From 94812cca98fbb157e5f64a15a85a2b852d289feb Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 6 Apr 2023 13:15:50 +0200
Subject: [PATCH 1/4] Backport SourcePath from the lazy-trees branch

This introduces the SourcePath type from lazy-trees as an abstraction
for accessing files from inputs that may not be materialized in the
real filesystem (e.g. Git repositories). Currently, however, it's just
a wrapper around CanonPath, so it shouldn't change any behaviour. (On
lazy-trees, SourcePath is a <InputAccessor, CanonPath> tuple.)
---
 src/libcmd/common-eval-args.cc         |  10 +-
 src/libcmd/common-eval-args.hh         |   3 +-
 src/libcmd/editor-for.cc               |   7 +-
 src/libcmd/editor-for.hh               |   3 +-
 src/libcmd/installable-flake.cc        |   3 +-
 src/libcmd/installables.cc             |   2 +-
 src/libcmd/repl.cc                     |  12 +-
 src/libexpr/attr-path.cc               |  26 +++--
 src/libexpr/attr-path.hh               |   2 +-
 src/libexpr/eval-cache.cc              |  10 +-
 src/libexpr/eval.cc                    |  94 ++++++++--------
 src/libexpr/eval.hh                    |  56 +++++-----
 src/libexpr/flake/flake.cc             |   4 +-
 src/libexpr/parser.y                   |  52 ++++-----
 src/libexpr/paths.cc                   |  10 ++
 src/libexpr/primops.cc                 | 122 ++++++++++----------
 src/libexpr/tests/libexpr.hh           |   2 +-
 src/libexpr/tests/local.mk             |   2 +-
 src/libexpr/value-to-json.cc           |   5 +-
 src/libexpr/value-to-xml.cc            |   2 +-
 src/libexpr/value.hh                   |  28 ++++-
 src/libfetchers/input-accessor.cc      | 100 +++++++++++++++++
 src/libfetchers/input-accessor.hh      | 149 +++++++++++++++++++++++++
 src/libutil/canon-path.cc              |   5 +
 src/libutil/canon-path.hh              |   2 +
 src/nix-build/nix-build.cc             |   6 +-
 src/nix-env/nix-env.cc                 |  70 ++++++------
 src/nix-env/user-env.cc                |   6 +-
 src/nix-instantiate/nix-instantiate.cc |  10 +-
 src/nix/eval.cc                        |   2 +-
 src/nix/flake.cc                       |   4 +-
 src/nix/main.cc                        |   6 +-
 src/nix/prefetch.cc                    |  11 +-
 src/nix/upgrade-nix.cc                 |   2 +-
 tests/plugins/local.mk                 |   2 +-
 35 files changed, 567 insertions(+), 263 deletions(-)
 create mode 100644 src/libexpr/paths.cc
 create mode 100644 src/libfetchers/input-accessor.cc
 create mode 100644 src/libfetchers/input-accessor.hh

diff --git a/src/libcmd/common-eval-args.cc b/src/libcmd/common-eval-args.cc
index 5b6477c82..ff3abd534 100644
--- a/src/libcmd/common-eval-args.cc
+++ b/src/libcmd/common-eval-args.cc
@@ -153,7 +153,7 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     for (auto & i : autoArgs) {
         auto v = state.allocValue();
         if (i.second[0] == 'E')
-            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), absPath(".")));
+            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), state.rootPath(CanonPath::fromCwd())));
         else
             v->mkString(((std::string_view) i.second).substr(1));
         res.insert(state.symbols.create(i.first), v);
@@ -161,19 +161,19 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     return res.finish();
 }
 
-Path lookupFileArg(EvalState & state, std::string_view s)
+SourcePath lookupFileArg(EvalState & state, std::string_view s)
 {
     if (EvalSettings::isPseudoUrl(s)) {
         auto storePath = fetchers::downloadTarball(
             state.store, EvalSettings::resolvePseudoUrl(s), "source", false).first.storePath;
-        return state.store->toRealPath(storePath);
+        return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }
 
     else if (hasPrefix(s, "flake:")) {
         experimentalFeatureSettings.require(Xp::Flakes);
         auto flakeRef = parseFlakeRef(std::string(s.substr(6)), {}, true, false);
         auto storePath = flakeRef.resolve(state.store).fetchTree(state.store).first.storePath;
-        return state.store->toRealPath(storePath);
+        return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }
 
     else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
@@ -182,7 +182,7 @@ Path lookupFileArg(EvalState & state, std::string_view s)
     }
 
     else
-        return absPath(std::string(s));
+        return state.rootPath(CanonPath::fromCwd(s));
 }
 
 }
diff --git a/src/libcmd/common-eval-args.hh b/src/libcmd/common-eval-args.hh
index b69db11dd..83edcfb85 100644
--- a/src/libcmd/common-eval-args.hh
+++ b/src/libcmd/common-eval-args.hh
@@ -8,6 +8,7 @@ namespace nix {
 class Store;
 class EvalState;
 class Bindings;
+struct SourcePath;
 
 struct MixEvalArgs : virtual Args
 {
@@ -25,6 +26,6 @@ private:
     std::map<std::string, std::string> autoArgs;
 };
 
-Path lookupFileArg(EvalState & state, std::string_view s);
+SourcePath lookupFileArg(EvalState & state, std::string_view s);
 
 }
diff --git a/src/libcmd/editor-for.cc b/src/libcmd/editor-for.cc
index f674f32bd..a17c6f12a 100644
--- a/src/libcmd/editor-for.cc
+++ b/src/libcmd/editor-for.cc
@@ -3,8 +3,11 @@
 
 namespace nix {
 
-Strings editorFor(const Path & file, uint32_t line)
+Strings editorFor(const SourcePath & file, uint32_t line)
 {
+    auto path = file.getPhysicalPath();
+    if (!path)
+        throw Error("cannot open '%s' in an editor because it has no physical path", file);
     auto editor = getEnv("EDITOR").value_or("cat");
     auto args = tokenizeString<Strings>(editor);
     if (line > 0 && (
@@ -13,7 +16,7 @@ Strings editorFor(const Path & file, uint32_t line)
         editor.find("vim") != std::string::npos ||
         editor.find("kak") != std::string::npos))
         args.push_back(fmt("+%d", line));
-    args.push_back(file);
+    args.push_back(path->abs());
     return args;
 }
 
diff --git a/src/libcmd/editor-for.hh b/src/libcmd/editor-for.hh
index f752bd849..c4873921f 100644
--- a/src/libcmd/editor-for.hh
+++ b/src/libcmd/editor-for.hh
@@ -2,11 +2,12 @@
 ///@file
 
 #include "types.hh"
+#include "input-accessor.hh"
 
 namespace nix {
 
 /* Helper function to generate args that invoke $EDITOR on
    filename:lineno. */
-Strings editorFor(const Path & file, uint32_t line);
+Strings editorFor(const SourcePath & file, uint32_t line);
 
 }
diff --git a/src/libcmd/installable-flake.cc b/src/libcmd/installable-flake.cc
index a3352af76..19e982df1 100644
--- a/src/libcmd/installable-flake.cc
+++ b/src/libcmd/installable-flake.cc
@@ -96,8 +96,7 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
         auto v = attr->forceValue();
 
         if (v.type() == nPath) {
-            PathSet context;
-            auto storePath = state->copyPathToStore(context, Path(v.path));
+            auto storePath = v.path().fetchToStore(state->store);
             return {{
                 .path = DerivedPath::Opaque {
                     .path = std::move(storePath),
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 67549b280..1873d175a 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -427,7 +427,7 @@ Installables SourceExprCommand::parseInstallables(
         else if (file)
             state->evalFile(lookupFileArg(*state, *file), *vFile);
         else {
-            auto e = state->parseExprFromString(*expr, absPath("."));
+            auto e = state->parseExprFromString(*expr, state->rootPath(CanonPath::fromCwd()));
             state->eval(e, *vFile);
         }
 
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index e3afb1531..9002fa555 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -54,8 +54,6 @@ struct NixRepl
     , gc
     #endif
 {
-    std::string curDir;
-
     size_t debugTraceIndex;
 
     Strings loadedFiles;
@@ -113,7 +111,6 @@ NixRepl::NixRepl(const Strings & searchPath, nix::ref<Store> store, ref<EvalStat
     , staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
     , historyFile(getDataDir() + "/nix/repl-history")
 {
-    curDir = absPath(".");
 }
 
 
@@ -590,7 +587,7 @@ bool NixRepl::processLine(std::string line)
         Value v;
         evalString(arg, v);
 
-        const auto [path, line] = [&] () -> std::pair<Path, uint32_t> {
+        const auto [path, line] = [&] () -> std::pair<SourcePath, uint32_t> {
             if (v.type() == nPath || v.type() == nString) {
                 PathSet context;
                 auto path = state->coerceToPath(noPos, v, context, "while evaluating the filename to edit");
@@ -598,7 +595,7 @@ bool NixRepl::processLine(std::string line)
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
                 if (auto path = std::get_if<Path>(&pos.origin))
-                    return {*path, pos.line};
+                    return {SourcePath(CanonPath(*path)), pos.line};
                 else
                     throw Error("'%s' cannot be shown in an editor", pos);
             } else {
@@ -872,8 +869,7 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 
 Expr * NixRepl::parseString(std::string s)
 {
-    Expr * e = state->parseExprFromString(std::move(s), curDir, staticEnv);
-    return e;
+    return state->parseExprFromString(std::move(s), state->rootPath(CanonPath::fromCwd()), staticEnv);
 }
 
 
@@ -930,7 +926,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
         break;
 
     case nPath:
-        str << ANSI_GREEN << v.path << ANSI_NORMAL; // !!! escaping?
+        str << ANSI_GREEN << v.path().to_string() << ANSI_NORMAL; // !!! escaping?
         break;
 
     case nNull:
diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 7c0705091..8ae4270e6 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -106,7 +106,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(EvalState & state, const std::strin
 }
 
