diff --git a/.version b/.version
index 7208c2182..f398a2061 100644
--- a/.version
+++ b/.version
@@ -1 +1 @@
-2.4
\ No newline at end of file
+3.0
\ No newline at end of file
diff --git a/mk/precompiled-headers.mk b/mk/precompiled-headers.mk
index 1c0452dc2..500c99e4a 100644
--- a/mk/precompiled-headers.mk
+++ b/mk/precompiled-headers.mk
@@ -21,13 +21,13 @@ clean-files += $(GCH) $(PCH)
 
 ifeq ($(PRECOMPILE_HEADERS), 1)
 
-  ifeq ($(CXX), g++)
+  ifeq ($(findstring g++,$(CXX)), g++)
 
     GLOBAL_CXXFLAGS_PCH += -include $(buildprefix)precompiled-headers.h -Winvalid-pch
 
     GLOBAL_ORDER_AFTER += $(GCH)
 
-  else ifeq ($(CXX), clang++)
+  else ifeq ($(findstring clang++,$(CXX)), clang++)
 
     GLOBAL_CXXFLAGS_PCH += -include-pch $(PCH) -Winvalid-pch
 
diff --git a/perl/lib/Nix/Store.xs b/perl/lib/Nix/Store.xs
index f14c3f73f..c412e4640 100644
--- a/perl/lib/Nix/Store.xs
+++ b/perl/lib/Nix/Store.xs
@@ -80,7 +80,7 @@ SV * queryReferences(char * path)
 SV * queryPathHash(char * path)
     PPCODE:
         try {
-            auto s = store()->queryPathInfo(store()->parseStorePath(path))->narHash.to_string(Base32, true);
+            auto s = store()->queryPathInfo(store()->parseStorePath(path))->narHash->to_string(Base32, true);
             XPUSHs(sv_2mortal(newSVpv(s.c_str(), 0)));
         } catch (Error & e) {
             croak("%s", e.what());
@@ -106,7 +106,7 @@ SV * queryPathInfo(char * path, int base32)
                 XPUSHs(&PL_sv_undef);
             else
                 XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(*info->deriver).c_str(), 0)));
-            auto s = info->narHash.to_string(base32 ? Base32 : Base16, true);
+            auto s = info->narHash->to_string(base32 ? Base32 : Base16, true);
             XPUSHs(sv_2mortal(newSVpv(s.c_str(), 0)));
             mXPUSHi(info->registrationTime);
             mXPUSHi(info->narSize);
@@ -304,7 +304,10 @@ SV * derivationFromPath(char * drvPath)
 
             HV * outputs = newHV();
             for (auto & i : drv.outputs)
-                hv_store(outputs, i.first.c_str(), i.first.size(), newSVpv(store()->printStorePath(i.second.path).c_str(), 0), 0);
+                hv_store(
+                    outputs, i.first.c_str(), i.first.size(),
+                    newSVpv(store()->printStorePath(i.second.path(*store(), drv.name)).c_str(), 0),
+                    0);
             hv_stores(hash, "outputs", newRV((SV *) outputs));
 
             AV * inputDrvs = newAV();
diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 919de8a4e..deb32484f 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -285,11 +285,10 @@ static std::shared_ptr<AttrDb> makeAttrDb(const Hash & fingerprint)
 }
 
 EvalCache::EvalCache(
-    bool useCache,
-    const Hash & fingerprint,
+    std::optional<std::reference_wrapper<const Hash>> useCache,
     EvalState & state,
     RootLoader rootLoader)
-    : db(useCache ? makeAttrDb(fingerprint) : nullptr)
+    : db(useCache ? makeAttrDb(*useCache) : nullptr)
     , state(state)
     , rootLoader(rootLoader)
 {
diff --git a/src/libexpr/eval-cache.hh b/src/libexpr/eval-cache.hh
index 674bb03c1..afee85fa9 100644
--- a/src/libexpr/eval-cache.hh
+++ b/src/libexpr/eval-cache.hh
@@ -4,6 +4,7 @@
 #include "hash.hh"
 #include "eval.hh"
 
+#include <functional>
 #include <variant>
 
 namespace nix::eval_cache {
@@ -26,8 +27,7 @@ class EvalCache : public std::enable_shared_from_this<EvalCache>
 public:
 
     EvalCache(
-        bool useCache,
-        const Hash & fingerprint,
+        std::optional<std::reference_wrapper<const Hash>> useCache,
         EvalState & state,
         RootLoader rootLoader);
 
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index 77f3abdeb..c2bb2888b 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -106,6 +106,6 @@ void emitTreeAttrs(
     EvalState & state,
     const fetchers::Tree & tree,
     const fetchers::Input & input,
-    Value & v);
+    Value & v, bool emptyRevFallback = false);
 
 }
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 9055f59a1..5d6e39aa0 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -39,7 +39,7 @@ DrvInfo::DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPat
     if (i == drv.outputs.end())
         throw Error("derivation '%s' does not have output '%s'", store->printStorePath(drvPath), outputName);
 
-    outPath = store->printStorePath(i->second.path);
+    outPath = store->printStorePath(i->second.path(*store, drv.name));
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 9f877f765..5059286b3 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -52,7 +52,7 @@ void EvalState::realiseContext(const PathSet & context)
                 DerivationOutputs::iterator i = drv.outputs.find(outputName);
                 if (i == drv.outputs.end())
                     throw Error("derivation '%s' does not have an output named '%s'", ctxS, outputName);
-                allowedPaths->insert(store->printStorePath(i->second.path));
+                allowedPaths->insert(store->printStorePath(i->second.path(*store, drv.name)));
             }
         }
     }
@@ -91,8 +91,17 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     Path realPath = state.checkSourcePath(state.toRealPath(path, context));
 
     // FIXME
-    if (state.store->isStorePath(path) && state.store->isValidPath(state.store->parseStorePath(path)) && isDerivation(path)) {
-        Derivation drv = readDerivation(*state.store, realPath);
+    auto isValidDerivationInStore = [&]() -> std::optional<StorePath> {
+        if (!state.store->isStorePath(path))
+            return std::nullopt;
+        auto storePath = state.store->parseStorePath(path);
+        if (!(state.store->isValidPath(storePath) && isDerivation(path)))
+            return std::nullopt;
+        return storePath;
+    };
+    if (auto optStorePath = isValidDerivationInStore()) {
+        auto storePath = *optStorePath;
+        Derivation drv = readDerivation(*state.store, realPath, Derivation::nameFromPath(storePath));
         Value & w = *state.allocValue();
         state.mkAttrs(w, 3 + drv.outputs.size());
         Value * v2 = state.allocAttr(w, state.sDrvPath);
@@ -106,7 +115,7 @@ 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, state.store->printStorePath(o.second.path), {"!" + o.first + "!" + path});
+            mkString(*v2, state.store->printStorePath(o.second.path(*state.store, drv.name)), {"!" + o.first + "!" + path});
             outputsVal->listElems()[outputs_index] = state.allocValue();
             mkString(*(outputsVal->listElems()[outputs_index++]), o.first);
         }
@@ -570,6 +579,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
     /* Build the derivation expression by processing the attributes. */
     Derivation drv;
+    drv.name = drvName;
 
     PathSet context;
 
@@ -764,11 +774,12 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         auto outPath = state.store->makeFixedOutputPath(ingestionMethod, h, drvName);
         if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);
         drv.outputs.insert_or_assign("out", DerivationOutput {
-            .path = std::move(outPath),
-            .hash = FixedOutputHash {
-                .method = ingestionMethod,
-                .hash = std::move(h),
-            },
+                .output = DerivationOutputFixed {
+                    .hash = FixedOutputHash {
+                        .method = ingestionMethod,
+                        .hash = std::move(h),
+                    },
+                },
         });
     }
 
@@ -783,8 +794,9 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             if (!jsonObject) drv.env[i] = "";
             drv.outputs.insert_or_assign(i,
                 DerivationOutput {
-                    .path = StorePath::dummy,
-                    .hash = std::optional<FixedOutputHash> {},
+                    .output = DerivationOutputInputAddressed {
+                        .path = StorePath::dummy,
+                    },
                 });
         }
 
@@ -795,8 +807,9 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             if (!jsonObject) drv.env[i] = state.store->printStorePath(outPath);
             drv.outputs.insert_or_assign(i,
                 DerivationOutput {
-                    .path = std::move(outPath),
-                    .hash = std::optional<FixedOutputHash>(),
+                    .output = DerivationOutputInputAddressed {
+                        .path = std::move(outPath),
+                    },
                 });
         }
     }
@@ -817,7 +830,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     mkString(*state.allocAttr(v, state.sDrvPath), drvPathS, {"=" + drvPathS});
     for (auto & i : drv.outputs) {
         mkString(*state.allocAttr(v, state.symbols.create(i.first)),
-            state.store->printStorePath(i.second.path), {"!" + i.first + "!" + drvPathS});
+            state.store->printStorePath(i.second.path(*state.store, drv.name)), {"!" + i.first + "!" + drvPathS});
     }
     v.attrs->sort();
 }
@@ -1111,7 +1124,7 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
 
 
 static void addPath(EvalState & state, const Pos & pos, const string & name, const Path & path_,
-    Value * filterFun, FileIngestionMethod method, const Hash & expectedHash, Value & v)
+    Value * filterFun, FileIngestionMethod method, const std::optional<Hash> expectedHash, Value & v)
 {
     const auto path = evalSettings.pureEval && expectedHash ?
         path_ :
@@ -1142,7 +1155,7 @@ static void addPath(EvalState & state, const Pos & pos, const string & name, con
 
     std::optional<StorePath> expectedStorePath;
     if (expectedHash)
-        expectedStorePath = state.store->makeFixedOutputPath(method, expectedHash, name);
+        expectedStorePath = state.store->makeFixedOutputPath(method, *expectedHash, name);
     Path dstPath;
     if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
         dstPath = state.store->printStorePath(settings.readOnlyMode
@@ -1176,7 +1189,7 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
             .errPos = pos
         });
 
-    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, Hash(), v);
+    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v);
 }
 
 static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value & v)
@@ -1186,7 +1199,7 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
     string name;
     Value * filterFun = nullptr;
     auto method = FileIngestionMethod::Recursive;
-    Hash expectedHash;
+    Hash expectedHash(htSHA256);
 
     for (auto & attr : *args[0]->attrs) {
         const string & n(attr.name);
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
deleted file mode 100644
index 5013e74f0..000000000
--- a/src/libexpr/primops/fetchGit.cc
+++ /dev/null
@@ -1,91 +0,0 @@
-#include "primops.hh"
-#include "eval-inline.hh"
-#include "store-api.hh"
-#include "hash.hh"
-#include "fetchers.hh"
-#include "url.hh"
-
-namespace nix {
-
-static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Value & v)
-{
-    std::string url;
-    std::optional<std::string> ref;
-    std::optional<Hash> rev;
-    std::string name = "source";
-    bool fetchSubmodules = false;
-    PathSet context;
-
-    state.forceValue(*args[0]);
-
-    if (args[0]->type == tAttrs) {
-
-        state.forceAttrs(*args[0], pos);
-
-        for (auto & attr : *args[0]->attrs) {
-            string n(attr.name);
-            if (n == "url")
-                url = state.coerceToString(*attr.pos, *attr.value, context, false, false);
-            else if (n == "ref")
-                ref = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (n == "rev")
-                rev = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA1);
-            else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (n == "submodules")
-                fetchSubmodules = state.forceBool(*attr.value, *attr.pos);
-            else
-                throw EvalError({
-                    .hint = hintfmt("unsupported argument '%s' to 'fetchGit'", attr.name),
-                    .errPos = *attr.pos
-                });
-        }
-
-        if (url.empty())
-            throw EvalError({
-                .hint = hintfmt("'url' argument required"),
-                .errPos = pos
-            });
-
-    } else
-        url = state.coerceToString(pos, *args[0], context, false, false);
-
-    // FIXME: git externals probably can be used to bypass the URI
-    // whitelist. Ah well.
-    state.checkURI(url);
-
-    if (evalSettings.pureEval && !rev)
-        throw Error("in pure evaluation mode, 'fetchGit' requires a Git revision");
-
-    fetchers::Attrs attrs;
-    attrs.insert_or_assign("type", "git");
-    attrs.insert_or_assign("url", url.find("://") != std::string::npos ? url : "file://" + url);
-    if (ref) attrs.insert_or_assign("ref", *ref);
-    if (rev) attrs.insert_or_assign("rev", rev->gitRev());
-    if (fetchSubmodules) attrs.insert_or_assign("submodules", fetchers::Explicit<bool>{true});
-    auto input = fetchers::Input::fromAttrs(std::move(attrs));
-
-    // FIXME: use name?
-    auto [tree, input2] = input.fetch(state.store);
-
-    state.mkAttrs(v, 8);
-    auto storePath = state.store->printStorePath(tree.storePath);
-    mkString(*state.allocAttr(v, state.sOutPath), storePath, PathSet({storePath}));
-    // Backward compatibility: set 'rev' to
-    // 0000000000000000000000000000000000000000 for a dirty tree.
-    auto rev2 = input2.getRev().value_or(Hash(htSHA1));
-    mkString(*state.allocAttr(v, state.symbols.create("rev")), rev2.gitRev());
-    mkString(*state.allocAttr(v, state.symbols.create("shortRev")), rev2.gitShortRev());
-    // Backward compatibility: set 'revCount' to 0 for a dirty tree.
-    mkInt(*state.allocAttr(v, state.symbols.create("revCount")),
-        input2.getRevCount().value_or(0));
-    mkBool(*state.allocAttr(v, state.symbols.create("submodules")), fetchSubmodules);
-    v.attrs->sort();
-
-    if (state.allowedPaths)
-        state.allowedPaths->insert(tree.actualPath);
-}
-
-static RegisterPrimOp r("fetchGit", 1, prim_fetchGit);
-
-}
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 6a796f3d3..cddcf0e59 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -14,7 +14,8 @@ void emitTreeAttrs(
     EvalState & state,
     const fetchers::Tree & tree,
     const fetchers::Input & input,
-    Value & v)
+    Value & v,
+    bool emptyRevFallback)
 {
     assert(input.isImmutable());
 
