/nix-store/*.cc) \
-nix_LIBS = libexpr libmain libstore libutil
+nix_LIBS = libexpr libmain libstore libutil libnixrust
 nix_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) -lboost_context -lboost_thread -lboost_system
--- a/src/nix/log.cc
+++ b/src/nix/log.cc
@ -43,7 +43,7 @@ struct CmdLog : InstallableCommand
        RunPager pager;
        for (auto & sub : subs) {
-            auto log = b.drvPath != "" ? sub->getBuildLog(b.drvPath) : nullptr;
+            auto log = b.drvPath ? sub->getBuildLog(*b.drvPath) : nullptr;
            for (auto & output : b.outputs) {
                if (log) break;
                log = sub->getBuildLog(output.second);
--- a/src/nix/ls.cc
+++ b/src/nix/ls.cc
@ -67,7 +67,7 @@ struct MixLs : virtual Args, MixJSON
        if (st.type == FSAccessor::Type::tMissing)
            throw Error(format("path '%1%' does not exist") % path);
        doPath(st, path,
-            st.type == FSAccessor::Type::tDirectory ? "." : baseNameOf(path),
+            st.type == FSAccessor::Type::tDirectory ? "." : std::string(baseNameOf(path)),
            showDirectory);
    }
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@ -135,7 +135,7 @@ void mainWrapped(int argc, char * * argv)
    initGC();
    programPath = argv[0];
-    string programName = baseNameOf(programPath);
+    auto programName = std::string(baseNameOf(programPath));
    {
        auto legacy = (*RegisterLegacyCommand::commands)[programName];
--- a/src/nix/make-content-addressable.cc
+++ b/src/nix/make-content-addressable.cc
@ -29,34 +29,36 @@ struct CmdMakeContentAddressable : StorePathsCommand
            },
        };
    }
-    void run(ref<Store> store, Paths storePaths) override
+    void run(ref<Store> store, StorePaths storePaths) override
    {
-        auto paths = store->topoSortPaths(PathSet(storePaths.begin(), storePaths.end()));
+        auto paths = store->topoSortPaths(storePathsToSet(storePaths));
-        paths.reverse();
+        std::reverse(paths.begin(), paths.end());
-        std::map<Path, Path> remappings;
+        std::map<StorePath, StorePath> remappings;
        for (auto & path : paths) {
            auto pathS = store->printStorePath(path);
            auto oldInfo = store->queryPathInfo(path);
-            auto oldHashPart = storePathToHash(path);
+            auto oldHashPart = storePathToHash(pathS);
            auto name = storePathToName(path);
            StringSink sink;
            store->narFromPath(path, sink);
            StringMap rewrites;
-            ValidPathInfo info;
+            StorePathSet references;
            bool hasSelfReference = false;
            for (auto & ref : oldInfo->references) {
                if (ref == path)
-                    info.references.insert("self");
+                    hasSelfReference = true;
                else {
-                    auto replacement = get(remappings, ref, ref);
+                    auto i = remappings.find(ref);
                    auto replacement = i != remappings.end() ? i->second.clone() : ref.clone();
                    // FIXME: warn about unremapped paths?