 
-std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what)
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what)
 {
     Value * v2;
     try {
@@ -118,21 +118,25 @@ std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value &
 
     // FIXME: is it possible to extract the Pos object instead of doing this
     //        toString + parsing?
-    auto pos = state.forceString(*v2, noPos, "while evaluating the 'meta.position' attribute of a derivation");
+    PathSet context;
+    auto path = state.coerceToPath(noPos, *v2, context, "while evaluating the 'meta.position' attribute of a derivation");
 
-    auto colon = pos.rfind(':');
-    if (colon == std::string::npos)
-        throw ParseError("cannot parse meta.position attribute '%s'", pos);
+    auto fn = path.path.abs();
+
+    auto fail = [fn]() {
+        throw ParseError("cannot parse 'meta.position' attribute '%s'", fn);
+    };
 
-    std::string filename(pos, 0, colon);
-    unsigned int lineno;
     try {
-        lineno = std::stoi(std::string(pos, colon + 1, std::string::npos));
+        auto colon = fn.rfind(':');
+        if (colon == std::string::npos) fail();
+        std::string filename(fn, 0, colon);
+        auto lineno = std::stoi(std::string(fn, colon + 1, std::string::npos));
+        return {CanonPath(fn.substr(0, colon)), lineno};
     } catch (std::invalid_argument & e) {
-        throw ParseError("cannot parse line number '%s'", pos);
+        fail();
+        abort();
     }
-
-    return { std::move(filename), lineno };
 }
 
 
diff --git a/src/libexpr/attr-path.hh b/src/libexpr/attr-path.hh
index d0d05b1a1..dee811fe1 100644
--- a/src/libexpr/attr-path.hh
+++ b/src/libexpr/attr-path.hh
@@ -18,7 +18,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(
     Value & vIn);
 
 /* Heuristic to find the filename and lineno or a nix value. */
-std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
 
 std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);
 
diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 1219b2471..ec4ad2f5e 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -442,8 +442,10 @@ Value & AttrCursor::forceValue()
         if (v.type() == nString)
             cachedValue = {root->db->setString(getKey(), v.string.s, v.string.context),
                            string_t{v.string.s, {}}};
-        else if (v.type() == nPath)
-            cachedValue = {root->db->setString(getKey(), v.path), string_t{v.path, {}}};
+        else if (v.type() == nPath) {
+            auto path = v.path().path;
+            cachedValue = {root->db->setString(getKey(), path.abs()), string_t{path.abs(), {}}};
+        }
         else if (v.type() == nBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
         else if (v.type() == nInt)
@@ -580,7 +582,7 @@ std::string AttrCursor::getString()
     if (v.type() != nString && v.type() != nPath)
         root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 
-    return v.type() == nString ? v.string.s : v.path;
+    return v.type() == nString ? v.string.s : v.path().to_string();
 }
 
 string_t AttrCursor::getStringWithContext()
@@ -622,7 +624,7 @@ string_t AttrCursor::getStringWithContext()
     if (v.type() == nString)
         return {v.string.s, v.getContext(*root->state.store)};
     else if (v.type() == nPath)
-        return {v.path, {}};
+        return {v.path().to_string(), {}};
     else
         root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 }
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 584bbc879..3b2877602 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -118,7 +118,7 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
         str << "\"";
         break;
     case tPath:
-        str << path; // !!! escaping?
+        str << path().to_string(); // !!! escaping?
         break;
     case tNull:
         str << "null";
@@ -577,11 +577,11 @@ void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value &
     v.mkString(path, PathSet({path}));
 }
 
-Path EvalState::checkSourcePath(const Path & path_)
+SourcePath EvalState::checkSourcePath(const SourcePath & path_)
 {
     if (!allowedPaths) return path_;
 
-    auto i = resolvedPaths.find(path_);
+    auto i = resolvedPaths.find(path_.path.abs());
     if (i != resolvedPaths.end())
         return i->second;
 
@@ -591,9 +591,9 @@ Path EvalState::checkSourcePath(const Path & path_)
      * attacker can't append ../../... to a path that would be in allowedPaths
      * and thus leak symlink targets.
      */
-    Path abspath = canonPath(path_);
+    Path abspath = canonPath(path_.path.abs());
 
-    if (hasPrefix(abspath, corepkgsPrefix)) return abspath;
+    if (hasPrefix(abspath, corepkgsPrefix)) return CanonPath(abspath);
 
     for (auto & i : *allowedPaths) {
         if (isDirOrInDir(abspath, i)) {
@@ -611,11 +611,11 @@ Path EvalState::checkSourcePath(const Path & path_)
 
     /* Resolve symlinks. */
     debug("checking access to '%s'", abspath);
-    Path path = canonPath(abspath, true);
+    SourcePath path = CanonPath(canonPath(abspath, true));
 
     for (auto & i : *allowedPaths) {
-        if (isDirOrInDir(path, i)) {
-            resolvedPaths[path_] = path;
+        if (isDirOrInDir(path.path.abs(), i)) {
+            resolvedPaths.insert_or_assign(path_.path.abs(), path);
             return path;
         }
     }
@@ -643,12 +643,12 @@ void EvalState::checkURI(const std::string & uri)
     /* If the URI is a path, then check it against allowedPaths as
        well. */
     if (hasPrefix(uri, "/")) {
-        checkSourcePath(uri);
+        checkSourcePath(CanonPath(uri));
         return;
     }
 
     if (hasPrefix(uri, "file://")) {
-        checkSourcePath(std::string(uri, 7));
+        checkSourcePath(CanonPath(std::string(uri, 7)));
         return;
     }
 
@@ -933,9 +933,9 @@ void Value::mkStringMove(const char * s, const PathSet & context)
 }
 
 
-void Value::mkPath(std::string_view s)
+void Value::mkPath(const SourcePath & path)
 {
-    mkPath(makeImmutableString(s));
+    mkPath(makeImmutableString(path.path.abs()));
 }
 
 
@@ -1049,7 +1049,7 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
 }
 
 
-void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
+void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial)
 {
     auto path = checkSourcePath(path_);
 
@@ -1059,7 +1059,7 @@ void EvalState::evalFile(const Path & path_, Value & v, bool mustBeTrivial)
         return;
     }
 
-    Path resolvedPath = resolveExprPath(path);
+    auto resolvedPath = resolveExprPath(path);
     if ((i = fileEvalCache.find(resolvedPath)) != fileEvalCache.end()) {
         v = i->second;
         return;
@@ -1087,8 +1087,8 @@ void EvalState::resetFileCache()
 
 
 void EvalState::cacheFile(
-    const Path & path,
-    const Path & resolvedPath,
+    const SourcePath & path,
+    const SourcePath & resolvedPath,
     Expr * e,
     Value & v,
     bool mustBeTrivial)
@@ -1102,7 +1102,7 @@ void EvalState::cacheFile(
                 *e,
                 this->baseEnv,
                 e->getPos() ? static_cast<std::shared_ptr<AbstractPos>>(positions[e->getPos()]) : nullptr,
-                "while evaluating the file '%1%':", resolvedPath)
+                "while evaluating the file '%1%':", resolvedPath.to_string())
             : nullptr;
 
         // Enforce that 'flake.nix' is a direct attrset, not a
@@ -1112,7 +1112,7 @@ void EvalState::cacheFile(
             error("file '%s' must be an attribute set", path).debugThrow<EvalError>();
         eval(e, v);
     } catch (Error & e) {
-        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath);
+        addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath.to_string());
         throw;
     }
 
@@ -1947,7 +1947,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     else if (firstType == nPath) {
         if (!context.empty())
             state.error("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow<EvalError>();
-        v.mkPath(canonPath(str()));
+        v.mkPath(CanonPath(canonPath(str())));
     } else
         v.mkStringMove(c_str(), context);
 }
@@ -2136,8 +2136,14 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
     return {};
 }
 
-BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &context,
-    std::string_view errorCtx, bool coerceMore, bool copyToStore, bool canonicalizePath)
+BackedStringView EvalState::coerceToString(
+    const PosIdx pos,
+    Value & v,
+    PathSet & context,
+    std::string_view errorCtx,
+    bool coerceMore,
+    bool copyToStore,
+    bool canonicalizePath)
 {
     forceValue(v, pos);
 
@@ -2147,12 +2153,14 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &
     }
 
     if (v.type() == nPath) {
-        BackedStringView path(PathView(v.path));
-        if (canonicalizePath)
-            path = canonPath(*path);
-        if (copyToStore)
-            path = store->printStorePath(copyPathToStore(context, std::move(path).toOwned()));
-        return path;
+        return
+            !canonicalizePath && !copyToStore
+            ? // FIXME: hack to preserve path literals that end in a
+              // slash, as in /foo/${x}.
+              v._path
+            : copyToStore
+            ? store->printStorePath(copyPathToStore(context, v.path()))
+            : std::string(v.path().path.abs());
     }
 
     if (v.type() == nAttrs) {
@@ -2213,36 +2221,34 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &
 }
 
 
-StorePath EvalState::copyPathToStore(PathSet & context, const Path & path)
+StorePath EvalState::copyPathToStore(PathSet & context, const SourcePath & path)
 {
-    if (nix::isDerivation(path))
+    if (nix::isDerivation(path.path.abs()))
         error("file names are not allowed to end in '%1%'", drvExtension).debugThrow<EvalError>();
 
-    auto dstPath = [&]() -> StorePath
-    {
-        auto i = srcToStore.find(path);
-        if (i != srcToStore.end()) return i->second;
+    auto i = srcToStore.find(path);
 