@@ -34,10 +35,20 @@ void emitTreeAttrs(
     if (auto rev = input.getRev()) {
         mkString(*state.allocAttr(v, state.symbols.create("rev")), rev->gitRev());
         mkString(*state.allocAttr(v, state.symbols.create("shortRev")), rev->gitShortRev());
+    } else if (emptyRevFallback) {
+        // Backwards compat for `builtins.fetchGit`: dirty repos return an empty sha1 as rev
+        auto emptyHash = Hash(htSHA1);
+        mkString(*state.allocAttr(v, state.symbols.create("rev")), emptyHash.gitRev());
+        mkString(*state.allocAttr(v, state.symbols.create("shortRev")), emptyHash.gitRev());
     }
 
+    if (input.getType() == "git")
+        mkBool(*state.allocAttr(v, state.symbols.create("submodules")), maybeGetBoolAttr(input.attrs, "submodules").value_or(false));
+
     if (auto revCount = input.getRevCount())
         mkInt(*state.allocAttr(v, state.symbols.create("revCount")), *revCount);
+    else if (emptyRevFallback)
+        mkInt(*state.allocAttr(v, state.symbols.create("revCount")), 0);
 
     if (auto lastModified = input.getLastModified()) {
         mkInt(*state.allocAttr(v, state.symbols.create("lastModified")), *lastModified);
@@ -48,10 +59,26 @@ void emitTreeAttrs(
     v.attrs->sort();
 }
 
-static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)
+std::string fixURI(std::string uri, EvalState &state)
 {
-    settings.requireExperimentalFeature("flakes");
+    state.checkURI(uri);
+    return uri.find("://") != std::string::npos ? uri : "file://" + uri;
+}
 
+void addURI(EvalState &state, fetchers::Attrs &attrs, Symbol name, std::string v)
+{
+    string n(name);
+    attrs.emplace(name, n == "url" ? fixURI(v, state) : v);
+}
+
+static void fetchTree(
+    EvalState &state,
+    const Pos &pos,
+    Value **args,
+    Value &v,
+    const std::optional<std::string> type,
+    bool emptyRevFallback = false
+) {
     fetchers::Input input;
     PathSet context;
 
@@ -64,8 +91,15 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
 
         for (auto & attr : *args[0]->attrs) {
             state.forceValue(*attr.value);
-            if (attr.value->type == tString)
-                attrs.emplace(attr.name, attr.value->string.s);
+            if (attr.value->type == tPath || attr.value->type == tString)
+                addURI(
+                    state,
+                    attrs,
+                    attr.name,
+                    state.coerceToString(*attr.pos, *attr.value, context, false, false)
+                );
+            else if (attr.value->type == tString)
+                addURI(state, attrs, attr.name, attr.value->string.s);
             else if (attr.value->type == tBool)
                 attrs.emplace(attr.name, fetchers::Explicit<bool>{attr.value->boolean});
             else if (attr.value->type == tInt)
@@ -75,6 +109,9 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
                     attr.name, showType(*attr.value));
         }
 
+        if (type)
+            attrs.emplace("type", type.value());
+
         if (!attrs.count("type"))
             throw Error({
                 .hint = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
@@ -82,8 +119,18 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
             });
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
-    } else
-        input = fetchers::Input::fromURL(state.coerceToString(pos, *args[0], context, false, false));
+    } else {
+        auto url = fixURI(state.coerceToString(pos, *args[0], context, false, false), state);
+
+        if (type == "git") {
+            fetchers::Attrs attrs;
+            attrs.emplace("type", "git");
+            attrs.emplace("url", url);
+            input = fetchers::Input::fromAttrs(std::move(attrs));
+        } else {
+            input = fetchers::Input::fromURL(url);
+        }
+    }
 
     if (!evalSettings.pureEval && !input.isDirect())
         input = lookupInRegistries(state.store, input).first;
@@ -96,7 +143,13 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
     if (state.allowedPaths)
         state.allowedPaths->insert(tree.actualPath);
 
-    emitTreeAttrs(state, tree, input2, v);
+    emitTreeAttrs(state, tree, input2, v, emptyRevFallback);
+}
+
+static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    settings.requireExperimentalFeature("flakes");
+    fetchTree(state, pos, args, v, std::nullopt);
 }
 
 static RegisterPrimOp r("fetchTree", 1, prim_fetchTree);
@@ -159,7 +212,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             : hashFile(htSHA256, path);
         if (hash != *expectedHash)
             throw Error((unsigned int) 102, "hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
-                *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true));
+                *url, expectedHash->to_string(Base32, true), hash->to_string(Base32, true));
     }
 
     if (state.allowedPaths)
@@ -178,7 +231,13 @@ static void prim_fetchTarball(EvalState & state, const Pos & pos, Value * * args
     fetch(state, pos, args, v, "fetchTarball", true, "source");
 }
 
+static void prim_fetchGit(EvalState &state, const Pos &pos, Value **args, Value &v)
+{
+    fetchTree(state, pos, args, v, "git", true);
+}
+
 static RegisterPrimOp r2("__fetchurl", 1, prim_fetchurl);
 static RegisterPrimOp r3("fetchTarball", 1, prim_fetchTarball);
+static RegisterPrimOp r4("fetchGit", 1, prim_fetchGit);
 
 }
diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 2b6173df9..28db8aa9c 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -130,12 +130,12 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
         tree.actualPath = store->toRealPath(tree.storePath);
 
     auto narHash = store->queryPathInfo(tree.storePath)->narHash;
-    input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
+    input.attrs.insert_or_assign("narHash", narHash->to_string(SRI, true));
 
     if (auto prevNarHash = getNarHash()) {
         if (narHash != *prevNarHash)
-            throw Error("NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
-                to_string(), tree.actualPath, prevNarHash->to_string(SRI, true), narHash.to_string(SRI, true));
+            throw Error((unsigned int) 102, "NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
+                to_string(), tree.actualPath, prevNarHash->to_string(SRI, true), narHash->to_string(SRI, true));
     }
 
     if (auto prevLastModified = getLastModified()) {
diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index b791c125b..30150eeba 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -178,7 +178,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
     auto [fileHash, fileSize] = fileHashSink.finish();
     narInfo->fileHash = fileHash;
     narInfo->fileSize = fileSize;
-    narInfo->url = "nar/" + narInfo->fileHash.to_string(Base32, false) + ".nar"
+    narInfo->url = "nar/" + narInfo->fileHash->to_string(Base32, false) + ".nar"
         + (compression == "xz" ? ".xz" :
            compression == "bzip2" ? ".bz2" :
            compression == "br" ? ".br" :
@@ -372,7 +372,7 @@ StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath
        method for very large paths, but `copyPath' is mainly used for
        small files. */
     StringSink sink;
-    Hash h;
+    std::optional<Hash> h;
     if (method == FileIngestionMethod::Recursive) {
         dumpPath(srcPath, sink, filter);
         h = hashString(hashAlgo, *sink.s);
@@ -382,7 +382,7 @@ StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath
         h = hashString(hashAlgo, s);
     }
 
-    ValidPathInfo info(makeFixedOutputPath(method, h, name));
+    ValidPathInfo info(makeFixedOutputPath(method, *h, name));
 
     auto source = StringSource { *sink.s };
     addToStore(info, source, repair, CheckSigs);
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 62294a08c..d26f96b74 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1047,7 +1047,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath, const BasicDerivation
 {
     this->drv = std::make_unique<BasicDerivation>(BasicDerivation(drv));
     state = &DerivationGoal::haveDerivation;
-    name = fmt("building of %s", worker.store.showPaths(drv.outputPaths()));
+    name = fmt("building of %s", worker.store.showPaths(drv.outputPaths(worker.store)));
     trace("created");
 
     mcExpectedBuilds = std::make_unique<MaintainCount<uint64_t>>(worker.expectedBuilds);
@@ -1182,7 +1182,7 @@ void DerivationGoal::haveDerivation()
     retrySubstitution = false;
 
     for (auto & i : drv->outputs)
-        worker.store.addTempRoot(i.second.path);
+        worker.store.addTempRoot(i.second.path(worker.store, drv->name));
 
     /* Check what outputs paths are not already valid. */
     auto invalidOutputs = checkPathValidity(false, buildMode == bmRepair);
@@ -1290,12 +1290,12 @@ void DerivationGoal::repairClosure()
     StorePathSet outputClosure;
     for (auto & i : drv->outputs) {
         if (!wantOutput(i.first, wantedOutputs)) continue;
-        worker.store.computeFSClosure(i.second.path, outputClosure);
+        worker.store.computeFSClosure(i.second.path(worker.store, drv->name), outputClosure);
     }
 
     /* Filter out our own outputs (which we have already checked). */
     for (auto & i : drv->outputs)
-        outputClosure.erase(i.second.path);
+        outputClosure.erase(i.second.path(worker.store, drv->name));
 
     /* Get all dependencies of this derivation so that we know which
        derivation is responsible for which path in the output
@@ -1307,7 +1307,7 @@ void DerivationGoal::repairClosure()
         if (i.isDerivation()) {
             Derivation drv = worker.store.derivationFromPath(i);
             for (auto & j : drv.outputs)
-                outputsToDrv.insert_or_assign(j.second.path, i);
+                outputsToDrv.insert_or_assign(j.second.path(worker.store, drv.name), i);
         }
 
     /* Check each path (slow!). */
@@ -1379,7 +1379,7 @@ void DerivationGoal::inputsRealised()
             for (auto & j : i.second) {
                 auto k = inDrv.outputs.find(j);
                 if (k != inDrv.outputs.end())
-                    worker.store.computeFSClosure(k->second.path, inputPaths);
+                    worker.store.computeFSClosure(k->second.path(worker.store, inDrv.name), inputPaths);
                 else
                     throw Error(
                         "derivation '%s' requires non-existent output '%s' from input derivation '%s'",
@@ -1432,7 +1432,7 @@ void DerivationGoal::tryToBuild()
        goal can start a build, and if not, the main loop will sleep a
        few seconds and then retry this goal. */
     PathSet lockFiles;
-    for (auto & outPath : drv->outputPaths())
+    for (auto & outPath : drv->outputPaths(worker.store))
         lockFiles.insert(worker.store.Store::toRealPath(outPath));
 
     if (!outputLocks.lockPaths(lockFiles, "", false)) {
@@ -1460,16 +1460,16 @@ void DerivationGoal::tryToBuild()
         return;
     }
 