                    info.references.insert(replacement);
                    if (replacement != ref)
-                        rewrites[storePathToHash(ref)] = storePathToHash(replacement);
+                        rewrites.insert_or_assign(store->printStorePath(ref), store->printStorePath(replacement));
                    references.insert(std::move(replacement));
                }
            }
@ -65,24 +67,26 @@ struct CmdMakeContentAddressable : StorePathsCommand
            HashModuloSink hashModuloSink(htSHA256, oldHashPart);
            hashModuloSink((unsigned char *) sink.s->data(), sink.s->size());
-            info.narHash = hashModuloSink.finish().first;
+            auto narHash = hashModuloSink.finish().first;
            ValidPathInfo info(store->makeFixedOutputPath(true, narHash, path.name(), references, hasSelfReference));
            info.references = std::move(references);
            if (hasSelfReference) info.references.insert(info.path.clone());
            info.narHash = narHash;
            info.narSize = sink.s->size();
            replaceInSet(info.references, path, std::string("self"));
            info.path = store->makeFixedOutputPath(true, info.narHash, name, info.references);
            replaceInSet(info.references, std::string("self"), info.path);
            info.ca = makeFixedOutputCA(true, info.narHash);
-            printError("rewrote '%s' to '%s'", path, info.path);
+            printError("rewrote '%s' to '%s'", pathS, store->printStorePath(info.path));
            auto source = sinkToSource([&](Sink & nextSink) {
-                RewritingSink rsink2(oldHashPart, storePathToHash(info.path), nextSink);
+                RewritingSink rsink2(oldHashPart, storePathToHash(store->printStorePath(info.path)), nextSink);
                rsink2((unsigned char *) sink.s->data(), sink.s->size());
                rsink2.flush();
            });
            store->addToStore(info, *source);
-            remappings[path] = info.path;
+            remappings.insert_or_assign(std::move(path), std::move(info.path));
        }
    }
 };
--- a/src/nix/path-info.cc
+++ b/src/nix/path-info.cc
@ -78,36 +78,36 @@ struct CmdPathInfo : StorePathsCommand, MixJSON
        std::cout << fmt("\t%6.1f%c", res, idents.at(power));
    }
-    void run(ref<Store> store, Paths storePaths) override
+    void run(ref<Store> store, StorePaths storePaths) override
    {
        size_t pathLen = 0;
        for (auto & storePath : storePaths)
-            pathLen = std::max(pathLen, storePath.size());
+            pathLen = std::max(pathLen, store->printStorePath(storePath).size());
        if (json) {
            JSONPlaceholder jsonRoot(std::cout);
            store->pathInfoToJSON(jsonRoot,
                // FIXME: preserve order?
-                PathSet(storePaths.begin(), storePaths.end()),
+                storePathsToSet(storePaths),
                true, showClosureSize, AllowInvalid);
        }
        else {
-            for (auto storePath : storePaths) {
+            for (auto & storePath : storePaths) {
                auto info = store->queryPathInfo(storePath);
-                storePath = info->path; // FIXME: screws up padding
+                auto storePathS = store->printStorePath(storePath);
-                std::cout << storePath;
+                std::cout << storePathS;
                if (showSize || showClosureSize || showSigs)
-                    std::cout << std::string(std::max(0, (int) pathLen - (int) storePath.size()), ' ');
+                    std::cout << std::string(std::max(0, (int) pathLen - (int) storePathS.size()), ' ');
                if (showSize)
                    printSize(info->narSize);
                if (showClosureSize)
-                    printSize(store->getClosureSize(storePath).first);
+                    printSize(store->getClosureSize(info->path).first);
                if (showSigs) {
                    std::cout << '\t';
--- a/src/nix/progress-bar.cc
+++ b/src/nix/progress-bar.cc
@ -24,6 +24,13 @@ static uint64_t getI(const std::vector<Logger::Field> & fields, size_t n)
    return fields[n].i;
 }
 static std::string_view storePathToName(std::string_view path)
 {
    auto base = baseNameOf(path);
    auto i = base.find('-');
    return i == std::string::npos ? base.substr(0, 0) : base.substr(i + 1);
 }
 class ProgressBar : public Logger
 {
 private:
@ -148,7 +155,7 @@ public:
        if (type == actBuild) {
            auto name = storePathToName(getS(fields, 0));
            if (hasSuffix(name, ".drv"))
-                name.resize(name.size() - 4);
+                name = name.substr(name.size() - 4);
            i->s = fmt("building " ANSI_BOLD "%s" ANSI_NORMAL, name);
            auto machineName = getS(fields, 1);
            if (machineName != "")
@ -173,7 +180,7 @@ public:
        if (type == actPostBuildHook) {
            auto name = storePathToName(getS(fields, 0));
            if (hasSuffix(name, ".drv"))
-                name.resize(name.size() - 4);
+                name = name.substr(name.size() - 4);
            i->s = fmt("post-build " ANSI_BOLD "%s" ANSI_NORMAL, name);
            i->name = DrvName(name).name;
        }