-        auto dstPath = settings.readOnlyMode
-            ? store->computeStorePathForPath(std::string(baseNameOf(path)), checkSourcePath(path)).first
-            : store->addToStore(std::string(baseNameOf(path)), checkSourcePath(path), FileIngestionMethod::Recursive, htSHA256, defaultPathFilter, repair);
-        allowPath(dstPath);
-        srcToStore.insert_or_assign(path, dstPath);
-        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
-        return dstPath;
-    }();
+    auto dstPath = i != srcToStore.end()
+        ? i->second
+        : [&]() {
+            auto dstPath = path.fetchToStore(store, path.baseName(), nullptr, repair);
+            allowPath(dstPath);
+            srcToStore.insert_or_assign(path, dstPath);
+            printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
+            return dstPath;
+        }();
 
     context.insert(store->printStorePath(dstPath));
     return dstPath;
 }
 
 
-Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
+SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
     auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (path == "" || path[0] != '/')
         error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
-    return path;
+    return CanonPath(path);
 }
 
 
@@ -2285,7 +2291,7 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
             return strcmp(v1.string.s, v2.string.s) == 0;
 
         case nPath:
-            return strcmp(v1.path, v2.path) == 0;
+            return strcmp(v1._path, v2._path) == 0;
 
         case nNull:
             return true;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index a1b54951e..1e8f8391c 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -8,6 +8,7 @@
 #include "symbol-table.hh"
 #include "config.hh"
 #include "experimental-features.hh"
+#include "input-accessor.hh"
 
 #include <map>
 #include <optional>
@@ -56,15 +57,11 @@ std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const Stati
 void copyContext(const Value & v, PathSet & context);
 
 
-/* Cache for calls to addToStore(); maps source paths to the store
-   paths. */
-typedef std::map<Path, StorePath> SrcToStore;
-
-
 std::string printValue(const EvalState & state, const Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
+// FIXME: maybe change this to an std::variant<SourcePath, URL>.
 typedef std::pair<std::string, std::string> SearchPathElem;
 typedef std::list<SearchPathElem> SearchPath;
 
@@ -217,21 +214,24 @@ public:
     }
 
 private:
-    SrcToStore srcToStore;
+
+    /* Cache for calls to addToStore(); maps source paths to the store
+       paths. */
+    std::map<SourcePath, StorePath> srcToStore;
 
     /* A cache from path names to parse trees. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *>>> FileParseCache;
+    typedef std::map<SourcePath, Expr *, std::less<SourcePath>, traceable_allocator<std::pair<const SourcePath, Expr *>>> FileParseCache;
 #else
-    typedef std::map<Path, Expr *> FileParseCache;
+    typedef std::map<SourcePath, Expr *> FileParseCache;
 #endif
     FileParseCache fileParseCache;
 
     /* A cache from path names to values. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value>>> FileEvalCache;
+    typedef std::map<SourcePath, Value, std::less<SourcePath>, traceable_allocator<std::pair<const SourcePath, Value>>> FileEvalCache;
 #else
-    typedef std::map<Path, Value> FileEvalCache;
+    typedef std::map<SourcePath, Value> FileEvalCache;
 #endif
     FileEvalCache fileEvalCache;
 
@@ -240,7 +240,7 @@ private:
     std::map<std::string, std::pair<bool, std::string>> searchPathResolved;
 
     /* Cache used by checkSourcePath(). */
-    std::unordered_map<Path, Path> resolvedPaths;
+    std::unordered_map<Path, SourcePath> resolvedPaths;
 
     /* Cache used by prim_match(). */
     std::shared_ptr<RegexCache> regexCache;
@@ -265,6 +265,12 @@ public:
 
     SearchPath getSearchPath() { return searchPath; }
 
+    /**
+     * Return a `SourcePath` that refers to `path` in the root
+     * filesystem.
+     */
+    SourcePath rootPath(CanonPath path);
+
     /* Allow access to a path. */
     void allowPath(const Path & path);
 
@@ -277,7 +283,7 @@ public:
 
     /* Check whether access to a path is allowed and throw an error if
        not. Otherwise return the canonicalised path. */
-    Path checkSourcePath(const Path & path);
+    SourcePath checkSourcePath(const SourcePath & path);
 
     void checkURI(const std::string & uri);
 
@@ -291,24 +297,24 @@ public:
     Path toRealPath(const Path & path, const PathSet & context);
 
     /* Parse a Nix expression from the specified file. */
-    Expr * parseExprFromFile(const Path & path);
-    Expr * parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromFile(const SourcePath & path);
+    Expr * parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
-    Expr * parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv);
-    Expr * parseExprFromString(std::string s, const Path & basePath);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath);
 
     Expr * parseStdin();
 
     /* Evaluate an expression read from the given file to normal
        form. Optionally enforce that the top-level expression is
        trivial (i.e. doesn't require arbitrary computation). */
-    void evalFile(const Path & path, Value & v, bool mustBeTrivial = false);
+    void evalFile(const SourcePath & path, Value & v, bool mustBeTrivial = false);
 
     /* Like `evalFile`, but with an already parsed expression. */
     void cacheFile(
-        const Path & path,
-        const Path & resolvedPath,
+        const SourcePath & path,
+        const SourcePath & resolvedPath,
         Expr * e,
         Value & v,
         bool mustBeTrivial = false);
@@ -316,8 +322,8 @@ public:
     void resetFileCache();
 
     /* Look up a file in the search path. */
-    Path findFile(const std::string_view path);
-    Path findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
+    SourcePath findFile(const std::string_view path);
+    SourcePath findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
 
     /* If the specified search path element is a URI, download it. */
     std::pair<bool, std::string> resolveSearchPathElem(const SearchPathElem & elem);
@@ -383,12 +389,12 @@ public:
         bool coerceMore = false, bool copyToStore = true,
         bool canonicalizePath = true);
 
-    StorePath copyPathToStore(PathSet & context, const Path & path);
+    StorePath copyPathToStore(PathSet & context, const SourcePath & path);
 
     /* Path coercion.  Converts strings, paths and derivations to a
        path.  The result is guaranteed to be a canonicalised, absolute
        path.  Nothing is copied to the store. */
-    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
+    SourcePath coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
 
     /* Like coerceToPath, but the result must be a store path. */
     StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
@@ -444,7 +450,7 @@ private:
         char * text,
         size_t length,
         Pos::Origin origin,
-        Path basePath,
+        const SourcePath & basePath,
         std::shared_ptr<StaticEnv> & staticEnv);
 
 public:
@@ -556,7 +562,7 @@ std::string_view showType(ValueType type);
 std::string showType(const Value & v);
 
 /* If `path' refers to a directory, then append "/default.nix". */
-Path resolveExprPath(Path path);
+SourcePath resolveExprPath(const SourcePath & path);
 
 struct InvalidPathError : EvalError
 {
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 81e94848a..2b858ee98 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -218,7 +218,7 @@ static Flake getFlake(
         throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
 
     Value vInfo;
-    state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack
+    state.evalFile(CanonPath(flakeFile), vInfo, true); // FIXME: symlink attack
 
     expectType(state, nAttrs, vInfo, state.positions.add({flakeFile}, 1, 1));
 
@@ -731,7 +731,7 @@ void callFlake(EvalState & state,
         state.vCallFlake = allocRootValue(state.allocValue());
         state.eval(state.parseExprFromString(
             #include "call-flake.nix.gen.hh"
-            , "/"), **state.vCallFlake);
+            , CanonPath::root), **state.vCallFlake);
     }
 
     state.callFunction(**state.vCallFlake, *vLocks, *vTmp1, noPos);
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 97e615c37..8b67665db 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -31,7 +31,7 @@ namespace nix {
         EvalState & state;
         SymbolTable & symbols;
         Expr * result;
-        Path basePath;
+        SourcePath basePath;
         PosTable::Origin origin;
         std::optional<ErrorInfo> error;
     };
@@ -509,7 +509,7 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, data->basePath));
+    Path path(absPath({$1.p, $1.l}, data->basePath.path.abs()));
     /* add back in the trailing '/' to the first segment */
     if ($1.p[$1.l-1] == '/' && $1.l > 1)
       path += "/";
@@ -651,7 +651,7 @@ Expr * EvalState::parse(
     char * text,
     size_t length,
     Pos::Origin origin,
-    Path basePath,
+    const SourcePath & basePath,
     std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
@@ -675,48 +675,36 @@ Expr * EvalState::parse(
 }
 
 
-Path resolveExprPath(Path path)
+SourcePath resolveExprPath(const SourcePath & path)
 {
-    assert(path[0] == '/');
-
-    unsigned int followCount = 0, maxFollow = 1024;
-
     /* If `path' is a symlink, follow it.  This is so that relative
        path references work. */
-    struct stat st;
-    while (true) {
-        // Basic cycle/depth limit to avoid infinite loops.
-        if (++followCount >= maxFollow)
-            throw Error("too many symbolic links encountered while traversing the path '%s'", path);
-        st = lstat(path);
-        if (!S_ISLNK(st.st_mode)) break;
-        path = absPath(readLink(path), dirOf(path));
-    }
+    auto path2 = path.resolveSymlinks();
 
     /* If `path' refers to a directory, append `/default.nix'. */
-    if (S_ISDIR(st.st_mode))
-        path = canonPath(path + "/default.nix");
+    if (path2.lstat().type == InputAccessor::tDirectory)
+        return path2 + "default.nix";
 