-    missingPaths = drv->outputPaths();
+    missingPaths = drv->outputPaths(worker.store);
     if (buildMode != bmCheck)
         for (auto & i : validPaths) missingPaths.erase(i);
 
     /* If any of the outputs already exist but are not valid, delete
        them. */
     for (auto & i : drv->outputs) {
-        if (worker.store.isValidPath(i.second.path)) continue;
-        debug("removing invalid path '%s'", worker.store.printStorePath(i.second.path));
-        deletePath(worker.store.Store::toRealPath(i.second.path));
+        if (worker.store.isValidPath(i.second.path(worker.store, drv->name))) continue;
+        debug("removing invalid path '%s'", worker.store.printStorePath(i.second.path(worker.store, drv->name)));
+        deletePath(worker.store.Store::toRealPath(i.second.path(worker.store, drv->name)));
     }
 
     /* Don't do a remote build if the derivation has the attribute
@@ -1692,7 +1692,7 @@ void DerivationGoal::buildDone()
                 fmt("running post-build-hook '%s'", settings.postBuildHook),
                 Logger::Fields{worker.store.printStorePath(drvPath)});
             PushActivity pact(act.id);
-            auto outputPaths = drv->outputPaths();
+            auto outputPaths = drv->outputPaths(worker.store);
             std::map<std::string, std::string> hookEnvironment = getEnv();
 
             hookEnvironment.emplace("DRV_PATH", worker.store.printStorePath(drvPath));
@@ -1920,7 +1920,7 @@ StorePathSet DerivationGoal::exportReferences(const StorePathSet & storePaths)
         if (j.isDerivation()) {
             Derivation drv = worker.store.derivationFromPath(j);
             for (auto & k : drv.outputs)
-                worker.store.computeFSClosure(k.second.path, paths);
+                worker.store.computeFSClosure(k.second.path(worker.store, drv.name), paths);
         }
     }
 
@@ -2015,7 +2015,7 @@ void DerivationGoal::startBuilder()
 
     /* Substitute output placeholders with the actual output paths. */
     for (auto & output : drv->outputs)
-        inputRewrites[hashPlaceholder(output.first)] = worker.store.printStorePath(output.second.path);
+        inputRewrites[hashPlaceholder(output.first)] = worker.store.printStorePath(output.second.path(worker.store, drv->name));
 
     /* Construct the environment passed to the builder. */
     initEnv();
@@ -2200,7 +2200,7 @@ void DerivationGoal::startBuilder()
            (typically the dependencies of /bin/sh).  Throw them
            out. */
         for (auto & i : drv->outputs)
-            dirsInChroot.erase(worker.store.printStorePath(i.second.path));
+            dirsInChroot.erase(worker.store.printStorePath(i.second.path(worker.store, drv->name)));
 
 #elif __APPLE__
         /* We don't really have any parent prep work to do (yet?)
@@ -2613,7 +2613,7 @@ void DerivationGoal::writeStructuredAttrs()
     /* Add an "outputs" object containing the output paths. */
     nlohmann::json outputs;
     for (auto & i : drv->outputs)
-        outputs[i.first] = rewriteStrings(worker.store.printStorePath(i.second.path), inputRewrites);
+        outputs[i.first] = rewriteStrings(worker.store.printStorePath(i.second.path(worker.store, drv->name)), inputRewrites);
     json["outputs"] = outputs;
 
     /* Handle exportReferencesGraph. */
@@ -2817,7 +2817,7 @@ struct RestrictedStore : public LocalFSStore
                 auto drv = derivationFromPath(path.path);
                 for (auto & output : drv.outputs)
                     if (wantOutput(output.first, path.outputs))
-                        newPaths.insert(output.second.path);
+                        newPaths.insert(output.second.path(*this, drv.name));
             } else if (!goal.isAllowed(path.path))
                 throw InvalidPath("cannot build unknown path '%s' in recursive Nix", printStorePath(path.path));
         }
@@ -3579,7 +3579,7 @@ StorePathSet parseReferenceSpecifiers(Store & store, const BasicDerivation & drv
         if (store.isStorePath(i))
             result.insert(store.parseStorePath(i));
         else if (drv.outputs.count(i))
-            result.insert(drv.outputs.find(i)->second.path);
+            result.insert(drv.outputs.find(i)->second.path(store, drv.name));
         else throw BuildError("derivation contains an illegal reference specifier '%s'", i);
     }
     return result;
@@ -3617,7 +3617,7 @@ void DerivationGoal::registerOutputs()
     if (hook) {
         bool allValid = true;
         for (auto & i : drv->outputs)
-            if (!worker.store.isValidPath(i.second.path)) allValid = false;
+            if (!worker.store.isValidPath(i.second.path(worker.store, drv->name))) allValid = false;
         if (allValid) return;
     }
 
@@ -3638,23 +3638,23 @@ void DerivationGoal::registerOutputs()
        Nix calls. */
     StorePathSet referenceablePaths;
     for (auto & p : inputPaths) referenceablePaths.insert(p);
-    for (auto & i : drv->outputs) referenceablePaths.insert(i.second.path);
+    for (auto & i : drv->outputs) referenceablePaths.insert(i.second.path(worker.store, drv->name));
     for (auto & p : addedPaths) referenceablePaths.insert(p);
 
     /* Check whether the output paths were created, and grep each
        output path to determine what other paths it references.  Also make all
        output paths read-only. */
     for (auto & i : drv->outputs) {
-        auto path = worker.store.printStorePath(i.second.path);
-        if (!missingPaths.count(i.second.path)) continue;
+        auto path = worker.store.printStorePath(i.second.path(worker.store, drv->name));
+        if (!missingPaths.count(i.second.path(worker.store, drv->name))) continue;
 
         Path actualPath = path;
         if (needsHashRewrite()) {
-            auto r = redirectedOutputs.find(i.second.path);
+            auto r = redirectedOutputs.find(i.second.path(worker.store, drv->name));
             if (r != redirectedOutputs.end()) {
                 auto redirected = worker.store.Store::toRealPath(r->second);
                 if (buildMode == bmRepair
-                    && redirectedBadOutputs.count(i.second.path)
+                    && redirectedBadOutputs.count(i.second.path(worker.store, drv->name))
                     && pathExists(redirected))
                     replaceValidPath(path, redirected);
                 if (buildMode == bmCheck)
@@ -3723,7 +3723,9 @@ void DerivationGoal::registerOutputs()
 
         if (fixedOutput) {
 
-            if (i.second.hash->method == FileIngestionMethod::Flat) {
+            FixedOutputHash outputHash = std::get<DerivationOutputFixed>(i.second.output).hash;
+
+            if (outputHash.method == FileIngestionMethod::Flat) {
                 /* The output path should be a regular file without execute permission. */
                 if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0)
                     throw BuildError(
@@ -3734,13 +3736,13 @@ void DerivationGoal::registerOutputs()
 
             /* Check the hash. In hash mode, move the path produced by
                the derivation to its content-addressed location. */
-            Hash h2 = i.second.hash->method == FileIngestionMethod::Recursive
-                ? hashPath(*i.second.hash->hash.type, actualPath).first
-                : hashFile(*i.second.hash->hash.type, actualPath);
+            Hash h2 = outputHash.method == FileIngestionMethod::Recursive
+                ? hashPath(outputHash.hash.type, actualPath).first
+                : hashFile(outputHash.hash.type, actualPath);
 
-            auto dest = worker.store.makeFixedOutputPath(i.second.hash->method, h2, i.second.path.name());
+            auto dest = worker.store.makeFixedOutputPath(outputHash.method, h2, i.second.path(worker.store, drv->name).name());
 
-            if (i.second.hash->hash != h2) {
+            if (outputHash.hash != h2) {
 
                 /* Throw an error after registering the path as
                    valid. */
@@ -3748,7 +3750,7 @@ void DerivationGoal::registerOutputs()
                 delayedException = std::make_exception_ptr(
                     BuildError("hash mismatch in fixed-output derivation '%s':\n  wanted: %s\n  got:    %s",
                         worker.store.printStorePath(dest),
-                        i.second.hash->hash.to_string(SRI, true),
+                        outputHash.hash.to_string(SRI, true),
                         h2.to_string(SRI, true)));
 
                 Path actualDest = worker.store.Store::toRealPath(dest);
@@ -3770,7 +3772,7 @@ void DerivationGoal::registerOutputs()
                 assert(worker.store.parseStorePath(path) == dest);
 
             ca = FixedOutputHash {
-                .method = i.second.hash->method,
+                .method = outputHash.method,
                 .hash = h2,
             };
         }
@@ -3785,8 +3787,10 @@ void DerivationGoal::registerOutputs()
            time.  The hash is stored in the database so that we can
            verify later on whether nobody has messed with the store. */
         debug("scanning for references inside '%1%'", path);
-        HashResult hash;
-        auto references = worker.store.parseStorePathSet(scanForReferences(actualPath, worker.store.printStorePathSet(referenceablePaths), hash));
+        // HashResult hash;
+        auto pathSetAndHash = scanForReferences(actualPath, worker.store.printStorePathSet(referenceablePaths));
+        auto references = worker.store.parseStorePathSet(pathSetAndHash.first);
+        HashResult hash = pathSetAndHash.second;
 
         if (buildMode == bmCheck) {
             if (!worker.store.isValidPath(worker.store.parseStorePath(path))) continue;
@@ -3894,7 +3898,7 @@ void DerivationGoal::registerOutputs()
     /* If this is the first round of several, then move the output out of the way. */
     if (nrRounds > 1 && curRound == 1 && curRound < nrRounds && keepPreviousRound) {
         for (auto & i : drv->outputs) {
-            auto path = worker.store.printStorePath(i.second.path);
+            auto path = worker.store.printStorePath(i.second.path(worker.store, drv->name));
             Path prev = path + checkSuffix;
             deletePath(prev);
             Path dst = path + checkSuffix;
@@ -3912,7 +3916,7 @@ void DerivationGoal::registerOutputs()
        if the result was not determistic? */
     if (curRound == nrRounds) {
         for (auto & i : drv->outputs) {
-            Path prev = worker.store.printStorePath(i.second.path) + checkSuffix;
+            Path prev = worker.store.printStorePath(i.second.path(worker.store, drv->name)) + checkSuffix;
             deletePath(prev);
         }
     }
@@ -4213,9 +4217,9 @@ StorePathSet DerivationGoal::checkPathValidity(bool returnValid, bool checkHash)
     for (auto & i : drv->outputs) {
         if (!wantOutput(i.first, wantedOutputs)) continue;
         bool good =
-            worker.store.isValidPath(i.second.path) &&
-            (!checkHash || worker.pathContentsGood(i.second.path));
-        if (good == returnValid) result.insert(i.second.path);
+            worker.store.isValidPath(i.second.path(worker.store, drv->name)) &&
+            (!checkHash || worker.pathContentsGood(i.second.path(worker.store, drv->name)));
+        if (good == returnValid) result.insert(i.second.path(worker.store, drv->name));
     }
     return result;
 }
@@ -5008,7 +5012,7 @@ bool Worker::pathContentsGood(const StorePath & path)
     if (!pathExists(store.printStorePath(path)))
         res = false;
     else {
-        HashResult current = hashPath(*info->narHash.type, store.printStorePath(path));
+        HashResult current = hashPath(info->narHash->type, store.printStorePath(path));
         Hash nullHash(htSHA256);
         res = info->narHash == nullHash || info->narHash == current.first;
     }
diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index e630cf6f1..03bb77488 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -63,16 +63,19 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
     auto & output = drv.outputs.begin()->second;
 
     /* Try the hashed mirrors first. */
-    if (output.hash && output.hash->method == FileIngestionMethod::Flat)
-        for (auto hashedMirror : settings.hashedMirrors.get())
-            try {
-                if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
-                auto & h = output.hash->hash;
-                fetch(hashedMirror + printHashType(*h.type) + "/" + h.to_string(Base16, false));
-                return;
-            } catch (Error & e) {
-                debug(e.what());
+    if (auto hash = std::get_if<DerivationOutputFixed>(&output.output)) {
+        if (hash->hash.method == FileIngestionMethod::Flat) {
+            for (auto hashedMirror : settings.hashedMirrors.get()) {
+                try {
+                    if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
+                    fetch(hashedMirror + printHashType(hash->hash.hash.type) + "/" + hash->hash.hash.to_string(Base16, false));
+                    return;
+                } catch (Error & e) {
+                    debug(e.what());
+                }
             }
+        }
+    }
 