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@ -413,7 +413,7 @@ Path NixRepl::getDerivationPath(Value & v) {
    if (!drvInfo)
        throw Error("expression does not evaluate to a derivation, so I can't build it");
    Path drvPath = drvInfo->queryDrvPath();
-    if (drvPath == "" || !state.store->isValidPath(drvPath))
+    if (drvPath == "" || !state.store->isValidPath(state.store->parseStorePath(drvPath)))
        throw Error("expression did not evaluate to a valid derivation");
    return drvPath;
 }
@ -521,10 +521,10 @@ bool NixRepl::processLine(string line)
               but doing it in a child makes it easier to recover from
               problems / SIGINT. */
            if (runProgram(settings.nixBinDir + "/nix", Strings{"build", "--no-link", drvPath}) == 0) {
-                Derivation drv = readDerivation(drvPath);
+                auto drv = readDerivation(*state.store, drvPath);
                std::cout << std::endl << "this derivation produced the following outputs:" << std::endl;
                for (auto & i : drv.outputs)
-                    std::cout << format("  %1% -> %2%") % i.first % i.second.path << std::endl;
+                    std::cout << fmt("  %s -> %s\n", i.first, state.store->printStorePath(i.second.path));
            }
        } else if (command == ":i") {
            runProgram(settings.nixBinDir + "/nix-env", Strings{"-i", drvPath});
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@ -119,24 +119,24 @@ struct CmdRun : InstallablesCommand
                unsetenv(var.c_str());
        }
-        std::unordered_set<Path> done;
+        std::unordered_set<StorePath> done;
-        std::queue<Path> todo;
+        std::queue<StorePath> todo;
-        for (auto & path : outPaths) todo.push(path);
+        for (auto & path : outPaths) todo.push(path.clone());
        auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");
        while (!todo.empty()) {
-            Path path = todo.front();
+            auto path = todo.front().clone();
            todo.pop();
-            if (!done.insert(path).second) continue;
+            if (!done.insert(path.clone()).second) continue;
            if (true)
-                unixPath.push_front(path + "/bin");
+                unixPath.push_front(store->printStorePath(path) + "/bin");
-            auto propPath = path + "/nix-support/propagated-user-env-packages";
+            auto propPath = store->printStorePath(path) + "/nix-support/propagated-user-env-packages";
            if (accessor->stat(propPath).type == FSAccessor::tRegular) {
                for (auto & p : tokenizeString<Paths>(readFile(propPath)))
-                    todo.push(p);
+                    todo.push(store->parseStorePath(p));
            }
        }
--- a/src/nix/show-derivation.cc
+++ b/src/nix/show-derivation.cc
@ -46,9 +46,9 @@ struct CmdShowDerivation : InstallablesCommand
        auto drvPaths = toDerivations(store, installables, true);
        if (recursive) {
-            PathSet closure;
+            StorePathSet closure;
            store->computeFSClosure(drvPaths, closure);
-            drvPaths = closure;
+            drvPaths = std::move(closure);
        }
        {
@ -56,17 +56,19 @@ struct CmdShowDerivation : InstallablesCommand
        JSONObject jsonRoot(std::cout, true);
        for (auto & drvPath : drvPaths) {
-            if (!isDerivation(drvPath)) continue;
+            if (!drvPath.isDerivation()) continue;
-            auto drvObj(jsonRoot.object(drvPath));
+            auto drvPathS = store->printStorePath(drvPath);
-            auto drv = readDerivation(drvPath);
+            auto drvObj(jsonRoot.object(drvPathS));
            auto drv = readDerivation(*store, drvPathS);
            {
                auto outputsObj(drvObj.object("outputs"));
                for (auto & output : drv.outputs) {
                    auto outputObj(outputsObj.object(output.first));
-                    outputObj.attr("path", output.second.path);
+                    outputObj.attr("path", store->printStorePath(output.second.path));
                    if (output.second.hash != "") {
                        outputObj.attr("hashAlgo", output.second.hashAlgo);
                        outputObj.attr("hash", output.second.hash);
@ -77,13 +79,13 @@ struct CmdShowDerivation : InstallablesCommand
            {
                auto inputsList(drvObj.list("inputSrcs"));
                for (auto & input : drv.inputSrcs)
-                    inputsList.elem(input);
+                    inputsList.elem(store->printStorePath(input));
            }
            {
                auto inputDrvsObj(drvObj.object("inputDrvs"));
                for (auto & input : drv.inputDrvs) {
-                    auto inputList(inputDrvsObj.list(input.first));
+                    auto inputList(inputDrvsObj.list(store->printStorePath(input.first)));
                    for (auto & outputId : input.second)
                        inputList.elem(outputId);
                }
--- a/src/nix/sigs.cc
+++ b/src/nix/sigs.cc
@ -27,7 +27,7 @@ struct CmdCopySigs : StorePathsCommand
        return "copy path signatures from substituters (like binary caches)";
    }
-    void run(ref<Store> store, Paths storePaths) override