-    return path;
+    return path2;
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path)
+Expr * EvalState::parseExprFromFile(const SourcePath & path)
 {
     return parseExprFromFile(path, staticBaseEnv);
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv)
 {
-    auto buffer = readFile(path);
-    // readFile should have left some extra space for terminators
+    auto buffer = path.readFile();
+    // readFile hopefully have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), path, dirOf(path), staticEnv);
+    return parse(buffer.data(), buffer.size(), path.path.abs(), path.parent(), staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s_, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     auto s = make_ref<std::string>(std::move(s_));
     s->append("\0\0", 2);
@@ -724,7 +712,7 @@ Expr * EvalState::parseExprFromString(std::string s_, const Path & basePath, std
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath)
+Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath)
 {
     return parseExprFromString(std::move(s), basePath, staticBaseEnv);
 }
@@ -737,7 +725,7 @@ Expr * EvalState::parseStdin()
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
     auto s = make_ref<std::string>(std::move(buffer));
-    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, absPath("."), staticBaseEnv);
+    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, rootPath(CanonPath::fromCwd()), staticBaseEnv);
 }
 
 
@@ -757,13 +745,13 @@ void EvalState::addToSearchPath(const std::string & s)
 }
 
 
-Path EvalState::findFile(const std::string_view path)
+SourcePath EvalState::findFile(const std::string_view path)
 {
     return findFile(searchPath, path);
 }
 
 
-Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos)
+SourcePath EvalState::findFile(SearchPath & searchPath, const std::string_view path, const PosIdx pos)
 {
     for (auto & i : searchPath) {
         std::string suffix;
@@ -779,11 +767,11 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
         auto r = resolveSearchPathElem(i);
         if (!r.first) continue;
         Path res = r.second + suffix;
-        if (pathExists(res)) return canonPath(res);
+        if (pathExists(res)) return CanonPath(canonPath(res));
     }
 
     if (hasPrefix(path, "nix/"))
-        return concatStrings(corepkgsPrefix, path.substr(4));
+        return CanonPath(concatStrings(corepkgsPrefix, path.substr(4)));
 
     debugThrow(ThrownError({
         .msg = hintfmt(evalSettings.pureEval
diff --git a/src/libexpr/paths.cc b/src/libexpr/paths.cc
new file mode 100644
index 000000000..1d690b722
--- /dev/null
+++ b/src/libexpr/paths.cc
@@ -0,0 +1,10 @@
+#include "eval.hh"
+
+namespace nix {
+
+SourcePath EvalState::rootPath(CanonPath path)
+{
+    return std::move(path);
+}
+
+}
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 72faeada8..c77d45bd3 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -110,7 +110,7 @@ struct RealisePathFlags {
     bool checkForPureEval = true;
 };
 
-static Path realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
+static SourcePath realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
 {
     PathSet context;
 
@@ -119,7 +119,7 @@ static Path realisePath(EvalState & state, const PosIdx pos, Value & v, const Re
     try {
         StringMap rewrites = state.realiseContext(context);
 
-        auto realPath = state.toRealPath(rewriteStrings(path, rewrites), context);
+        auto realPath = state.rootPath(CanonPath(state.toRealPath(rewriteStrings(path.path.abs(), rewrites), context)));
 
         return flags.checkForPureEval
             ? state.checkSourcePath(realPath)
@@ -166,13 +166,14 @@ static void mkOutputString(
 static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * vScope, Value & v)
 {
     auto path = realisePath(state, pos, vPath);
+    auto path2 = path.path.abs();
 
     // FIXME
     auto isValidDerivationInStore = [&]() -> std::optional<StorePath> {
-        if (!state.store->isStorePath(path))
+        if (!state.store->isStorePath(path2))
             return std::nullopt;
-        auto storePath = state.store->parseStorePath(path);
-        if (!(state.store->isValidPath(storePath) && isDerivation(path)))
+        auto storePath = state.store->parseStorePath(path2);
+        if (!(state.store->isValidPath(storePath) && isDerivation(path2)))
             return std::nullopt;
         return storePath;
     };
@@ -181,7 +182,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         auto storePath = *optStorePath;
         Derivation drv = state.store->readDerivation(storePath);
         auto attrs = state.buildBindings(3 + drv.outputs.size());
-        attrs.alloc(state.sDrvPath).mkString(path, {"=" + path});
+        attrs.alloc(state.sDrvPath).mkString(path2, {"=" + path2});
         attrs.alloc(state.sName).mkString(drv.env["name"]);
         auto & outputsVal = attrs.alloc(state.sOutputs);
         state.mkList(outputsVal, drv.outputs.size());
@@ -198,7 +199,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
             state.vImportedDrvToDerivation = allocRootValue(state.allocValue());
             state.eval(state.parseExprFromString(
                 #include "imported-drv-to-derivation.nix.gen.hh"
-                , "/"), **state.vImportedDrvToDerivation);
+                , CanonPath::root), **state.vImportedDrvToDerivation);
         }
 
         state.forceFunction(**state.vImportedDrvToDerivation, pos, "while evaluating imported-drv-to-derivation.nix.gen.hh");
@@ -206,10 +207,10 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         state.forceAttrs(v, pos, "while calling imported-drv-to-derivation.nix.gen.hh");
     }
 
-    else if (path == corepkgsPrefix + "fetchurl.nix") {
+    else if (path2 == corepkgsPrefix + "fetchurl.nix") {
         state.eval(state.parseExprFromString(
             #include "fetchurl.nix.gen.hh"
-            , "/"), v);
+            , CanonPath::root), v);
     }
 
     else {
@@ -330,7 +331,7 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 
     std::string sym(state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.importNative"));
 
-    void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
+    void *handle = dlopen(path.path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
         state.debugThrowLastTrace(EvalError("could not open '%1%': %2%", path, dlerror()));
 
@@ -378,7 +379,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
     try {
-        parsed = state.parseExprFromString(std::move(output), "/");
+        parsed = state.parseExprFromString(std::move(output), state.rootPath(CanonPath::root));
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "while parsing the output from '%1%'", program);
         throw;
@@ -585,7 +586,7 @@ struct CompareValues
                 case nString:
                     return strcmp(v1->string.s, v2->string.s) < 0;
                 case nPath:
-                    return strcmp(v1->path, v2->path) < 0;
+                    return strcmp(v1->_path, v2->_path) < 0;
                 case nList:
                     // Lexicographic comparison
                     for (size_t i = 0;; i++) {
@@ -1428,8 +1429,8 @@ static RegisterPrimOp primop_placeholder({
 static void prim_toPath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.toPath");
-    v.mkString(canonPath(path), context);
+    auto path = state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.toPath");
+    v.mkString(path.path.abs(), context);
 }
 
 static RegisterPrimOp primop_toPath({
@@ -1459,21 +1460,22 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
         }));
 
     PathSet context;
-    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.storePath"));
+    auto path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.storePath")).path;
     /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
        directly in the store.  The latter condition is necessary so
        e.g. nix-push does the right thing. */
-    if (!state.store->isStorePath(path)) path = canonPath(path, true);
-    if (!state.store->isInStore(path))
+    if (!state.store->isStorePath(path.abs()))
+        path = CanonPath(canonPath(path.abs(), true));
+    if (!state.store->isInStore(path.abs()))
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("path '%1%' is not in the Nix store", path),
             .errPos = state.positions[pos]
         }));
-    auto path2 = state.store->toStorePath(path).first;
+    auto path2 = state.store->toStorePath(path.abs()).first;
     if (!settings.readOnlyMode)
         state.store->ensurePath(path2);
     context.insert(state.store->printStorePath(path2));
-    v.mkString(path, context);
+    v.mkString(path.abs(), context);
 }
 
 static RegisterPrimOp primop_storePath({
@@ -1504,7 +1506,7 @@ static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args,
     auto path = realisePath(state, pos, *args[0], { .checkForPureEval = false });
 
     try {
-        v.mkBool(pathExists(state.checkSourcePath(path)));
+        v.mkBool(state.checkSourcePath(path).pathExists());
     } catch (SysError & e) {
         /* Don't give away info from errors while canonicalising
            ‘path’ in restricted mode. */
@@ -1551,11 +1553,17 @@ static RegisterPrimOp primop_baseNameOf({
 static void prim_dirOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto path = state.coerceToString(pos, *args[0], context,
-            "while evaluating the first argument passed to builtins.dirOf",
+    state.forceValue(*args[0], pos);
+    if (args[0]->type() == nPath) {
+        auto path = args[0]->path();
+        v.mkPath(path.path.isRoot() ? path : path.parent());
+    } else {
+        auto path = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to 'builtins.dirOf'",
             false, false);
-    auto dir = dirOf(*path);
-    if (args[0]->type() == nPath) v.mkPath(dir); else v.mkString(dir, context);
+        auto dir = dirOf(*path);
+        v.mkString(dir, context);
+    }
 }
 
 static RegisterPrimOp primop_dirOf({
@@ -1573,13 +1581,13 @@ static RegisterPrimOp primop_dirOf({
 static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
-    auto s = readFile(path);
+    auto s = path.readFile();
     if (s.find((char) 0) != std::string::npos)
         state.debugThrowLastTrace(Error("the contents of the file '%1%' cannot be represented as a Nix string", path));
     StorePathSet refs;
-    if (state.store->isInStore(path)) {
+    if (state.store->isInStore(path.path.abs())) {
         try {
-            refs = state.store->queryPathInfo(state.store->toStorePath(path).first)->references;
+            refs = state.store->queryPathInfo(state.store->toStorePath(path.path.abs()).first)->references;
         } catch (Error &) { // FIXME: should be InvalidPathError
         }
         // Re-scan references to filter down to just the ones that actually occur in the file.
@@ -1660,7 +1668,7 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, V
 
     auto path = realisePath(state, pos, *args[1]);
 
-    v.mkString(hashFile(*ht, path).to_string(Base16, false));
+    v.mkString(hashString(*ht, path.readFile()).to_string(Base16, false));
 }
 
 static RegisterPrimOp primop_hashFile({
@@ -1674,26 +1682,20 @@ static RegisterPrimOp primop_hashFile({
     .fun = prim_hashFile,
 });
 