     /* Otherwise try the specified URL. */
     fetch(mainUrl);
diff --git a/src/libstore/content-address.cc b/src/libstore/content-address.cc
index 6cb69d0a9..f83b98a98 100644
--- a/src/libstore/content-address.cc
+++ b/src/libstore/content-address.cc
@@ -3,7 +3,7 @@
 namespace nix {
 
 std::string FixedOutputHash::printMethodAlgo() const {
-    return makeFileIngestionPrefix(method) + printHashType(*hash.type);
+    return makeFileIngestionPrefix(method) + printHashType(hash.type);
 }
 
 std::string makeFileIngestionPrefix(const FileIngestionMethod m) {
@@ -46,7 +46,7 @@ ContentAddress parseContentAddress(std::string_view rawCa) {
         if (prefix == "text") {
             auto hashTypeAndHash = rawCa.substr(prefixSeparator+1, string::npos);
             Hash hash = Hash(string(hashTypeAndHash));
-            if (*hash.type != htSHA256) {
+            if (hash.type != htSHA256) {
                 throw Error("parseContentAddress: the text hash should have type SHA256");
             }
             return TextHash { hash };
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 7e16529a5..478ae39ca 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -86,7 +86,7 @@ struct TunnelLogger : public Logger
     }
 
     /* startWork() means that we're starting an operation for which we
-      want to send out stderr to the client. */
+       want to send out stderr to the client. */
     void startWork()
     {
         auto state(state_.lock());
@@ -289,7 +289,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         auto hash = store->queryPathInfo(path)->narHash;
         logger->stopWork();
-        to << hash.to_string(Base16, false);
+        to << hash->to_string(Base16, false);
         break;
     }
 
@@ -451,7 +451,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     case wopBuildDerivation: {
         auto drvPath = store->parseStorePath(readString(from));
         BasicDerivation drv;
-        readDerivation(from, *store, drv);
+        readDerivation(from, *store, drv, Derivation::nameFromPath(drvPath));
         BuildMode buildMode = (BuildMode) readInt(from);
         logger->startWork();
         if (!trusted)
@@ -632,7 +632,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             if (GET_PROTOCOL_MINOR(clientVersion) >= 17)
                 to << 1;
             to << (info->deriver ? store->printStorePath(*info->deriver) : "")
-               << info->narHash.to_string(Base16, false);
+               << info->narHash->to_string(Base16, false);
             writeStorePaths(*store, to, info->references);
             to << info->registrationTime << info->narSize;
             if (GET_PROTOCOL_MINOR(clientVersion) >= 16) {
@@ -703,24 +703,84 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (!trusted)
             info.ultimate = false;
 
-        std::unique_ptr<Source> source;
-        if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
-            source = std::make_unique<TunnelSource>(from, to);
-        else {
-            StringSink saved;
-            TeeSource tee { from, saved };
-            ParseSink ether;
-            parseDump(ether, tee);
-            source = std::make_unique<StringSource>(std::move(*saved.s));
+        if (GET_PROTOCOL_MINOR(clientVersion) >= 23) {
+
+            struct FramedSource : Source
+            {
+                Source & from;
+                bool eof = false;
+                std::vector<unsigned char> pending;
+                size_t pos = 0;
+
+                FramedSource(Source & from) : from(from)
+                { }
+
+                ~FramedSource()
+                {
+                    if (!eof) {
+                        while (true) {
+                            auto n = readInt(from);
+                            if (!n) break;
+                            std::vector<unsigned char> data(n);
+                            from(data.data(), n);
+                        }
+                    }
+                }
+
+                size_t read(unsigned char * data, size_t len) override
+                {
+                    if (eof) throw EndOfFile("reached end of FramedSource");
+
+                    if (pos >= pending.size()) {
+                        size_t len = readInt(from);
+                        if (!len) {
+                            eof = true;
+                            return 0;
+                        }
+                        pending = std::vector<unsigned char>(len);
+                        pos = 0;
+                        from(pending.data(), len);
+                    }
+
+                    auto n = std::min(len, pending.size() - pos);
+                    memcpy(data, pending.data() + pos, n);
+                    pos += n;
+                    return n;
+                }
+            };
+
+            logger->startWork();
+
+            {
+                FramedSource source(from);
+                store->addToStore(info, source, (RepairFlag) repair,
+                    dontCheckSigs ? NoCheckSigs : CheckSigs);
+            }
+
+            logger->stopWork();
         }
 
-        logger->startWork();
+        else {
+            std::unique_ptr<Source> source;
+            if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
+                source = std::make_unique<TunnelSource>(from, to);
+            else {
+                StringSink saved;
+                TeeSource tee { from, saved };
+                ParseSink ether;
+                parseDump(ether, tee);
+                source = std::make_unique<StringSource>(std::move(*saved.s));
+            }
 
-        // FIXME: race if addToStore doesn't read source?
-        store->addToStore(info, *source, (RepairFlag) repair,
-            dontCheckSigs ? NoCheckSigs : CheckSigs);
+            logger->startWork();
+
+            // FIXME: race if addToStore doesn't read source?
+            store->addToStore(info, *source, (RepairFlag) repair,
+                dontCheckSigs ? NoCheckSigs : CheckSigs);
+
+            logger->stopWork();
+        }
 
-        logger->stopWork();
         break;
     }
 
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 870901421..7d0a5abeb 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -7,6 +7,22 @@
 
 namespace nix {
 
+// FIXME Put this somewhere?
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
+
+StorePath DerivationOutput::path(const Store & store, std::string_view drvName) const
+{
+    return std::visit(overloaded {
+        [](DerivationOutputInputAddressed doi) {
+            return doi.path;
+        },
+        [&](DerivationOutputFixed dof) {
+            return store.makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
+        }
+    }, output);
+}
+
 
 bool BasicDerivation::isBuiltin() const
 {
@@ -99,7 +115,6 @@ static DerivationOutput parseDerivationOutput(const Store & store, std::istrings
     expect(str, ","); const auto hash = parseString(str);
     expect(str, ")");
 
-    std::optional<FixedOutputHash> fsh;
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
@@ -107,22 +122,29 @@ static DerivationOutput parseDerivationOutput(const Store & store, std::istrings
             hashAlgo = string(hashAlgo, 2);
         }
         const HashType hashType = parseHashType(hashAlgo);
-        fsh = FixedOutputHash {
-            .method = std::move(method),
-            .hash = Hash(hash, hashType),
-        };
-    }
 
-    return DerivationOutput {
-        .path = std::move(path),
-        .hash = std::move(fsh),
-    };
+        return DerivationOutput {
+            .output = DerivationOutputFixed {
+                .hash = FixedOutputHash {
+                    .method = std::move(method),
+                    .hash = Hash(hash, hashType),
+                },
+            }
+        };
+    } else
+        return DerivationOutput {
+            .output = DerivationOutputInputAddressed {
+                .path = std::move(path),
+            }
+        };
 }
 
 
-static Derivation parseDerivation(const Store & store, std::string && s)
+static Derivation parseDerivation(const Store & store, std::string && s, std::string_view name)
 {
     Derivation drv;
+    drv.name = name;
+
     std::istringstream str(std::move(s));
     expect(str, "Derive([");
 
@@ -166,10 +188,10 @@ static Derivation parseDerivation(const Store & store, std::string && s)
 }
 
 
-Derivation readDerivation(const Store & store, const Path & drvPath)
+Derivation readDerivation(const Store & store, const Path & drvPath, std::string_view name)
 {
     try {
-        return parseDerivation(store, readFile(drvPath));
+        return parseDerivation(store, readFile(drvPath), name);
     } catch (FormatError & e) {
         throw Error("error parsing derivation '%1%': %2%", drvPath, e.msg());
     }
@@ -187,7 +209,7 @@ Derivation Store::readDerivation(const StorePath & drvPath)
 {
     auto accessor = getFSAccessor();
     try {
-        return parseDerivation(*this, accessor->readFile(printStorePath(drvPath)));
+        return parseDerivation(*this, accessor->readFile(printStorePath(drvPath)), Derivation::nameFromPath(drvPath));
     } catch (FormatError & e) {
         throw Error("error parsing derivation '%s': %s", printStorePath(drvPath), e.msg());
     }
@@ -255,10 +277,14 @@ string Derivation::unparse(const Store & store, bool maskOutputs,
     for (auto & i : outputs) {
         if (first) first = false; else s += ',';
         s += '('; printUnquotedString(s, i.first);
-        s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(i.second.path));
-        s += ','; printUnquotedString(s, i.second.hash ? i.second.hash->printMethodAlgo() : "");
-        s += ','; printUnquotedString(s,
-            i.second.hash ? i.second.hash->hash.to_string(Base16, false) : "");
+        s += ','; printUnquotedString(s, maskOutputs ? "" : store.printStorePath(i.second.path(store, name)));
+        if (auto hash = std::get_if<DerivationOutputFixed>(&i.second.output)) {
+            s += ','; printUnquotedString(s, hash->hash.printMethodAlgo());
+            s += ','; printUnquotedString(s, hash->hash.hash.to_string(Base16, false));
+        } else {
+            s += ','; printUnquotedString(s, "");
+            s += ','; printUnquotedString(s, "");
+        }
         s += ')';
     }
 
@@ -314,7 +340,7 @@ bool BasicDerivation::isFixedOutput() const
 {
     return outputs.size() == 1 &&
         outputs.begin()->first == "out" &&
-        outputs.begin()->second.hash;
+        std::holds_alternative<DerivationOutputFixed>(outputs.begin()->second.output);
 }
 
 
@@ -346,10 +372,11 @@ Hash hashDerivationModulo(Store & store, const Derivation & drv, bool maskOutput
     /* Return a fixed hash for fixed-output derivations. */
     if (drv.isFixedOutput()) {
         DerivationOutputs::const_iterator i = drv.outputs.begin();
+        auto hash = std::get<DerivationOutputFixed>(i->second.output);
         return hashString(htSHA256, "fixed:out:"
-            + i->second.hash->printMethodAlgo() + ":"
-            + i->second.hash->hash.to_string(Base16, false) + ":"
-            + store.printStorePath(i->second.path));
+            + hash.hash.printMethodAlgo() + ":"
+            + hash.hash.hash.to_string(Base16, false) + ":"
+            + store.printStorePath(i->second.path(store, drv.name)));
     }
 
     /* For other derivations, replace the inputs paths with recursive
@@ -383,11 +410,11 @@ bool wantOutput(const string & output, const std::set<string> & wanted)
 }
 
 
-StorePathSet BasicDerivation::outputPaths() const
+StorePathSet BasicDerivation::outputPaths(const Store & store) const
 {
     StorePathSet paths;
     for (auto & i : outputs)
-        paths.insert(i.second.path);
+        paths.insert(i.second.path(store, name));
     return paths;
 }
 
@@ -397,7 +424,6 @@ static DerivationOutput readDerivationOutput(Source & in, const Store & store)
     auto hashAlgo = readString(in);
     auto hash = readString(in);
 
-    std::optional<FixedOutputHash> fsh;
     if (hashAlgo != "") {
         auto method = FileIngestionMethod::Flat;
         if (string(hashAlgo, 0, 2) == "r:") {
@@ -405,16 +431,20 @@ static DerivationOutput readDerivationOutput(Source & in, const Store & store)
             hashAlgo = string(hashAlgo, 2);
         }
         auto hashType = parseHashType(hashAlgo);
-        fsh = FixedOutputHash {
-            .method = std::move(method),
-            .hash = Hash(hash, hashType),
+        return DerivationOutput {
+            .output = DerivationOutputFixed {
+                .hash = FixedOutputHash {
+                    .method = std::move(method),
+                    .hash = Hash(hash, hashType),
+                },
+            }
+        };
+    } else
+        return DerivationOutput {
+            .output = DerivationOutputInputAddressed {
+                .path = std::move(path),
+            }
         };
-    }
-
-    return DerivationOutput {
-        .path = std::move(path),
-        .hash = std::move(fsh),
-    };
 }
 
 StringSet BasicDerivation::outputNames() const
@@ -426,8 +456,19 @@ StringSet BasicDerivation::outputNames() const
 }
 