+    void run(ref<Store> store, StorePaths storePaths) override
    {
        if (substituterUris.empty())
            throw UsageError("you must specify at least one substituter using '-s'");
@ -44,18 +44,20 @@ struct CmdCopySigs : StorePathsCommand
        //logger->setExpected(doneLabel, storePaths.size());
-        auto doPath = [&](const Path & storePath) {
+        auto doPath = [&](const Path & storePathS) {
            //Activity act(*logger, lvlInfo, format("getting signatures for '%s'") % storePath);
            checkInterrupt();
            auto storePath = store->parseStorePath(storePathS);
            auto info = store->queryPathInfo(storePath);
            StringSet newSigs;
            for (auto & store2 : substituters) {
                try {
-                    auto info2 = store2->queryPathInfo(storePath);
+                    auto info2 = store2->queryPathInfo(info->path);
                    /* Don't import signatures that don't match this
                       binary. */
@ -80,11 +82,11 @@ struct CmdCopySigs : StorePathsCommand
        };
        for (auto & storePath : storePaths)
-            pool.enqueue(std::bind(doPath, storePath));
+            pool.enqueue(std::bind(doPath, store->printStorePath(storePath)));
        pool.process();
-        printInfo(format("imported %d signatures") % added);
+        printInfo("imported %d signatures", added);
    }
 };
@ -109,7 +111,7 @@ struct CmdSignPaths : StorePathsCommand
        return "sign the specified paths";
    }
-    void run(ref<Store> store, Paths storePaths) override
+    void run(ref<Store> store, StorePaths storePaths) override
    {
        if (secretKeyFile.empty())
            throw UsageError("you must specify a secret key file using '-k'");
@ -123,7 +125,7 @@ struct CmdSignPaths : StorePathsCommand
            auto info2(*info);
            info2.sigs.clear();
-            info2.sign(secretKey);
+            info2.sign(*store, secretKey);
            assert(!info2.sigs.empty());
            if (!info->sigs.count(*info2.sigs.begin())) {
@ -132,7 +134,7 @@ struct CmdSignPaths : StorePathsCommand
            }
        }
-        printInfo(format("added %d signatures") % added);
+        printInfo("added %d signatures", added);
    }
 };
--- a/src/nix/upgrade-nix.cc
+++ b/src/nix/upgrade-nix.cc
@ -58,13 +58,9 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
        printInfo("upgrading Nix in profile '%s'", profileDir);
-        Path storePath;
+        auto storePath = getLatestNix(store);
        {
            Activity act(*logger, lvlInfo, actUnknown, "querying latest Nix version");
            storePath = getLatestNix(store);
        }
-        auto version = DrvName(storePathToName(storePath)).version;
+        auto version = DrvName(storePath.name()).version;
        if (dryRun) {
            stopProgressBar();
@ -73,13 +69,13 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
        }
        {
-            Activity act(*logger, lvlInfo, actUnknown, fmt("downloading '%s'...", storePath));
+            Activity act(*logger, lvlInfo, actUnknown, fmt("downloading '%s'...", store->printStorePath(storePath)));
            store->ensurePath(storePath);
        }
        {
-            Activity act(*logger, lvlInfo, actUnknown, fmt("verifying that '%s' works...", storePath));
+            Activity act(*logger, lvlInfo, actUnknown, fmt("verifying that '%s' works...", store->printStorePath(storePath)));
-            auto program = storePath + "/bin/nix-env";
+            auto program = store->printStorePath(storePath) + "/bin/nix-env";
            auto s = runProgram(program, false, {"--version"});
            if (s.find("Nix") == std::string::npos)
                throw Error("could not verify that '%s' works", program);
@ -88,9 +84,10 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
        stopProgressBar();
        {
-            Activity act(*logger, lvlInfo, actUnknown, fmt("installing '%s' into profile '%s'...", storePath, profileDir));
+            Activity act(*logger, lvlInfo, actUnknown,
                fmt("installing '%s' into profile '%s'...", store->printStorePath(storePath), profileDir));
            runProgram(settings.nixBinDir + "/nix-env", false,
-                {"--profile", profileDir, "-i", storePath, "--no-sandbox"});
+                {"--profile", profileDir, "-i", store->printStorePath(storePath), "--no-sandbox"});
        }
        printError(ANSI_GREEN "upgrade to version %s done" ANSI_NORMAL, version);
@ -129,15 +126,17 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
            !hasSuffix(userEnv, "user-environment"))
            throw Error("directory '%s' does not appear to be part of a Nix profile", where);
-        if (!store->isValidPath(userEnv))
+        if (!store->isValidPath(store->parseStorePath(userEnv)))
            throw Error("directory '%s' is not in the Nix store", userEnv);
        return profileDir;
    }
    /* Return the store path of the latest stable Nix. */
-    Path getLatestNix(ref<Store> store)
+    StorePath getLatestNix(ref<Store> store)
    {
        Activity act(*logger, lvlInfo, actUnknown, "querying latest Nix version");
        // FIXME: use nixos.org?