-
-/* Stringize a directory entry enum. Used by `readFileType' and `readDir'. */
-static const char * dirEntTypeToString(unsigned char dtType)
+static std::string_view fileTypeToString(InputAccessor::Type type)
 {
-    /* Enum DT_(DIR|LNK|REG|UNKNOWN) */
-    switch(dtType) {
-        case DT_REG: return "regular";   break;
-        case DT_DIR: return "directory"; break;
-        case DT_LNK: return "symlink";   break;
-        default:     return "unknown";   break;
-    }
-    return "unknown";  /* Unreachable */
+    return
+        type == InputAccessor::Type::tRegular ? "regular" :
+        type == InputAccessor::Type::tDirectory ? "directory" :
+        type == InputAccessor::Type::tSymlink ? "symlink" :
+        "unknown";
 }
 
-
 static void prim_readFileType(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
     /* Retrieve the directory entry type and stringize it. */
-    v.mkString(dirEntTypeToString(getFileType(path)));
+    v.mkString(fileTypeToString(path.lstat().type));
 }
 
 static RegisterPrimOp primop_readFileType({
@@ -1714,8 +1716,7 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Va
     // Retrieve directory entries for all nodes in a directory.
     // This is similar to `getFileType` but is optimized to reduce system calls
     // on many systems.
-    DirEntries entries = readDirectory(path);
-
+    auto entries = path.readDirectory();
     auto attrs = state.buildBindings(entries.size());
 
     // If we hit unknown directory entry types we may need to fallback to
@@ -1724,22 +1725,21 @@ static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Va
     // `builtins.readFileType` application.
     Value * readFileType = nullptr;
 
-    for (auto & ent : entries) {
-        auto & attr = attrs.alloc(ent.name);
-        if (ent.type == DT_UNKNOWN) {
+    for (auto & [name, type] : entries) {
+        auto & attr = attrs.alloc(name);
+        if (!type) {
             // Some filesystems or operating systems may not be able to return
             // detailed node info quickly in this case we produce a thunk to
             // query the file type lazily.
             auto epath = state.allocValue();
-            Path path2 = path + "/" + ent.name;
-            epath->mkString(path2);
+            epath->mkPath(path + name);
             if (!readFileType)
                 readFileType = &state.getBuiltin("readFileType");
             attr.mkApp(readFileType, epath);
         } else {
             // This branch of the conditional is much more likely.
             // Here we just stringize the directory entry type.
-            attr.mkString(dirEntTypeToString(ent.type));
+            attr.mkString(fileTypeToString(*type));
         }
     }
 
@@ -2045,7 +2045,7 @@ static RegisterPrimOp primop_toFile({
 static void addPath(
     EvalState & state,
     const PosIdx pos,
-    const std::string & name,
+    std::string_view name,
     Path path,
     Value * filterFun,
     FileIngestionMethod method,
@@ -2073,7 +2073,7 @@ static void addPath(
 
         path = evalSettings.pureEval && expectedHash
             ? path
-            : state.checkSourcePath(path);
+            : state.checkSourcePath(CanonPath(path)).path.abs();
 
         PathFilter filter = filterFun ? ([&](const Path & path) {
             auto st = lstat(path);
@@ -2120,9 +2120,10 @@ static void addPath(
 static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[1], context, "while evaluating the second argument (the path to filter) passed to builtins.filterSource");
+    auto path = state.coerceToPath(pos, *args[1], context,
+        "while evaluating the second argument (the path to filter) passed to builtins.filterSource");
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filterSource");
-    addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
+    addPath(state, pos, path.baseName(), path.path.abs(), args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
 }
 
 static RegisterPrimOp primop_filterSource({
@@ -2182,18 +2183,19 @@ static RegisterPrimOp primop_filterSource({
 
 static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.path");
-    Path path;
+    std::optional<SourcePath> path;
     std::string name;
     Value * filterFun = nullptr;
     auto method = FileIngestionMethod::Recursive;
     std::optional<Hash> expectedHash;
     PathSet context;
 
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to 'builtins.path'");
+
     for (auto & attr : *args[0]->attrs) {
         auto n = state.symbols[attr.name];
         if (n == "path")
-            path = state.coerceToPath(attr.pos, *attr.value, context, "while evaluating the `path` attribute passed to builtins.path");
+            path.emplace(state.coerceToPath(attr.pos, *attr.value, context, "while evaluating the 'path' attribute passed to 'builtins.path'"));
         else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.path");
         else if (n == "filter")
@@ -2208,15 +2210,15 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
                 .errPos = state.positions[attr.pos]
             }));
     }
-    if (path.empty())
+    if (!path)
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("missing required 'path' attribute in the first argument to builtins.path"),
             .errPos = state.positions[pos]
         }));
     if (name.empty())
-        name = baseNameOf(path);
+        name = path->baseName();
 
-    addPath(state, pos, name, path, filterFun, method, expectedHash, v, context);
+    addPath(state, pos, name, path->path.abs(), filterFun, method, expectedHash, v, context);
 }
 
 static RegisterPrimOp primop_path({
@@ -4151,7 +4153,7 @@ void EvalState::createBaseEnv()
         // the parser needs two NUL bytes as terminators; one of them
         // is implied by being a C string.
         "\0";
-    eval(parse(code, sizeof(code), derivationNixPath, "/", staticBaseEnv), *vDerivation);
+    eval(parse(code, sizeof(code), derivationNixPath, {CanonPath::root}, staticBaseEnv), *vDerivation);
 }
 
 
diff --git a/src/libexpr/tests/libexpr.hh b/src/libexpr/tests/libexpr.hh
index 69c932f05..b8e65aafe 100644
--- a/src/libexpr/tests/libexpr.hh
+++ b/src/libexpr/tests/libexpr.hh
@@ -28,7 +28,7 @@ namespace nix {
             }
             Value eval(std::string input, bool forceValue = true) {
                 Value v;
-                Expr * e = state.parseExprFromString(input, "");
+                Expr * e = state.parseExprFromString(input, state.rootPath(CanonPath::root));
                 assert(e);
                 state.eval(e, v);
                 if (forceValue)
diff --git a/src/libexpr/tests/local.mk b/src/libexpr/tests/local.mk
index 3e5504f71..331a5ead6 100644
--- a/src/libexpr/tests/local.mk
+++ b/src/libexpr/tests/local.mk
@@ -12,7 +12,7 @@ libexpr-tests_SOURCES := \
     $(wildcard $(d)/*.cc) \
     $(wildcard $(d)/value/*.cc)
 
-libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests
+libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests -I src/libfetchers
 
 libexpr-tests_LIBS = libstore-tests libutils-tests libexpr libutil libstore libfetchers
 
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index c35c876e3..d40a77302 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -36,9 +36,10 @@ json printValueAsJSON(EvalState & state, bool strict,
 
         case nPath:
             if (copyToStore)
-                out = state.store->printStorePath(state.copyPathToStore(context, v.path));
+                out = state.store->printStorePath(
+                    state.copyPathToStore(context, v.path()));
             else
-                out = v.path;
+                out = v.path().path.abs();
             break;
 
         case nNull:
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index 341c8922f..a9fcb803e 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -78,7 +78,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             break;
 
         case nPath:
-            doc.writeEmptyElement("path", singletonAttrs("value", v.path));
+            doc.writeEmptyElement("path", singletonAttrs("value", v.path().to_string()));
             break;
 
         case nNull:
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index bfae4ee94..eb6f56d07 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -5,6 +5,7 @@
 
 #include "symbol-table.hh"
 #include "value/context.hh"
+#include "input-accessor.hh"
 
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
@@ -171,7 +172,7 @@ public:
             const char * * context; // must be in sorted order
         } string;
 
-        const char * path;
+        const char * _path;
         Bindings * attrs;
         struct {
             size_t size;
@@ -251,15 +252,20 @@ public:
 
     void mkStringMove(const char * s, const PathSet & context);
 
-    inline void mkPath(const char * s)
+    inline void mkString(const Symbol & s)
+    {
+        mkString(((const std::string &) s).c_str());
+    }
+
+    void mkPath(const SourcePath & path);
+
+    inline void mkPath(const char * path)
     {
         clearValue();
         internalType = tPath;
-        path = s;
+        _path = path;
     }
 