 
-Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv)
+std::string_view BasicDerivation::nameFromPath(const StorePath & drvPath) {
+    auto nameWithSuffix = drvPath.name();
+    constexpr std::string_view extension = ".drv";
+    assert(hasSuffix(nameWithSuffix, extension));
+    nameWithSuffix.remove_suffix(extension.size());
+    return nameWithSuffix;
+}
+
+
+Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv, std::string_view name)
 {
+    drv.name = name;
+
     drv.outputs.clear();
     auto nr = readNum<size_t>(in);
     for (size_t n = 0; n < nr; n++) {
@@ -456,10 +497,10 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
     out << drv.outputs.size();
     for (auto & i : drv.outputs) {
         out << i.first
-            << store.printStorePath(i.second.path);
-        if (i.second.hash) {
-            out << i.second.hash->printMethodAlgo()
-                << i.second.hash->hash.to_string(Base16, false);
+            << store.printStorePath(i.second.path(store, drv.name));
+        if (auto hash = std::get_if<DerivationOutputFixed>(&i.second.output)) {
+            out << hash->hash.printMethodAlgo()
+                << hash->hash.hash.to_string(Base16, false);
         } else {
             out << "" << "";
         }
diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index 00cf1bb08..133ffe50e 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -13,10 +13,20 @@ namespace nix {
 
 /* Abstract syntax of derivations. */
 
-struct DerivationOutput
+struct DerivationOutputInputAddressed
 {
     StorePath path;
-    std::optional<FixedOutputHash> hash; /* hash used for expected hash computation */
+};
+
+struct DerivationOutputFixed
+{
+    FixedOutputHash hash; /* hash used for expected hash computation */
+};
+
+struct DerivationOutput
+{
+    std::variant<DerivationOutputInputAddressed, DerivationOutputFixed> output;
+    StorePath path(const Store & store, std::string_view drvName) const;
 };
 
 typedef std::map<string, DerivationOutput> DerivationOutputs;
@@ -35,6 +45,7 @@ struct BasicDerivation
     Path builder;
     Strings args;
     StringPairs env;
+    std::string name;
 
     BasicDerivation() { }
     virtual ~BasicDerivation() { };
@@ -45,10 +56,12 @@ struct BasicDerivation
     bool isFixedOutput() const;
 
     /* Return the output paths of a derivation. */
-    StorePathSet outputPaths() const;
+    StorePathSet outputPaths(const Store & store) const;
 
     /* Return the output names of a derivation. */
     StringSet outputNames() const;
+
+    static std::string_view nameFromPath(const StorePath & storePath);
 };
 
 struct Derivation : BasicDerivation
@@ -72,7 +85,7 @@ StorePath writeDerivation(ref<Store> store,
     const Derivation & drv, std::string_view name, RepairFlag repair = NoRepair);
 
 /* Read a derivation from a file. */
-Derivation readDerivation(const Store & store, const Path & drvPath);
+Derivation readDerivation(const Store & store, const Path & drvPath, std::string_view name);
 
 // FIXME: remove
 bool isDerivation(const string & fileName);
@@ -89,7 +102,7 @@ bool wantOutput(const string & output, const std::set<string> & wanted);
 struct Source;
 struct Sink;
 
-Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv);
+Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv, std::string_view name);
 void writeDerivation(Sink & out, const Store & store, const BasicDerivation & drv);
 
 std::string hashPlaceholder(const std::string & outputName);
diff --git a/src/libstore/export-import.cc b/src/libstore/export-import.cc
index b963d64d7..a0fc22264 100644
--- a/src/libstore/export-import.cc
+++ b/src/libstore/export-import.cc
@@ -38,9 +38,9 @@ void Store::exportPath(const StorePath & path, Sink & sink)
        filesystem corruption from spreading to other machines.
        Don't complain if the stored hash is zero (unknown). */
     Hash hash = hashSink.currentHash().first;
-    if (hash != info->narHash && info->narHash != Hash(*info->narHash.type))
+    if (hash != info->narHash && info->narHash != Hash(info->narHash->type))
         throw Error("hash of path '%s' has changed from '%s' to '%s'!",
-            printStorePath(path), info->narHash.to_string(Base32, true), hash.to_string(Base32, true));
+            printStorePath(path), info->narHash->to_string(Base32, true), hash.to_string(Base32, true));
 
     teeSink
         << exportMagic
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index a8bd8a972..5d7566121 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -113,7 +113,7 @@ struct LegacySSHStore : public Store
 
             if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4) {
                 auto s = readString(conn->from);
-                info->narHash = s.empty() ? Hash() : Hash(s);
+                info->narHash = s.empty() ? std::optional<Hash>{} : Hash{s};
                 info->ca = parseContentAddressOpt(readString(conn->from));
                 info->sigs = readStrings<StringSet>(conn->from);
             }
@@ -138,7 +138,7 @@ struct LegacySSHStore : public Store
                 << cmdAddToStoreNar
                 << printStorePath(info.path)
                 << (info.deriver ? printStorePath(*info.deriver) : "")
-                << info.narHash.to_string(Base16, false);
+                << info.narHash->to_string(Base16, false);
             writeStorePaths(*this, conn->to, info.references);
             conn->to
                 << info.registrationTime
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 9ea71170f..eb367d778 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -560,19 +560,12 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
         DerivationOutputs::const_iterator out = drv.outputs.find("out");
         if (out == drv.outputs.end())
             throw Error("derivation '%s' does not have an output named 'out'", printStorePath(drvPath));
-
-        check(
-            makeFixedOutputPath(
-                out->second.hash->method,
-                out->second.hash->hash,
-                drvName),
-            out->second.path, "out");
     }
 
     else {
         Hash h = hashDerivationModulo(*this, drv, true);
         for (auto & i : drv.outputs)
-            check(makeOutputPath(i.first, h, drvName), i.second.path, i.first);
+            check(makeOutputPath(i.first, h, drvName), i.second.path(*this, drv.name), i.first);
     }
 }
 
@@ -586,7 +579,7 @@ uint64_t LocalStore::addValidPath(State & state,
 
     state.stmtRegisterValidPath.use()
         (printStorePath(info.path))
-        (info.narHash.to_string(Base16, true))
+        (info.narHash->to_string(Base16, true))
         (info.registrationTime == 0 ? time(0) : info.registrationTime)
         (info.deriver ? printStorePath(*info.deriver) : "", (bool) info.deriver)
         (info.narSize, info.narSize != 0)
@@ -614,7 +607,7 @@ uint64_t LocalStore::addValidPath(State & state,
             state.stmtAddDerivationOutput.use()
                 (id)
                 (i.first)
-                (printStorePath(i.second.path))
+                (printStorePath(i.second.path(*this, drv.name)))
                 .exec();
         }
     }
@@ -686,7 +679,7 @@ void LocalStore::updatePathInfo(State & state, const ValidPathInfo & info)
 {
     state.stmtUpdatePathInfo.use()
         (info.narSize, info.narSize != 0)
-        (info.narHash.to_string(Base16, true))
+        (info.narHash->to_string(Base16, true))
         (info.ultimate ? 1 : 0, info.ultimate)
         (concatStringsSep(" ", info.sigs), !info.sigs.empty())
         (renderContentAddress(info.ca), (bool) info.ca)
@@ -900,7 +893,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
         StorePathSet paths;
 
         for (auto & i : infos) {
-            assert(i.narHash.type == htSHA256);
+            assert(i.narHash && i.narHash->type == htSHA256);
             if (isValidPath_(*state, i.path))
                 updatePathInfo(*state, i);
             else
@@ -1013,7 +1006,7 @@ 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",
-                    printStorePath(info.path), info.narHash.to_string(Base32, true), hashResult.first.to_string(Base32, true));
+                    printStorePath(info.path), info.narHash->to_string(Base32, true), hashResult.first.to_string(Base32, true));
 
             if (hashResult.second != info.narSize)
                 throw Error("size mismatch importing path '%s';\n  wanted: %s\n  got:   %s",
@@ -1309,9 +1302,9 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 
                 std::unique_ptr<AbstractHashSink> hashSink;
                 if (!info->ca || !info->references.count(info->path))
-                    hashSink = std::make_unique<HashSink>(*info->narHash.type);
+                    hashSink = std::make_unique<HashSink>(info->narHash->type);
                 else
-                    hashSink = std::make_unique<HashModuloSink>(*info->narHash.type, std::string(info->path.hashPart()));
+                    hashSink = std::make_unique<HashModuloSink>(info->narHash->type, std::string(info->path.hashPart()));
 
                 dumpPath(Store::toRealPath(i), *hashSink);
                 auto current = hashSink->finish();
@@ -1320,7 +1313,7 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
                     logError({
                         .name = "Invalid hash - path modified",
                         .hint = hintfmt("path '%s' was modified! expected hash '%s', got '%s'",
-                        printStorePath(i), info->narHash.to_string(Base32, true), current.first.to_string(Base32, true))
+                        printStorePath(i), info->narHash->to_string(Base32, true), current.first.to_string(Base32, true))
                     });
                     if (repair) repairPath(i); else errors = true;
                 } else {
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index e68edb38c..c4d22a634 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -198,8 +198,8 @@ void Store::queryMissing(const std::vector<StorePathWithOutputs> & targets,
             PathSet invalid;
             for (auto & j : drv->outputs)
                 if (wantOutput(j.first, path.outputs)
-                    && !isValidPath(j.second.path))
-                    invalid.insert(printStorePath(j.second.path));
+                    && !isValidPath(j.second.path(*this, drv->name)))
+                    invalid.insert(printStorePath(j.second.path(*this, drv->name)));
             if (invalid.empty()) return;
 
             if (settings.useSubstitutes && parsedDrv.substitutesAllowed()) {
diff --git a/src/libstore/nar-info-disk-cache.cc b/src/libstore/nar-info-disk-cache.cc
index 012dea6ea..9ddb9957f 100644
--- a/src/libstore/nar-info-disk-cache.cc
+++ b/src/libstore/nar-info-disk-cache.cc
@@ -230,9 +230,9 @@ public:
                     (std::string(info->path.name()))
                     (narInfo ? narInfo->url : "", narInfo != 0)
                     (narInfo ? narInfo->compression : "", narInfo != 0)
-                    (narInfo && narInfo->fileHash ? narInfo->fileHash.to_string(Base32, true) : "", narInfo && narInfo->fileHash)
+                    (narInfo && narInfo->fileHash ? narInfo->fileHash->to_string(Base32, true) : "", narInfo && narInfo->fileHash)
                     (narInfo ? narInfo->fileSize : 0, narInfo != 0 && narInfo->fileSize)
-                    (info->narHash.to_string(Base32, true))
+                    (info->narHash->to_string(Base32, true))
                     (info->narSize)
                     (concatStringsSep(" ", info->shortRefs()))
                     (info->deriver ? std::string(info->deriver->to_string()) : "", (bool) info->deriver)
diff --git a/src/libstore/nar-info.cc b/src/libstore/nar-info.cc
index 04550ed97..ca471463c 100644
--- a/src/libstore/nar-info.cc
+++ b/src/libstore/nar-info.cc
@@ -7,15 +7,14 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
     : ValidPathInfo(StorePath(StorePath::dummy)) // FIXME: hack
 {
     auto corrupt = [&]() {
-        throw Error("NAR info file '%1%' is corrupt", whence);
+        return Error("NAR info file '%1%' is corrupt", whence);
     };
 
     auto parseHashField = [&](const string & s) {
         try {
             return Hash(s);
         } catch (BadHash &) {
-            corrupt();
-            return Hash(); // never reached
+            throw corrupt();
         }
     };
 
@@ -25,12 +24,12 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
     while (pos < s.size()) {
 
         size_t colon = s.find(':', pos);
-        if (colon == std::string::npos) corrupt();
+        if (colon == std::string::npos) throw corrupt();
 
         std::string name(s, pos, colon - pos);
 
         size_t eol = s.find('\n', colon + 2);
-        if (eol == std::string::npos) corrupt();
+        if (eol == std::string::npos) throw corrupt();
 
         std::string value(s, colon + 2, eol - colon - 2);
 