        auto req = DownloadRequest(storePathsUrl);
        auto res = getDownloader()->download(req);
@ -148,7 +147,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
        Bindings & bindings(*state->allocBindings(0));
        auto v2 = findAlongAttrPath(*state, settings.thisSystem, bindings, *v);
-        return state->forceString(*v2);
+        return store->parseStorePath(state->forceString(*v2));
    }
 };
--- a/src/nix/verify.cc
+++ b/src/nix/verify.cc
@ -49,7 +49,7 @@ struct CmdVerify : StorePathsCommand
        };
    }
-    void run(ref<Store> store, Paths storePaths) override
+    void run(ref<Store> store, StorePaths storePaths) override
    {
        std::vector<ref<Store>> substituters;
        for (auto & s : substituterUris)
@ -80,7 +80,7 @@ struct CmdVerify : StorePathsCommand
                MaintainCount<std::atomic<size_t>> mcActive(active);
                update();
-                auto info = store->queryPathInfo(storePath);
+                auto info = store->queryPathInfo(store->parseStorePath(storePath));
                if (!noContents) {
@ -88,7 +88,7 @@ struct CmdVerify : StorePathsCommand
                    if (info->ca == "")
                        hashSink = std::make_unique<HashSink>(info->narHash.type);
                    else
-                        hashSink = std::make_unique<HashModuloSink>(info->narHash.type, storePathToHash(info->path));
+                        hashSink = std::make_unique<HashModuloSink>(info->narHash.type, storePathToHash(store->printStorePath(info->path)));
                    store->narFromPath(info->path, *hashSink);
@ -96,10 +96,10 @@ struct CmdVerify : StorePathsCommand
                    if (hash.first != info->narHash) {
                        corrupted++;
-                        act2.result(resCorruptedPath, info->path);
+                        act2.result(resCorruptedPath, store->printStorePath(info->path));
                        printError(
-                            format("path '%s' was modified! expected hash '%s', got '%s'")
+                            "path '%s' was modified! expected hash '%s', got '%s'",
-                            % info->path % info->narHash.to_string() % hash.first.to_string());
+                            store->printStorePath(info->path), info->narHash.to_string(), hash.first.to_string());
                    }
                }
@ -120,7 +120,7 @@ struct CmdVerify : StorePathsCommand
                        auto doSigs = [&](StringSet sigs) {
                            for (auto sig : sigs) {
                                if (!sigsSeen.insert(sig).second) continue;
-                                if (validSigs < ValidPathInfo::maxSigs && info->checkSignature(publicKeys, sig))
+                                if (validSigs < ValidPathInfo::maxSigs && info->checkSignature(*store, publicKeys, sig))
                                    validSigs++;
                            }
                        };
@ -147,8 +147,8 @@ struct CmdVerify : StorePathsCommand
                    if (!good) {
                        untrusted++;
-                        act2.result(resUntrustedPath, info->path);
+                        act2.result(resUntrustedPath, store->printStorePath(info->path));
-                        printError(format("path '%s' is untrusted") % info->path);
+                        printError("path '%s' is untrusted", store->printStorePath(info->path));
                    }
                }
@ -164,7 +164,7 @@ struct CmdVerify : StorePathsCommand
        };
        for (auto & storePath : storePaths)
-            pool.enqueue(std::bind(doPath, storePath));
+            pool.enqueue(std::bind(doPath, store->printStorePath(storePath)));
        pool.process();
--- a/src/nix/why-depends.cc
+++ b/src/nix/why-depends.cc
@ -73,9 +73,9 @@ struct CmdWhyDepends : SourceExprCommand
        auto packagePath = toStorePath(store, Build, package);
        auto dependency = parseInstallable(*this, store, _dependency, false);
        auto dependencyPath = toStorePath(store, NoBuild, dependency);
-        auto dependencyPathHash = storePathToHash(dependencyPath);
+        auto dependencyPathHash = storePathToHash(store->printStorePath(dependencyPath));
-        PathSet closure;
+        StorePathSet closure;
        store->computeFSClosure({packagePath}, closure, false, false);
        if (!closure.count(dependencyPath)) {
@ -91,28 +91,28 @@ struct CmdWhyDepends : SourceExprCommand
        struct Node
        {
-            Path path;
+            StorePath path;
-            PathSet refs;
+            StorePathSet refs;
-            PathSet rrefs;
+            StorePathSet rrefs;
            size_t dist = inf;
            Node * prev = nullptr;
            bool queued = false;
            bool visited = false;
        };
-        std::map<Path, Node> graph;
+        std::map<StorePath, Node> graph;
        for (auto & path : closure)
-            graph.emplace(path, Node{path, store->queryPathInfo(path)->references});
+            graph.emplace(path.clone(), Node{path.clone(), cloneStorePathSet(store->queryPathInfo(path)->references)});
        // Transpose the graph.