-    void mkPath(std::string_view s);
-
     inline void mkNull()
     {
         clearValue();
@@ -400,6 +406,18 @@ public:
         auto begin = listElems();
         return ConstListIterable { begin, begin + listSize() };
     }
+
+    SourcePath path() const
+    {
+        assert(internalType == tPath);
+        return SourcePath{CanonPath(_path)};
+    }
+
+    std::string_view str() const
+    {
+        assert(internalType == tString);
+        return std::string_view(string.s);
+    }
 };
 
 
diff --git a/src/libfetchers/input-accessor.cc b/src/libfetchers/input-accessor.cc
new file mode 100644
index 000000000..f9909c218
--- /dev/null
+++ b/src/libfetchers/input-accessor.cc
@@ -0,0 +1,100 @@
+#include "input-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path)
+{
+    str << path.to_string();
+    return str;
+}
+
+std::string_view SourcePath::baseName() const
+{
+    return path.baseName().value_or("source");
+}
+
+SourcePath SourcePath::parent() const
+{
+    auto p = path.parent();
+    assert(p);
+    return std::move(*p);
+}
+
+InputAccessor::Stat SourcePath::lstat() const
+{
+    auto st = nix::lstat(path.abs());
+    return InputAccessor::Stat {
+        .type =
+            S_ISREG(st.st_mode) ? InputAccessor::tRegular :
+            S_ISDIR(st.st_mode) ? InputAccessor::tDirectory :
+            S_ISLNK(st.st_mode) ? InputAccessor::tSymlink :
+            InputAccessor::tMisc,
+        .isExecutable = S_ISREG(st.st_mode) && st.st_mode & S_IXUSR
+    };
+}
+
+std::optional<InputAccessor::Stat> SourcePath::maybeLstat() const
+{
+    // FIXME: merge these into one operation.
+    if (!pathExists())
+        return {};
+    return lstat();
+}
+
+InputAccessor::DirEntries SourcePath::readDirectory() const
+{
+    InputAccessor::DirEntries res;
+    for (auto & entry : nix::readDirectory(path.abs())) {
+        std::optional<InputAccessor::Type> type;
+        switch (entry.type) {
+        case DT_REG: type = InputAccessor::Type::tRegular; break;
+        case DT_LNK: type = InputAccessor::Type::tSymlink; break;
+        case DT_DIR: type = InputAccessor::Type::tDirectory; break;
+        }
+        res.emplace(entry.name, type);
+    }
+    return res;
+}
+
+StorePath SourcePath::fetchToStore(
+    ref<Store> store,
+    std::string_view name,
+    PathFilter * filter,
+    RepairFlag repair) const
+{
+    return
+        settings.readOnlyMode
+        ? store->computeStorePathForPath(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter).first
+        : store->addToStore(name, path.abs(), FileIngestionMethod::Recursive, htSHA256, filter ? *filter : defaultPathFilter, repair);
+}
+
+SourcePath SourcePath::resolveSymlinks() const
+{
+    SourcePath res(CanonPath::root);
+
+    int linksAllowed = 1024;
+
+    for (auto & component : path) {
+        res.path.push(component);
+        while (true) {
+            if (auto st = res.maybeLstat()) {
+                if (!linksAllowed--)
+                    throw Error("infinite symlink recursion in path '%s'", path);
+                if (st->type != InputAccessor::tSymlink) break;
+                auto target = res.readLink();
+                if (hasPrefix(target, "/"))
+                    res = CanonPath(target);
+                else {
+                    res.path.pop();
+                    res.path.extend(CanonPath(target));
+                }
+            } else
+                break;
+        }
+    }
+
+    return res;
+}
+
+}
diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
new file mode 100644
index 000000000..23c510d4d
--- /dev/null
+++ b/src/libfetchers/input-accessor.hh
@@ -0,0 +1,149 @@
+#pragma once
+
+#include "ref.hh"
+#include "types.hh"
+#include "archive.hh"
+#include "canon-path.hh"
+#include "repair-flag.hh"
+
+namespace nix {
+
+class StorePath;
+class Store;
+
+struct InputAccessor
+{
+    enum Type { tRegular, tSymlink, tDirectory, tMisc };
+
+    struct Stat
+    {
+        Type type = tMisc;
+        //uint64_t fileSize = 0; // regular files only
+        bool isExecutable = false; // regular files only
+    };
+
+    typedef std::optional<Type> DirEntry;
+
+    typedef std::map<std::string, DirEntry> DirEntries;
+};
+
+/**
+ * An abstraction for accessing source files during
+ * evaluation. Currently, it's just a wrapper around `CanonPath` that
+ * accesses files in the regular filesystem, but in the future it will
+ * support fetching files in other ways.
+ */
+struct SourcePath
+{
+    CanonPath path;
+
+    SourcePath(CanonPath path)
+        : path(std::move(path))
+    { }
+
+    std::string_view baseName() const;
+
+    /**
+     * Construct the parent of this `SourcePath`. Aborts if `this`
+     * denotes the root.
+     */
+    SourcePath parent() const;
+
+    /**
+     * If this `SourcePath` denotes a regular file (not a symlink),
+     * return its contents; otherwise throw an error.
+     */
+    std::string readFile() const
+    { return nix::readFile(path.abs()); }
+
+    /**
+     * Return whether this `SourcePath` denotes a file (of any type)
+     * that exists
+    */
+    bool pathExists() const
+    { return nix::pathExists(path.abs()); }
+
+    /**
+     * Return stats about this `SourcePath`, or throw an exception if
+     * it doesn't exist.
+     */
+    InputAccessor::Stat lstat() const;
+
+    /**
+     * Return stats about this `SourcePath`, or std::nullopt if it
+     * doesn't exist.
+     */
+    std::optional<InputAccessor::Stat> maybeLstat() const;
+
+    /**
+     * If this `SourcePath` denotes a directory (not a symlink),
+     * return its directory entries; otherwise throw an error.
+     */
+    InputAccessor::DirEntries readDirectory() const;
+
+    /**
+     * If this `SourcePath` denotes a symlink, return its target;
+     * otherwise throw an error.
+     */
+    std::string readLink() const
+    { return nix::readLink(path.abs()); }
+
+    /**
+     * Dump this `SourcePath` to `sink` as a NAR archive.
+     */
+    void dumpPath(
+        Sink & sink,
+        PathFilter & filter = defaultPathFilter) const
+    { return nix::dumpPath(path.abs(), sink, filter); }
+
+    /**
+     * Copy this `SourcePath` to the Nix store.
+     */
+    StorePath fetchToStore(
+        ref<Store> store,
+        std::string_view name = "source",
+        PathFilter * filter = nullptr,
+        RepairFlag repair = NoRepair) const;
+
+    /**
+     * Return the location of this path in the "real" filesystem, if
+     * it has a physical location.
+     */
+    std::optional<CanonPath> getPhysicalPath() const
+    { return path; }
+
+    std::string to_string() const
+    { return path.abs(); }
+
+    SourcePath operator + (const CanonPath & x) const
+    { return {path + x}; }
+
+    SourcePath operator + (std::string_view c) const
+    {  return {path + c}; }
+
+    bool operator == (const SourcePath & x) const
+    {
+        return path == x.path;
+    }
+
+    bool operator != (const SourcePath & x) const
+    {
+        return path != x.path;
+    }
+
+    bool operator < (const SourcePath & x) const
+    {
+        return path < x.path;
+    }
+
+    /**
+     * Resolve any symlinks in this `SourcePath` (including its
+     * parents). The result is a `SourcePath` in which no element is a
+     * symlink.
+     */
+    SourcePath resolveSymlinks() const;
+};
+
+std::ostream & operator << (std::ostream & str, const SourcePath & path);
+
+}
diff --git a/src/libutil/canon-path.cc b/src/libutil/canon-path.cc
index ddf6db6d1..040464532 100644
--- a/src/libutil/canon-path.cc
+++ b/src/libutil/canon-path.cc
@@ -13,6 +13,11 @@ CanonPath::CanonPath(std::string_view raw, const CanonPath & root)
     : path(absPath((Path) raw, root.abs()))
 { }
 
+CanonPath CanonPath::fromCwd(std::string_view path)
+{
+    return CanonPath(unchecked_t(), absPath((Path) path));
+}
+
 std::optional<CanonPath> CanonPath::parent() const
 {
     if (isRoot()) return std::nullopt;
diff --git a/src/libutil/canon-path.hh b/src/libutil/canon-path.hh
index 76e48c4f2..30f43b5a4 100644
--- a/src/libutil/canon-path.hh
+++ b/src/libutil/canon-path.hh
@@ -46,6 +46,8 @@ public:
         : path(std::move(path))
     { }
 
+    static CanonPath fromCwd(std::string_view path = ".");
+
     static CanonPath root;
 
     /**
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index bc7e7eb18..251f00edf 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -289,7 +289,7 @@ static void main_nix_build(int argc, char * * argv)
     else
         for (auto i : left) {
             if (fromArgs)
-                exprs.push_back(state->parseExprFromString(std::move(i), absPath(".")));
+                exprs.push_back(state->parseExprFromString(std::move(i), state->rootPath(CanonPath::fromCwd())));
             else {
                 auto absolute = i;
                 try {
@@ -385,7 +385,9 @@ static void main_nix_build(int argc, char * * argv)
         if (!shell) {
 
             try {
-                auto expr = state->parseExprFromString("(import <nixpkgs> {}).bashInteractive", absPath("."));
+                auto expr = state->parseExprFromString(
+                    "(import <nixpkgs> {}).bashInteractive",
+                    state->rootPath(CanonPath::fromCwd()));
 
                 Value v;
                 state->eval(expr, v);
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index f076ffdb0..0081bab25 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -44,7 +44,7 @@ typedef enum {
 struct InstallSourceInfo
 {
     InstallSourceType type;
-    Path nixExprPath; /* for srcNixExprDrvs, srcNixExprs */
+    std::shared_ptr<SourcePath> nixExprPath; /* for srcNixExprDrvs, srcNixExprs */
     Path profile; /* for srcProfile */
     std::string systemFilter; /* for srcNixExprDrvs */
     Bindings * autoArgs;
@@ -92,9 +92,11 @@ static bool parseInstallSourceOptions(Globals & globals,
 }
 
 
-static bool isNixExpr(const Path & path, struct stat & st)
+static bool isNixExpr(const SourcePath & path, struct InputAccessor::Stat & st)
 {
-    return S_ISREG(st.st_mode) || (S_ISDIR(st.st_mode) && pathExists(path + "/default.nix"));
+    return
+        st.type == InputAccessor::tRegular
+        || (st.type == InputAccessor::tDirectory && (path + "default.nix").pathExists());
 }
 
 
@@ -102,10 +104,10 @@ static constexpr size_t maxAttrs = 1024;
 
 
 static void getAllExprs(EvalState & state,
-    const Path & path, StringSet & seen, BindingsBuilder & attrs)
+    const SourcePath & path, StringSet & seen, BindingsBuilder & attrs)
 {
     StringSet namesSorted;
-    for (auto & i : readDirectory(path)) namesSorted.insert(i.name);
+    for (auto & [name, _] : path.readDirectory()) namesSorted.insert(name);
 
     for (auto & i : namesSorted) {
         /* Ignore the manifest.nix used by profiles.  This is
@@ -113,13 +115,16 @@ static void getAllExprs(EvalState & state,
            are implemented using profiles). */
         if (i == "manifest.nix") continue;
 