@@ -45,16 +44,16 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "FileHash")
             fileHash = parseHashField(value);
         else if (name == "FileSize") {
-            if (!string2Int(value, fileSize)) corrupt();
+            if (!string2Int(value, fileSize)) throw corrupt();
         }
         else if (name == "NarHash")
             narHash = parseHashField(value);
         else if (name == "NarSize") {
-            if (!string2Int(value, narSize)) corrupt();
+            if (!string2Int(value, narSize)) throw corrupt();
         }
         else if (name == "References") {
             auto refs = tokenizeString<Strings>(value, " ");
-            if (!references.empty()) corrupt();
+            if (!references.empty()) throw corrupt();
             for (auto & r : refs)
                 references.insert(StorePath(r));
         }
@@ -67,7 +66,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "Sig")
             sigs.insert(value);
         else if (name == "CA") {
-            if (ca) corrupt();
+            if (ca) throw corrupt();
             // FIXME: allow blank ca or require skipping field?
             ca = parseContentAddressOpt(value);
         }
@@ -77,7 +76,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
 
     if (compression == "") compression = "bzip2";
 
-    if (!havePath || url.empty() || narSize == 0 || !narHash) corrupt();
+    if (!havePath || url.empty() || narSize == 0 || !narHash) throw corrupt();
 }
 
 std::string NarInfo::to_string(const Store & store) const
@@ -87,11 +86,11 @@ std::string NarInfo::to_string(const Store & store) const
     res += "URL: " + url + "\n";
     assert(compression != "");
     res += "Compression: " + compression + "\n";
-    assert(fileHash.type == htSHA256);
-    res += "FileHash: " + fileHash.to_string(Base32, true) + "\n";
+    assert(fileHash && fileHash->type == htSHA256);
+    res += "FileHash: " + fileHash->to_string(Base32, true) + "\n";
     res += "FileSize: " + std::to_string(fileSize) + "\n";
-    assert(narHash.type == htSHA256);
-    res += "NarHash: " + narHash.to_string(Base32, true) + "\n";
+    assert(narHash && narHash->type == htSHA256);
+    res += "NarHash: " + narHash->to_string(Base32, true) + "\n";
     res += "NarSize: " + std::to_string(narSize) + "\n";
 
     res += "References: " + concatStringsSep(" ", shortRefs()) + "\n";
diff --git a/src/libstore/nar-info.hh b/src/libstore/nar-info.hh
index 373c33427..eff19f0ef 100644
--- a/src/libstore/nar-info.hh
+++ b/src/libstore/nar-info.hh
@@ -10,7 +10,7 @@ struct NarInfo : ValidPathInfo
 {
     std::string url;
     std::string compression;
-    Hash fileHash;
+    std::optional<Hash> fileHash;
     uint64_t fileSize = 0;
     std::string system;
 
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index 968e2e425..62a3cda61 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -76,8 +76,8 @@ void RefScanSink::operator () (const unsigned char * data, size_t len)
 }
 
 
-PathSet scanForReferences(const string & path,
-    const PathSet & refs, HashResult & hash)
+std::pair<PathSet, HashResult> scanForReferences(const string & path,
+    const PathSet & refs)
 {
     RefScanSink refsSink;
     HashSink hashSink { htSHA256 };
@@ -111,9 +111,9 @@ PathSet scanForReferences(const string & path,
         found.insert(j->second);
     }
 
-    hash = hashSink.finish();
+    auto hash = hashSink.finish();
 
-    return found;
+    return std::pair<PathSet, HashResult>(found, hash);
 }
 
 
diff --git a/src/libstore/references.hh b/src/libstore/references.hh
index c38bdd720..598a3203a 100644
--- a/src/libstore/references.hh
+++ b/src/libstore/references.hh
@@ -5,8 +5,7 @@
 
 namespace nix {
 
-PathSet scanForReferences(const Path & path, const PathSet & refs,
-    HashResult & hash);
+std::pair<PathSet, HashResult> scanForReferences(const Path & path, const PathSet & refs);
 
 struct RewritingSink : Sink
 {
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 9af4364b7..616081aa1 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -498,14 +498,89 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
         conn->to << wopAddToStoreNar
                  << printStorePath(info.path)
                  << (info.deriver ? printStorePath(*info.deriver) : "")
-                 << info.narHash.to_string(Base16, false);
+                 << info.narHash->to_string(Base16, false);
         writeStorePaths(*this, conn->to, info.references);
         conn->to << info.registrationTime << info.narSize
                  << info.ultimate << info.sigs << renderContentAddress(info.ca)
                  << repair << !checkSigs;
-        bool tunnel = GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21;
-        if (!tunnel) copyNAR(source, conn->to);
-        conn.processStderr(0, tunnel ? &source : nullptr);
+
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 23) {
+
+            std::exception_ptr ex;
+
+            struct FramedSink : BufferedSink
+            {
+                ConnectionHandle & conn;
+                std::exception_ptr & ex;
+
+                FramedSink(ConnectionHandle & conn, std::exception_ptr & ex) : conn(conn), ex(ex)
+                { }
+
+                ~FramedSink()
+                {
+                    try {
+                        conn->to << 0;
+                        conn->to.flush();
+                    } catch (...) {
+                        ignoreException();
+                    }
+                }
+
+                void write(const unsigned char * data, size_t len) override
+                {
+                    /* Don't send more data if the remote has
+                       encountered an error. */
+                    if (ex) {
+                        auto ex2 = ex;
+                        ex = nullptr;
+                        std::rethrow_exception(ex2);
+                    }
+                    conn->to << len;
+                    conn->to(data, len);
+                };
+            };
+
+            /* Handle log messages / exceptions from the remote on a
+               separate thread. */
+            std::thread stderrThread([&]()
+            {
+                try {
+                    conn.processStderr();
+                } catch (...) {
+                    ex = std::current_exception();
+                }
+            });
+
+            Finally joinStderrThread([&]()
+            {
+                if (stderrThread.joinable()) {
+                    stderrThread.join();
+                    if (ex) {
+                        try {
+                            std::rethrow_exception(ex);
+                        } catch (...) {
+                            ignoreException();
+                        }
+                    }
+                }
+            });
+
+            {
+                FramedSink sink(conn, ex);
+                copyNAR(source, sink);
+                sink.flush();
+            }
+
+            stderrThread.join();
+            if (ex)
+                std::rethrow_exception(ex);
+
+        } else if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 21) {
+            conn.processStderr(0, &source);
+        } else {
+            copyNAR(source, conn->to);
+            conn.processStderr(0, nullptr);
+        }
     }
 }
 
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 1b7dff085..67935f3ba 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -343,13 +343,10 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
             std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1)
                 .count();
 
-        auto size = istream->tellg();
-
-        printInfo("uploaded 's3://%s/%s' (%d bytes) in %d ms",
-            bucketName, path, size, duration);
+        printInfo("uploaded 's3://%s/%s' in %d ms",
+            bucketName, path, duration);
 
         stats.putTimeMs += duration;
-        stats.putBytes += size;
         stats.put++;
     }
 
diff --git a/src/libstore/s3-binary-cache-store.hh b/src/libstore/s3-binary-cache-store.hh
index 4d43fe4d2..b2b75d498 100644
--- a/src/libstore/s3-binary-cache-store.hh
+++ b/src/libstore/s3-binary-cache-store.hh
@@ -19,7 +19,6 @@ public:
     struct Stats
     {
         std::atomic<uint64_t> put{0};
-        std::atomic<uint64_t> putBytes{0};
         std::atomic<uint64_t> putTimeMs{0};
         std::atomic<uint64_t> get{0};
         std::atomic<uint64_t> getBytes{0};
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index ab21b0bd5..c804399d2 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -538,7 +538,7 @@ string Store::makeValidityRegistration(const StorePathSet & paths,
         auto info = queryPathInfo(i);
 
         if (showHash) {
-            s += info->narHash.to_string(Base16, false) + "\n";
+            s += info->narHash->to_string(Base16, false) + "\n";
             s += (format("%1%\n") % info->narSize).str();
         }
 
@@ -570,7 +570,7 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
             auto info = queryPathInfo(storePath);
 
             jsonPath
-                .attr("narHash", info->narHash.to_string(hashBase, true))
+                .attr("narHash", info->narHash->to_string(hashBase, true))
                 .attr("narSize", info->narSize);
 
             {
@@ -613,7 +613,7 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
                     if (!narInfo->url.empty())
                         jsonPath.attr("url", narInfo->url);
                     if (narInfo->fileHash)
-                        jsonPath.attr("downloadHash", narInfo->fileHash.to_string(hashBase, true));
+                        jsonPath.attr("downloadHash", narInfo->fileHash->to_string(hashBase, true));
                     if (narInfo->fileSize)
                         jsonPath.attr("downloadSize", narInfo->fileSize);
                     if (showClosureSize)
@@ -729,9 +729,9 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
 {
     auto valid = dstStore->queryValidPaths(storePaths, substitute);
 
-    PathSet missing;
+    StorePathSet missing;
     for (auto & path : storePaths)
-        if (!valid.count(path)) missing.insert(srcStore->printStorePath(path));
+        if (!valid.count(path)) missing.insert(path);
 
     if (missing.empty()) return;
 
@@ -748,29 +748,27 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
 
     ThreadPool pool;
 
-    processGraph<Path>(pool,
-        PathSet(missing.begin(), missing.end()),
+    processGraph<StorePath>(pool,
+        StorePathSet(missing.begin(), missing.end()),
 
-        [&](const Path & storePath) {
-            if (dstStore->isValidPath(dstStore->parseStorePath(storePath))) {
+        [&](const StorePath & storePath) {
+            if (dstStore->isValidPath(storePath)) {
                 nrDone++;
                 showProgress();
-                return PathSet();
+                return StorePathSet();
             }
 
-            auto info = srcStore->queryPathInfo(srcStore->parseStorePath(storePath));
+            auto info = srcStore->queryPathInfo(storePath);
 
             bytesExpected += info->narSize;
             act.setExpected(actCopyPath, bytesExpected);
 
-            return srcStore->printStorePathSet(info->references);
+            return info->references;
         },
 
-        [&](const Path & storePathS) {
+        [&](const StorePath & storePath) {
             checkInterrupt();
 
-            auto storePath = dstStore->parseStorePath(storePathS);
-
             if (!dstStore->isValidPath(storePath)) {
                 MaintainCount<decltype(nrRunning)> mc(nrRunning);
                 showProgress();
@@ -780,7 +778,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & st
                     nrFailed++;
                     if (!settings.keepGoing)
                         throw e;
-                    logger->log(lvlError, fmt("could not copy %s: %s", storePathS, e.what()));
+                    logger->log(lvlError, fmt("could not copy %s: %s", dstStore->printStorePath(storePath), e.what()));
                     showProgress();
                     return;
                 }
@@ -854,7 +852,7 @@ std::string ValidPathInfo::fingerprint(const Store & store) const
             store.printStorePath(path));
     return
         "1;" + store.printStorePath(path) + ";"
-        + narHash.to_string(Base32, true) + ";"
+        + narHash->to_string(Base32, true) + ";"
         + std::to_string(narSize) + ";"
         + concatStringsSep(",", store.printStorePathSet(references));
 }
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index d1cb2035f..a7288d0cc 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -115,7 +115,8 @@ struct ValidPathInfo
 {
     StorePath path;
     std::optional<StorePath> deriver;
-    Hash narHash;
+    // TODO document this
+    std::optional<Hash> narHash;
     StorePathSet references;
     time_t registrationTime = 0;
     uint64_t narSize = 0; // 0 = unknown
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index 8b538f6da..dcba73116 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x116
+#define PROTOCOL_VERSION 0x117
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 5578a618e..2b0390da4 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -15,19 +15,22 @@
 
 namespace nix {
 
+static size_t regularHashSize(HashType type) {
+    switch (type) {
+    case htMD5: return md5HashSize;
+    case htSHA1: return sha1HashSize;
+    case htSHA256: return sha256HashSize;
+    case htSHA512: return sha512HashSize;
+    }
+    abort();
+}
 
 std::set<std::string> hashTypes = { "md5", "sha1", "sha256", "sha512" };
 
 
 void Hash::init()
 {
-    assert(type);
-    switch (*type) {
-    case htMD5: hashSize = md5HashSize; break;
-    case htSHA1: hashSize = sha1HashSize; break;
-    case htSHA256: hashSize = sha256HashSize; break;
-    case htSHA512: hashSize = sha512HashSize; break;
-    }
+    hashSize = regularHashSize(type);
     assert(hashSize <= maxHashSize);
     memset(hash, 0, maxHashSize);
 }
@@ -108,17 +111,11 @@ string printHash16or32(const Hash & hash)
 }
 