        for (auto & node : graph)
            for (auto & ref : node.second.refs)
-                graph[ref].rrefs.insert(node.first);
+                graph.find(ref)->second.rrefs.insert(node.first.clone());
        /* Run Dijkstra's shortest path algorithm to get the distance
           of every path in the closure to 'dependency'. */
-        graph[dependencyPath].dist = 0;
+        graph[dependencyPath.clone()].dist = 0;
        std::priority_queue<Node *> queue;
@ -151,12 +151,14 @@ struct CmdWhyDepends : SourceExprCommand
        struct BailOut { };
        printNode = [&](Node & node, const string & firstPad, const string & tailPad) {
            auto pathS = store->printStorePath(node.path);
            assert(node.dist != inf);
            std::cout << fmt("%s%s%s%s" ANSI_NORMAL "\n",
                firstPad,
                node.visited ? "\e[38;5;244m" : "",
                firstPad != "" ? "=> " : "",
-                node.path);
+                pathS);
            if (node.path == dependencyPath && !all
                && packagePath != dependencyPath)
@ -175,7 +177,7 @@ struct CmdWhyDepends : SourceExprCommand
                auto & node2 = graph.at(ref);
                if (node2.dist == inf) continue;
                refs.emplace(node2.dist, &node2);
-                hashes.insert(storePathToHash(node2.path));
+                hashes.insert(storePathToHash(store->printStorePath(node2.path)));
            }
            /* For each reference, find the files and symlinks that
@ -187,7 +189,7 @@ struct CmdWhyDepends : SourceExprCommand
            visitPath = [&](const Path & p) {
                auto st = accessor->stat(p);
-                auto p2 = p == node.path ? "/" : std::string(p, node.path.size() + 1);
+                auto p2 = p == pathS ? "/" : std::string(p, pathS.size() + 1);
                auto getColour = [&](const std::string & hash) {
                    return hash == dependencyPathHash ? ANSI_GREEN : ANSI_BLUE;
@ -231,11 +233,11 @@ struct CmdWhyDepends : SourceExprCommand
            // FIXME: should use scanForReferences().
-            visitPath(node.path);
+            visitPath(pathS);
            RunPager pager;
            for (auto & ref : refs) {
-                auto hash = storePathToHash(ref.second->path);
+                auto hash = storePathToHash(store->printStorePath(ref.second->path));
                bool last = all ? ref == *refs.rbegin() : true;
--- a/src/resolve-system-dependencies/local.mk
+++ b/src/resolve-system-dependencies/local.mk
@ -6,6 +6,6 @@ resolve-system-dependencies_DIR := $(d)
 resolve-system-dependencies_INSTALL_DIR := $(libexecdir)/nix
-resolve-system-dependencies_LIBS := libstore libmain libutil
+resolve-system-dependencies_LIBS := libstore libmain libutil libnixrust
 resolve-system-dependencies_SOURCES := $(d)/resolve-system-dependencies.cc
--- a/src/resolve-system-dependencies/resolve-system-dependencies.cc
+++ b/src/resolve-system-dependencies/resolve-system-dependencies.cc
@ -179,8 +179,8 @@ int main(int argc, char ** argv)
        if (std::string(argv[1]) == "--test")
            impurePaths.insert(argv[2]);
        else {
-            auto drv = store->derivationFromPath(Path(argv[1]));
+            auto drv = store->derivationFromPath(store->parseStorePath(argv[1]));
-            impurePaths = tokenizeString<StringSet>(get(drv.env, "__impureHostDeps"));
+            impurePaths = tokenizeString<StringSet>(get(drv.env, "__impureHostDeps").value_or(""));
            impurePaths.insert("/usr/lib/libSystem.dylib");
        }