-        Path path2 = path + "/" + i;
+        SourcePath path2 = path + i;
 
-        struct stat st;
-        if (stat(path2.c_str(), &st) == -1)
+        InputAccessor::Stat st;
+        try {
+            st = path2.resolveSymlinks().lstat();
+        } catch (Error &) {
             continue; // ignore dangling symlinks in ~/.nix-defexpr
+        }
 
-        if (isNixExpr(path2, st) && (!S_ISREG(st.st_mode) || hasSuffix(path2, ".nix"))) {
+        if (isNixExpr(path2, st) && (st.type != InputAccessor::tRegular || hasSuffix(path2.baseName(), ".nix"))) {
             /* Strip off the `.nix' filename suffix (if applicable),
                otherwise the attribute cannot be selected with the
                `-A' option.  Useful if you want to stick a Nix
@@ -129,21 +134,20 @@ static void getAllExprs(EvalState & state,
                 attrName = std::string(attrName, 0, attrName.size() - 4);
             if (!seen.insert(attrName).second) {
                 std::string suggestionMessage = "";
-                if (path2.find("channels") != std::string::npos && path.find("channels") != std::string::npos) {
+                if (path2.path.abs().find("channels") != std::string::npos && path.path.abs().find("channels") != std::string::npos)
                     suggestionMessage = fmt("\nsuggestion: remove '%s' from either the root channels or the user channels", attrName);
-                }
                 printError("warning: name collision in input Nix expressions, skipping '%1%'"
                             "%2%", path2, suggestionMessage);
                 continue;
             }
             /* Load the expression on demand. */
             auto vArg = state.allocValue();
-            vArg->mkString(path2);
+            vArg->mkString(path2.path.abs());
             if (seen.size() == maxAttrs)
                 throw Error("too many Nix expressions in directory '%1%'", path);
             attrs.alloc(attrName).mkApp(&state.getBuiltin("import"), vArg);
         }
-        else if (S_ISDIR(st.st_mode))
+        else if (st.type == InputAccessor::tDirectory)
             /* `path2' is a directory (with no default.nix in it);
                recurse into it. */
             getAllExprs(state, path2, seen, attrs);
@@ -152,11 +156,9 @@ static void getAllExprs(EvalState & state,
 
 
 
-static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
+static void loadSourceExpr(EvalState & state, const SourcePath & path, Value & v)
 {
-    struct stat st;
-    if (stat(path.c_str(), &st) == -1)
-        throw SysError("getting information about '%1%'", path);
+    auto st = path.resolveSymlinks().lstat();
 
     if (isNixExpr(path, st))
         state.evalFile(path, v);
@@ -167,7 +169,7 @@ static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
        set flat, not nested, to make it easier for a user to have a
        ~/.nix-defexpr directory that includes some system-wide
        directory). */
-    else if (S_ISDIR(st.st_mode)) {
+    else if (st.type == InputAccessor::tDirectory) {
         auto attrs = state.buildBindings(maxAttrs);
         attrs.alloc("_combineChannels").mkList(0);
         StringSet seen;
@@ -179,7 +181,7 @@ static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
 }
 
 
-static void loadDerivations(EvalState & state, Path nixExprPath,
+static void loadDerivations(EvalState & state, const SourcePath & nixExprPath,
     std::string systemFilter, Bindings & autoArgs,
     const std::string & pathPrefix, DrvInfos & elems)
 {
@@ -390,7 +392,7 @@ static void queryInstSources(EvalState & state,
             /* Load the derivations from the (default or specified)
                Nix expression. */
             DrvInfos allElems;
-            loadDerivations(state, instSource.nixExprPath,
+            loadDerivations(state, *instSource.nixExprPath,
                 instSource.systemFilter, *instSource.autoArgs, "", allElems);
 
             elems = filterBySelector(state, allElems, args, newestOnly);
@@ -407,10 +409,10 @@ static void queryInstSources(EvalState & state,
         case srcNixExprs: {
 
             Value vArg;
-            loadSourceExpr(state, instSource.nixExprPath, vArg);
+            loadSourceExpr(state, *instSource.nixExprPath, vArg);
 
             for (auto & i : args) {
-                Expr * eFun = state.parseExprFromString(i, absPath("."));
+                Expr * eFun = state.parseExprFromString(i, state.rootPath(CanonPath::fromCwd()));
                 Value vFun, vTmp;
                 state.eval(eFun, vFun);
                 vTmp.mkApp(&vFun, &vArg);
@@ -462,7 +464,7 @@ static void queryInstSources(EvalState & state,
 
         case srcAttrPath: {
             Value vRoot;
-            loadSourceExpr(state, instSource.nixExprPath, vRoot);
+            loadSourceExpr(state, *instSource.nixExprPath, vRoot);
             for (auto & i : args) {
                 Value & v(*findAlongAttrPath(state, i, *instSource.autoArgs, vRoot).first);
                 getDerivations(state, v, "", *instSource.autoArgs, elems, true);
@@ -1030,7 +1032,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         installedElems = queryInstalled(*globals.state, globals.profile);
 
     if (source == sAvailable || compareVersions)
-        loadDerivations(*globals.state, globals.instSource.nixExprPath,
+        loadDerivations(*globals.state, *globals.instSource.nixExprPath,
             globals.instSource.systemFilter, *globals.instSource.autoArgs,
             attrPath, availElems);
 
@@ -1395,22 +1397,20 @@ static int main_nix_env(int argc, char * * argv)
         Globals globals;
 
         globals.instSource.type = srcUnknown;
-        {
-            Path nixExprPath = settings.useXDGBaseDirectories ? createNixStateDir() + "/defexpr" : getHome() + "/.nix-defexpr";
-            globals.instSource.nixExprPath = nixExprPath;
-        }
         globals.instSource.systemFilter = "*";
 
-        if (!pathExists(globals.instSource.nixExprPath)) {
+        Path nixExprPath = settings.useXDGBaseDirectories ? createNixStateDir() + "/defexpr" : getHome() + "/.nix-defexpr";
+
+        if (!pathExists(nixExprPath)) {
             try {
-                createDirs(globals.instSource.nixExprPath);
+                createDirs(nixExprPath);
                 replaceSymlink(
                     defaultChannelsDir(),
-                    globals.instSource.nixExprPath + "/channels");
+                    nixExprPath + "/channels");
                 if (getuid() != 0)
                     replaceSymlink(
                         rootChannelsDir(),
-                        globals.instSource.nixExprPath + "/channels_root");
+                        nixExprPath + "/channels_root");
             } catch (Error &) { }
         }
 
@@ -1517,8 +1517,10 @@ static int main_nix_env(int argc, char * * argv)
         globals.state = std::shared_ptr<EvalState>(new EvalState(myArgs.searchPath, store));
         globals.state->repair = repair;
 
-        if (file != "")
-            globals.instSource.nixExprPath = lookupFileArg(*globals.state, file);
+        globals.instSource.nixExprPath = std::make_shared<SourcePath>(
+            file != ""
+            ? lookupFileArg(*globals.state, file)
+            : globals.state->rootPath(CanonPath(nixExprPath)));
 
         globals.instSource.autoArgs = myArgs.getAutoArgs(*globals.state);
 
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 745e9e174..f70381bf2 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -19,10 +19,10 @@ DrvInfos queryInstalled(EvalState & state, const Path & userEnv)
     DrvInfos elems;
     if (pathExists(userEnv + "/manifest.json"))
         throw Error("profile '%s' is incompatible with 'nix-env'; please use 'nix profile' instead", userEnv);
-    Path manifestFile = userEnv + "/manifest.nix";
+    auto manifestFile = userEnv + "/manifest.nix";
     if (pathExists(manifestFile)) {
         Value v;
-        state.evalFile(manifestFile, v);
+        state.evalFile(state.rootPath(CanonPath(manifestFile)), v);
         Bindings & bindings(*state.allocBindings(0));
         getDerivations(state, v, "", bindings, elems, false);
     }
@@ -114,7 +114,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     Value envBuilder;
     state.eval(state.parseExprFromString(
         #include "buildenv.nix.gen.hh"
-            , "/"), envBuilder);
+            , state.rootPath(CanonPath::root)), envBuilder);
 