 
-HashType assertInitHashType(const Hash & h)
-{
-    assert(h.type);
-    return *h.type;
-}
-
 std::string Hash::to_string(Base base, bool includeType) const
 {
     std::string s;
     if (base == SRI || includeType) {
-        s += printHashType(assertInitHashType(*this));
+        s += printHashType(type);
         s += base == SRI ? '-' : ':';
     }
     switch (base) {
@@ -139,60 +136,66 @@ std::string Hash::to_string(Base base, bool includeType) const
 Hash::Hash(std::string_view s, HashType type) : Hash(s, std::optional { type }) { }
 Hash::Hash(std::string_view s) : Hash(s, std::optional<HashType>{}) { }
 
-Hash::Hash(std::string_view s, std::optional<HashType> type)
-    : type(type)
+Hash::Hash(std::string_view original, std::optional<HashType> optType)
 {
+    auto rest = original;
+
     size_t pos = 0;
     bool isSRI = false;
 
-    auto sep = s.find(':');
-    if (sep == string::npos) {
-        sep = s.find('-');
-        if (sep != string::npos) {
-            isSRI = true;
-        } else if (! type)
-            throw BadHash("hash '%s' does not include a type", s);
+    // Parse the has type before the separater, if there was one.
+    std::optional<HashType> optParsedType;
+    {
+        auto sep = rest.find(':');
+        if (sep == std::string_view::npos) {
+            sep = rest.find('-');
+            if (sep != std::string_view::npos)
+                isSRI = true;
+        }
+        if (sep != std::string_view::npos) {
+            auto hashRaw = rest.substr(0, sep);
+            optParsedType = parseHashType(hashRaw);
+            rest = rest.substr(sep + 1);
+        }
     }
 
-    if (sep != string::npos) {
-        string hts = string(s, 0, sep);
-        this->type = parseHashType(hts);
-        if (!this->type)
-            throw BadHash("unknown hash type '%s'", hts);
-        if (type && type != this->type)
-            throw BadHash("hash '%s' should have type '%s'", s, printHashType(*type));
-        pos = sep + 1;
+    // Either the string or user must provide the type, if they both do they
+    // must agree.
+    if (!optParsedType && !optType) {
+        throw BadHash("hash '%s' does not include a type, nor is the type otherwise known from context.", rest);
+    } else {
+        this->type = optParsedType ? *optParsedType : *optType;
+        if (optParsedType && optType && *optParsedType != *optType)
+            throw BadHash("hash '%s' should have type '%s'", original, printHashType(*optType));
     }
 
     init();
 
-    size_t size = s.size() - pos;
-
-    if (!isSRI && size == base16Len()) {
+    if (!isSRI && rest.size() == base16Len()) {
 
         auto parseHexDigit = [&](char c) {
             if (c >= '0' && c <= '9') return c - '0';
             if (c >= 'A' && c <= 'F') return c - 'A' + 10;
             if (c >= 'a' && c <= 'f') return c - 'a' + 10;
-            throw BadHash("invalid base-16 hash '%s'", s);
+            throw BadHash("invalid base-16 hash '%s'", original);
         };
 
         for (unsigned int i = 0; i < hashSize; i++) {
             hash[i] =
-                parseHexDigit(s[pos + i * 2]) << 4
-                | parseHexDigit(s[pos + i * 2 + 1]);
+                parseHexDigit(rest[pos + i * 2]) << 4
+                | parseHexDigit(rest[pos + i * 2 + 1]);
         }
     }
 
-    else if (!isSRI && size == base32Len()) {
+    else if (!isSRI && rest.size() == base32Len()) {
 
-        for (unsigned int n = 0; n < size; ++n) {
-            char c = s[pos + size - n - 1];
+        for (unsigned int n = 0; n < rest.size(); ++n) {
+            char c = rest[rest.size() - n - 1];
             unsigned char digit;
             for (digit = 0; digit < base32Chars.size(); ++digit) /* !!! slow */
                 if (base32Chars[digit] == c) break;
             if (digit >= 32)
-                throw BadHash("invalid base-32 hash '%s'", s);
+                throw BadHash("invalid base-32 hash '%s'", original);
             unsigned int b = n * 5;
             unsigned int i = b / 8;
             unsigned int j = b % 8;
@@ -202,21 +205,21 @@ Hash::Hash(std::string_view s, std::optional<HashType> type)
                 hash[i + 1] |= digit >> (8 - j);
             } else {
                 if (digit >> (8 - j))
-                    throw BadHash("invalid base-32 hash '%s'", s);
+                    throw BadHash("invalid base-32 hash '%s'", original);
             }
         }
     }
 
-    else if (isSRI || size == base64Len()) {
-        auto d = base64Decode(s.substr(pos));
+    else if (isSRI || rest.size() == base64Len()) {
+        auto d = base64Decode(rest);
         if (d.size() != hashSize)
-            throw BadHash("invalid %s hash '%s'", isSRI ? "SRI" : "base-64", s);
+            throw BadHash("invalid %s hash '%s'", isSRI ? "SRI" : "base-64", original);
         assert(hashSize);
         memcpy(hash, d.data(), hashSize);
     }
 
     else
-        throw BadHash("hash '%s' has wrong length for hash type '%s'", s, printHashType(*type));
+        throw BadHash("hash '%s' has wrong length for hash type '%s'", rest, printHashType(this->type));
 }
 
 Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht)
@@ -269,7 +272,7 @@ static void finish(HashType ht, Ctx & ctx, unsigned char * hash)
 }
 
 
-Hash hashString(HashType ht, const string & s)
+Hash hashString(HashType ht, std::string_view s)
 {
     Ctx ctx;
     Hash hash(ht);
@@ -336,7 +339,7 @@ HashResult hashPath(
 
 Hash compressHash(const Hash & hash, unsigned int newSize)
 {
-    Hash h;
+    Hash h(hash.type);
     h.hashSize = newSize;
     for (unsigned int i = 0; i < hash.hashSize; ++i)
         h.hash[i % newSize] ^= hash.hash[i];
@@ -344,7 +347,7 @@ Hash compressHash(const Hash & hash, unsigned int newSize)
 }
 
 
-std::optional<HashType> parseHashTypeOpt(const string & s)
+std::optional<HashType> parseHashTypeOpt(std::string_view s)
 {
     if (s == "md5") return htMD5;
     else if (s == "sha1") return htSHA1;
@@ -353,7 +356,7 @@ std::optional<HashType> parseHashTypeOpt(const string & s)
     else return std::optional<HashType> {};
 }
 
-HashType parseHashType(const string & s)
+HashType parseHashType(std::string_view s)
 {
     auto opt_h = parseHashTypeOpt(s);
     if (opt_h)
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index ad6093fca..98ee1bed0 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -27,14 +27,11 @@ enum Base : int { Base64, Base32, Base16, SRI };
 
 struct Hash
 {
-    static const unsigned int maxHashSize = 64;
-    unsigned int hashSize = 0;
-    unsigned char hash[maxHashSize] = {};
+    constexpr static size_t maxHashSize = 64;
+    size_t hashSize = 0;
+    uint8_t hash[maxHashSize] = {};
 
-    std::optional<HashType> type = {};
-
-    /* Create an unset hash object. */
-    Hash() { };
+    HashType type;
 
     /* Create a zero-filled hash object. */
     Hash(HashType type) : type(type) { init(); };
@@ -107,7 +104,7 @@ Hash newHashAllowEmpty(std::string hashStr, std::optional<HashType> ht);
 string printHash16or32(const Hash & hash);
 
 /* Compute the hash of the given string. */
-Hash hashString(HashType ht, const string & s);
+Hash hashString(HashType ht, std::string_view s);
 
 /* Compute the hash of the given file. */
 Hash hashFile(HashType ht, const Path & path);
@@ -123,10 +120,10 @@ HashResult hashPath(HashType ht, const Path & path,
 Hash compressHash(const Hash & hash, unsigned int newSize);
 
 /* Parse a string representing a hash type. */
-HashType parseHashType(const string & s);
+HashType parseHashType(std::string_view s);
 
 /* Will return nothing on parse error */
-std::optional<HashType> parseHashTypeOpt(const string & s);
+std::optional<HashType> parseHashTypeOpt(std::string_view s);
 
 /* And the reverse. */
 string printHashType(HashType ht);
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 93798a765..a0a8ff4d3 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -1581,7 +1581,7 @@ AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
 
     struct sockaddr_un addr;
     addr.sun_family = AF_UNIX;
-    if (path.size() >= sizeof(addr.sun_path))
+    if (path.size() + 1 >= sizeof(addr.sun_path))
         throw Error("socket path '%1%' is too long", path);
     strcpy(addr.sun_path, path.c_str());
 
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index 961e7fb6d..65d8ec6b6 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -154,7 +154,7 @@ static int _main(int argc, char * * argv)
         /* If an expected hash is given, the file may already exist in
            the store. */
         std::optional<Hash> expectedHash;
-        Hash hash;
+        Hash hash(ht);
         std::optional<StorePath> storePath;
         if (args.size() == 2) {
             expectedHash = Hash(args[1], ht);
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 23bb48d88..76bf5154a 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -77,7 +77,7 @@ static PathSet realisePath(StorePathWithOutputs path, bool build = true)
             if (i == drv.outputs.end())
                 throw Error("derivation '%s' does not have an output named '%s'",
                     store2->printStorePath(path.path), j);
-            auto outPath = store2->printStorePath(i->second.path);
+            auto outPath = store2->printStorePath(i->second.path(*store, drv.name));
             if (store2) {
                 if (gcRoot == "")
                     printGCWarning();
@@ -219,7 +219,7 @@ static StorePathSet maybeUseOutputs(const StorePath & storePath, bool useOutput,
         auto drv = store->derivationFromPath(storePath);
         StorePathSet outputs;
         for (auto & i : drv.outputs)
-            outputs.insert(i.second.path);
+            outputs.insert(i.second.path(*store, drv.name));
         return outputs;
     }
     else return {storePath};
@@ -313,7 +313,7 @@ static void opQuery(Strings opFlags, Strings opArgs)
                 if (forceRealise) realisePath({i2});
                 Derivation drv = store->derivationFromPath(i2);
                 for (auto & j : drv.outputs)
-                    cout << fmt("%1%\n", store->printStorePath(j.second.path));
+                    cout << fmt("%1%\n", store->printStorePath(j.second.path(*store, drv.name)));
             }
             break;
         }
@@ -372,8 +372,8 @@ static void opQuery(Strings opFlags, Strings opArgs)
                 for (auto & j : maybeUseOutputs(store->followLinksToStorePath(i), useOutput, forceRealise)) {
                     auto info = store->queryPathInfo(j);
                     if (query == qHash) {
-                        assert(info->narHash.type == htSHA256);
-                        cout << fmt("%s\n", info->narHash.to_string(Base32, true));
+                        assert(info->narHash && info->narHash->type == htSHA256);
+                        cout << fmt("%s\n", info->narHash->to_string(Base32, true));
                     } else if (query == qSize)
                         cout << fmt("%d\n", info->narSize);
                 }
@@ -725,7 +725,7 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
         auto path = store->followLinksToStorePath(i);
         printMsg(lvlTalkative, "checking path '%s'...", store->printStorePath(path));
         auto info = store->queryPathInfo(path);
-        HashSink sink(*info->narHash.type);
+        HashSink sink(info->narHash->type);
         store->narFromPath(path, sink);
         auto current = sink.finish();
         if (current.first != info->narHash) {
@@ -734,7 +734,7 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
                 .hint = hintfmt(
                     "path '%s' was modified! expected hash '%s', got '%s'",
                     store->printStorePath(path),
-                    info->narHash.to_string(Base32, true),
+                    info->narHash->to_string(Base32, true),
                     current.first.to_string(Base32, true))
             });
             status = 1;
@@ -864,7 +864,9 @@ static void opServe(Strings opFlags, Strings opArgs)
                         out << info->narSize // downloadSize
                             << info->narSize;
                         if (GET_PROTOCOL_MINOR(clientVersion) >= 4)
-                            out << (info->narHash ? info->narHash.to_string(Base32, true) : "") << renderContentAddress(info->ca) << info->sigs;
+                            out << (info->narHash ? info->narHash->to_string(Base32, true) : "")
+                                << renderContentAddress(info->ca)
+                                << info->sigs;
                     } catch (InvalidPath &) {
                     }
                 }
@@ -914,9 +916,9 @@ static void opServe(Strings opFlags, Strings opArgs)
 
                 if (!writeAllowed) throw Error("building paths is not allowed");
 
-                auto drvPath = store->parseStorePath(readString(in)); // informational only
+                auto drvPath = store->parseStorePath(readString(in));
                 BasicDerivation drv;
-                readDerivation(in, *store, drv);
+                readDerivation(in, *store, drv, Derivation::nameFromPath(drvPath));
 
                 getBuildSettings();
 
diff --git a/src/nix/add-to-store.cc b/src/nix/add-to-store.cc
index f9d6de16e..ad1f9e91f 100644
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@@ -50,7 +50,7 @@ struct CmdAddToStore : MixDryRun, StoreCommand
         info.narSize = sink.s->size();
         info.ca = std::optional { FixedOutputHash {
             .method = FileIngestionMethod::Recursive,
-            .hash = info.narHash,
+            .hash = *info.narHash,
         } };
 
         if (!dryRun) {
diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index a6d7d6add..a0c119e43 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -68,22 +68,22 @@ BuildEnvironment readEnvironment(const Path & path)
 
         std::smatch match;
 
-        if (std::regex_search(pos, file.cend(), match, declareRegex)) {
+        if (std::regex_search(pos, file.cend(), match, declareRegex, std::regex_constants::match_continuous)) {
             pos = match[0].second;
             exported.insert(match[1]);
         }
 
-        else if (std::regex_search(pos, file.cend(), match, varRegex)) {
+        else if (std::regex_search(pos, file.cend(), match, varRegex, std::regex_constants::match_continuous)) {
             pos = match[0].second;
             res.env.insert({match[1], Var { .exported = exported.count(match[1]) > 0, .value = match[2] }});
         }
 
-        else if (std::regex_search(pos, file.cend(), match, assocArrayRegex)) {
+        else if (std::regex_search(pos, file.cend(), match, assocArrayRegex, std::regex_constants::match_continuous)) {
             pos = match[0].second;
             res.env.insert({match[1], Var { .associative = true, .value = match[2] }});
         }
 
-        else if (std::regex_search(pos, file.cend(), match, functionRegex)) {
+        else if (std::regex_search(pos, file.cend(), match, functionRegex, std::regex_constants::match_continuous)) {
             res.bashFunctions = std::string(pos, file.cend());
             break;
         }
@@ -130,14 +130,16 @@ StorePath getDerivationEnvironment(ref<Store> store, const StorePath & drvPath)
     drvName += "-env";
     for (auto & output : drv.outputs)
         drv.env.erase(output.first);
-    drv.outputs = {{"out", DerivationOutput { .path = StorePath::dummy }}};
+    drv.outputs = {{"out", DerivationOutput { .output = DerivationOutputInputAddressed { .path = StorePath::dummy }}}};
     drv.env["out"] = "";
     drv.env["_outputs_saved"] = drv.env["outputs"];
     drv.env["outputs"] = "out";
     drv.inputSrcs.insert(std::move(getEnvShPath));
     Hash h = hashDerivationModulo(*store, drv, true);
     auto shellOutPath = store->makeOutputPath("out", h, drvName);
-    drv.outputs.insert_or_assign("out", DerivationOutput { .path = shellOutPath });
+    drv.outputs.insert_or_assign("out", DerivationOutput { .output = DerivationOutputInputAddressed {
+                .path = shellOutPath
+            } });
     drv.env["out"] = store->printStorePath(shellOutPath);
     auto shellDrvPath2 = writeDerivation(store, drv, drvName);
 
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index a13e5a3df..72aae7ef7 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -276,7 +276,7 @@ std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
 Installable::getCursors(EvalState & state, bool useEvalCache)
 {
     auto evalCache =
-        std::make_shared<nix::eval_cache::EvalCache>(false, Hash(), state,
+        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
             [&]() { return toValue(state).first; });
     return {{evalCache->getRoot(), ""}};
 }
@@ -304,8 +304,9 @@ struct InstallableStorePath : Installable
     {
         if (storePath.isDerivation()) {
             std::map<std::string, StorePath> outputs;
-            for (auto & [name, output] : store->readDerivation(storePath).outputs)
-                outputs.emplace(name, output.path);
+            auto drv = store->readDerivation(storePath);
+            for (auto & [name, output] : drv.outputs)
+                outputs.emplace(name, output.path(*store, drv.name));
             return {
                 Buildable {
                     .drvPath = storePath,
@@ -436,9 +437,11 @@ ref<eval_cache::EvalCache> openEvalCache(
     std::shared_ptr<flake::LockedFlake> lockedFlake,
     bool useEvalCache)
 {
-    return ref(std::make_shared<nix::eval_cache::EvalCache>(
-        useEvalCache && evalSettings.pureEval,
-        lockedFlake->getFingerprint(),
+	auto fingerprint = lockedFlake->getFingerprint();
+    return make_ref<nix::eval_cache::EvalCache>(
+        useEvalCache && evalSettings.pureEval
+            ? std::optional { std::cref(fingerprint) }
+            : std::nullopt,
         state,
         [&state, lockedFlake]()
         {
@@ -456,7 +459,7 @@ ref<eval_cache::EvalCache> openEvalCache(
             assert(aOutputs);
 
             return aOutputs->value;
-        }));
+        });
 }
 
 static std::string showAttrPaths(const std::vector<std::string> & paths)
diff --git a/src/nix/make-content-addressable.cc b/src/nix/make-content-addressable.cc
index 712043978..2fe2e2fb2 100644
--- a/src/nix/make-content-addressable.cc
+++ b/src/nix/make-content-addressable.cc
@@ -84,7 +84,7 @@ struct CmdMakeContentAddressable : StorePathsCommand, MixJSON
             info.narSize = sink.s->size();
             info.ca = FixedOutputHash {
                 .method = FileIngestionMethod::Recursive,
-                .hash = info.narHash,
+                .hash = *info.narHash,
             };
 
             if (!json)
diff --git a/src/nix/profile.cc b/src/nix/profile.cc
index c6cd88c49..7dcc0b6d4 100644
--- a/src/nix/profile.cc
+++ b/src/nix/profile.cc
@@ -133,7 +133,7 @@ struct ProfileManifest
         info.references = std::move(references);
         info.narHash = narHash;
         info.narSize = sink.s->size();
-        info.ca = FixedOutputHash { .method = FileIngestionMethod::Recursive, .hash = info.narHash };
+        info.ca = FixedOutputHash { .method = FileIngestionMethod::Recursive, .hash = *info.narHash };
 
         auto source = StringSource { *sink.s };
         store->addToStore(info, source);
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 8eb58f62a..fb9050d0d 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -483,10 +483,10 @@ 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) {
-                auto drv = readDerivation(*state->store, drvPath);
+                auto drv = readDerivation(*state->store, drvPath, Derivation::nameFromPath(state->store->parseStorePath(drvPath)));
                 std::cout << std::endl << "this derivation produced the following outputs:" << std::endl;
                 for (auto & i : drv.outputs)
-                    std::cout << fmt("  %s -> %s\n", i.first, state->store->printStorePath(i.second.path));
+                    std::cout << fmt("  %s -> %s\n", i.first, state->store->printStorePath(i.second.path(*state->store, drv.name)));
             }
         } else if (command == ":i") {
             runProgram(settings.nixBinDir + "/nix-env", Strings{"-i", drvPath});
diff --git a/src/nix/show-derivation.cc b/src/nix/show-derivation.cc
index b5434f982..9fd26e2d7 100644
--- a/src/nix/show-derivation.cc
+++ b/src/nix/show-derivation.cc
@@ -69,10 +69,10 @@ struct CmdShowDerivation : InstallablesCommand
                 auto outputsObj(drvObj.object("outputs"));
                 for (auto & output : drv.outputs) {
                     auto outputObj(outputsObj.object(output.first));
-                    outputObj.attr("path", store->printStorePath(output.second.path));
-                    if (output.second.hash) {
-                        outputObj.attr("hashAlgo", output.second.hash->printMethodAlgo());
-                        outputObj.attr("hash", output.second.hash->hash.to_string(Base16, false));
+                    outputObj.attr("path", store->printStorePath(output.second.path(*store, drv.name)));
+                    if (auto hash = std::get_if<DerivationOutputFixed>(&output.second.output)) {
+                        outputObj.attr("hashAlgo", hash->hash.printMethodAlgo());
+                        outputObj.attr("hash", hash->hash.hash.to_string(Base16, false));
                     }
                 }
             }
diff --git a/src/nix/verify.cc b/src/nix/verify.cc
index ce90b0f6d..fc7a9765c 100644
--- a/src/nix/verify.cc
+++ b/src/nix/verify.cc
@@ -91,15 +91,15 @@ struct CmdVerify : StorePathsCommand
 
                     std::unique_ptr<AbstractHashSink> hashSink;
                     if (!info->ca)
-                        hashSink = std::make_unique<HashSink>(*info->narHash.type);
+                        hashSink = std::make_unique<HashSink>(info->narHash->type);
                     else
-                        hashSink = std::make_unique<HashModuloSink>(*info->narHash.type, std::string(info->path.hashPart()));
+                        hashSink = std::make_unique<HashModuloSink>(info->narHash->type, std::string(info->path.hashPart()));
 
                     store->narFromPath(info->path, *hashSink);
 
                     auto hash = hashSink->finish();
 
-                    if (hash.first != info->narHash) {
+                    if (hash.first != *info->narHash) {
                         corrupted++;
                         act2.result(resCorruptedPath, store->printStorePath(info->path));
                         logError({
@@ -107,7 +107,7 @@ struct CmdVerify : StorePathsCommand
                             .hint = hintfmt(
                                 "path '%s' was modified! expected hash '%s', got '%s'",
                                 store->printStorePath(info->path),
-                                info->narHash.to_string(Base32, true),
+                                info->narHash->to_string(Base32, true),
                                 hash.first.to_string(Base32, true))
                         });
                     }
diff --git a/tests/fetchGit.sh b/tests/fetchGit.sh
index 9faa5d9f6..cedd796f7 100644
--- a/tests/fetchGit.sh
+++ b/tests/fetchGit.sh
@@ -32,6 +32,8 @@ rev2=$(git -C $repo rev-parse HEAD)
 # Fetch a worktree
 unset _NIX_FORCE_HTTP
 path0=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$TEST_ROOT/worktree).outPath")
+path0_=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; url = file://$TEST_ROOT/worktree; }).outPath")
+[[ $path0 = $path0_ ]]
 export _NIX_FORCE_HTTP=1
 [[ $(tail -n 1 $path0/hello) = "hello" ]]
 
@@ -102,6 +104,12 @@ git -C $repo commit -m 'Bla3' -a
 path4=$(nix eval --impure --refresh --raw --expr "(builtins.fetchGit file://$repo).outPath")
 [[ $path2 = $path4 ]]
 
+nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=\"; }).outPath" || status=$?
+[[ "$status" = "102" ]]
+
+path5=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-Hr8g6AqANb3xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath")
+[[ $path = $path5 ]]
+
 # tarball-ttl should be ignored if we specify a rev
 echo delft > $repo/hello
 git -C $repo add hello
diff --git a/tests/tarball.sh b/tests/tarball.sh
index b3ec16d40..88a1a07a0 100644
--- a/tests/tarball.sh
+++ b/tests/tarball.sh
@@ -27,10 +27,13 @@ test_tarball() {
 
     nix-build -o $TEST_ROOT/result -E "import (fetchTarball file://$tarball)"
 
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree file://$tarball)"
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; })"
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })"
-    nix-build --experimental-features flakes -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" 2>&1 | grep 'NAR hash mismatch in input'
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree file://$tarball)"
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; })"
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })"
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" 2>&1 | grep 'NAR hash mismatch in input'
+
+    nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" >&2
+    nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" 2>&1 | grep 'true'
 
     nix-instantiate --eval -E '1 + 2' -I fnord=file://no-such-tarball.tar$ext
     nix-instantiate --eval -E 'with <fnord/xyzzy>; 1 + 2' -I fnord=file://no-such-tarball$ext