     /* Construct a Nix expression that calls the user environment
        builder with the manifest as argument. */
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 6b5ba595d..2b381288a 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -168,9 +168,11 @@ static int main_nix_instantiate(int argc, char * * argv)
 
         if (findFile) {
             for (auto & i : files) {
-                Path p = state->findFile(i);
-                if (p == "") throw Error("unable to find '%1%'", i);
-                std::cout << p << std::endl;
+                auto p = state->findFile(i);
+                if (auto fn = p.getPhysicalPath())
+                    std::cout << fn->abs() << std::endl;
+                else
+                    throw Error("'%s' has no physical path", p);
             }
             return 0;
         }
@@ -184,7 +186,7 @@ static int main_nix_instantiate(int argc, char * * argv)
 
         for (auto & i : files) {
             Expr * e = fromArgs
-                ? state->parseExprFromString(i, absPath("."))
+                ? state->parseExprFromString(i, state->rootPath(CanonPath::fromCwd()))
                 : state->parseExprFromFile(resolveExprPath(state->checkSourcePath(lookupFileArg(*state, i))));
             processExpr(*state, attrPaths, parseOnly, strict, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index 43db5150c..c7af9c92c 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -66,7 +66,7 @@ struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
 
         if (apply) {
             auto vApply = state->allocValue();
-            state->eval(state->parseExprFromString(*apply, absPath(".")), *vApply);
+            state->eval(state->parseExprFromString(*apply, state->rootPath(CanonPath::fromCwd())), *vApply);
             auto vRes = state->allocValue();
             state->callFunction(*vApply, *v, *vRes, noPos);
             v = vRes;
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index cd4ee5921..a6c6b947f 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -440,8 +440,8 @@ struct CmdFlakeCheck : FlakeCommand
                     if (attr->name == state->symbols.create("path")) {
                         PathSet context;
                         auto path = state->coerceToPath(attr->pos, *attr->value, context, "");
-                        if (!store->isInStore(path))
-                            throw Error("template '%s' has a bad 'path' attribute");
+                        if (!path.pathExists())
+                            throw Error("template '%s' refers to a non-existent path '%s'", attrPath, path);
                         // TODO: recursively check the flake in 'path'.
                     }
                 } else
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 54c920b4e..df0a25214 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -197,14 +197,14 @@ static void showHelp(std::vector<std::string> subcommand, NixArgs & toplevel)
     auto vGenerateManpage = state.allocValue();
     state.eval(state.parseExprFromString(
         #include "generate-manpage.nix.gen.hh"
-        , "/"), *vGenerateManpage);
+        , CanonPath::root), *vGenerateManpage);
 
     auto vUtils = state.allocValue();
     state.cacheFile(
-        "/utils.nix", "/utils.nix",
+        CanonPath("/utils.nix"), CanonPath("/utils.nix"),
         state.parseExprFromString(
             #include "utils.nix.gen.hh"
-            , "/"),
+            , CanonPath::root),
         *vUtils);
 
     auto vDump = state.allocValue();
diff --git a/src/nix/prefetch.cc b/src/nix/prefetch.cc
index 51c8a3319..039608d48 100644
--- a/src/nix/prefetch.cc
+++ b/src/nix/prefetch.cc
@@ -27,7 +27,10 @@ std::string resolveMirrorUrl(EvalState & state, const std::string & url)
 
     Value vMirrors;
     // FIXME: use nixpkgs flake
-    state.eval(state.parseExprFromString("import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>", "."), vMirrors);
+    state.eval(state.parseExprFromString(
+            "import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>",
+            state.rootPath(CanonPath::root)),
+        vMirrors);
     state.forceAttrs(vMirrors, noPos, "while evaluating the set of all mirrors");
 
     auto mirrorList = vMirrors.attrs->find(state.symbols.create(mirrorName));
@@ -192,9 +195,11 @@ static int main_nix_prefetch_url(int argc, char * * argv)
                 throw UsageError("you must specify a URL");
             url = args[0];
         } else {
-            Path path = resolveExprPath(lookupFileArg(*state, args.empty() ? "." : args[0]));
             Value vRoot;
-            state->evalFile(path, vRoot);
+            state->evalFile(
+                resolveExprPath(
+                    lookupFileArg(*state, args.empty() ? "." : args[0])),
+                vRoot);
             Value & v(*findAlongAttrPath(*state, attrPath, autoArgs, vRoot).first);
             state->forceAttrs(v, noPos, "while evaluating the source attribute to prefetch");
 
diff --git a/src/nix/upgrade-nix.cc b/src/nix/upgrade-nix.cc
index 17796d6b8..9185ba407 100644
--- a/src/nix/upgrade-nix.cc
+++ b/src/nix/upgrade-nix.cc
@@ -140,7 +140,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
 
         auto state = std::make_unique<EvalState>(Strings(), store);
         auto v = state->allocValue();
-        state->eval(state->parseExprFromString(res.data, "/no-such-path"), *v);
+        state->eval(state->parseExprFromString(res.data, state->rootPath(CanonPath("/no-such-path"))), *v);
         Bindings & bindings(*state->allocBindings(0));
         auto v2 = findAlongAttrPath(*state, settings.thisSystem, bindings, *v).first;
 
diff --git a/tests/plugins/local.mk b/tests/plugins/local.mk
index 8182a6a83..40350aa96 100644
--- a/tests/plugins/local.mk
+++ b/tests/plugins/local.mk
@@ -8,4 +8,4 @@ libplugintest_ALLOW_UNDEFINED := 1
 
 libplugintest_EXCLUDE_FROM_LIBRARY_LIST := 1
 
-libplugintest_CXXFLAGS := -I src/libutil -I src/libstore -I src/libexpr
+libplugintest_CXXFLAGS := -I src/libutil -I src/libstore -I src/libexpr -I src/libfetchers

From a9759407e55fb02c6e306fdd9fcedd821e465024 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 6 Apr 2023 15:25:06 +0200
Subject: [PATCH 2/4] Origin: Use SourcePath

---
 src/libcmd/repl.cc          |  4 ++--
 src/libexpr/eval.cc         | 17 +++++++++--------
 src/libexpr/eval.hh         |  5 ++---
 src/libexpr/flake/flake.cc  |  2 +-
 src/libexpr/nixexpr.cc      |  6 +++---
 src/libexpr/nixexpr.hh      |  2 +-
 src/libexpr/parser.y        |  2 +-
 src/libexpr/primops.cc      |  3 +--
 src/libexpr/value-to-xml.cc |  4 ++--
 9 files changed, 22 insertions(+), 23 deletions(-)

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 9002fa555..c7fd0db91 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -594,8 +594,8 @@ bool NixRepl::processLine(std::string line)
                 return {path, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
-                if (auto path = std::get_if<Path>(&pos.origin))
-                    return {SourcePath(CanonPath(*path)), pos.line};
+                if (auto path = std::get_if<SourcePath>(&pos.origin))
+                    return {*path, pos.line};
                 else
                     throw Error("'%s' cannot be shown in an editor", pos);
             } else {
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 3b2877602..dec123b69 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -499,6 +499,7 @@ EvalState::EvalState(
     , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
+    , derivationInternal(rootPath(CanonPath("/builtin/derivation.nix")))
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , debugRepl(nullptr)
@@ -991,9 +992,9 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto pos = positions[p];
-    if (auto path = std::get_if<Path>(&pos.origin)) {
+    if (auto path = std::get_if<SourcePath>(&pos.origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(*path);
+        attrs.alloc(sFile).mkString(path->path.abs());
         attrs.alloc(sLine).mkInt(pos.line);
         attrs.alloc(sColumn).mkInt(pos.column);
         v.mkAttrs(attrs);
@@ -1373,8 +1374,8 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     } catch (Error & e) {
         if (pos2) {
             auto pos2r = state.positions[pos2];
-            auto origin = std::get_if<Path>(&pos2r.origin);
-            if (!(origin && *origin == state.derivationNixPath))
+            auto origin = std::get_if<SourcePath>(&pos2r.origin);
+            if (!(origin && *origin == state.derivationInternal))
                 state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
                     showAttrPath(state, env, attrPath));
         }
@@ -2417,8 +2418,8 @@ void EvalState::printStats()
                     else
                         obj["name"] = nullptr;
                     if (auto pos = positions[fun->pos]) {
-                        if (auto path = std::get_if<Path>(&pos.origin))
-                            obj["file"] = *path;
+                        if (auto path = std::get_if<SourcePath>(&pos.origin))
+                            obj["file"] = path->to_string();
                         obj["line"] = pos.line;
                         obj["column"] = pos.column;
                     }
@@ -2432,8 +2433,8 @@ void EvalState::printStats()
                 for (auto & i : attrSelects) {
                     json obj = json::object();
                     if (auto pos = positions[i.first]) {
-                        if (auto path = std::get_if<Path>(&pos.origin))
-                            obj["file"] = *path;
+                        if (auto path = std::get_if<SourcePath>(&pos.origin))
+                            obj["file"] = path->to_string();
                         obj["line"] = pos.line;
                         obj["column"] = pos.column;
                     }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 1e8f8391c..b9578321f 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -127,8 +127,6 @@ public:
     SymbolTable symbols;
     PosTable positions;
 
-    static inline std::string derivationNixPath = "//builtin/derivation.nix";
-
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
@@ -139,7 +137,6 @@ public:
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
         sPrefix,
         sOutputSpecified;
-    Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */
@@ -151,6 +148,8 @@ public:
 
     Bindings emptyBindings;
 
+    const SourcePath derivationInternal;
+
     /* Store used to materialise .drv files. */
     const ref<Store> store;
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 2b858ee98..4c571fd7d 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -220,7 +220,7 @@ static Flake getFlake(
     Value vInfo;
     state.evalFile(CanonPath(flakeFile), vInfo, true); // FIXME: symlink attack
 
-    expectType(state, nAttrs, vInfo, state.positions.add({flakeFile}, 1, 1));
+    expectType(state, nAttrs, vInfo, state.positions.add({CanonPath(flakeFile)}, 1, 1));
 
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index eb6f062b4..2c9d5754e 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -31,9 +31,9 @@ struct PosAdapter : AbstractPos
                 // Get rid of the null terminators added by the parser.
                 return std::string(s.source->c_str());
             },
-            [](const Path & path) -> std::optional<std::string> {
+            [](const SourcePath & path) -> std::optional<std::string> {
                 try {
-                    return readFile(path);
+                    return path.readFile();
                 } catch (Error &) {
                     return std::nullopt;
                 }
@@ -47,7 +47,7 @@ struct PosAdapter : AbstractPos
             [&](const Pos::none_tag &) { out << "«none»"; },
             [&](const Pos::Stdin &) { out << "«stdin»"; },
             [&](const Pos::String & s) { out << "«string»"; },
-            [&](const Path & path) { out << path; }
+            [&](const SourcePath & path) { out << path; }
         }, origin);
     }
 };
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 4079a7b24..d70280582 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -32,7 +32,7 @@ struct Pos
     struct Stdin { ref<std::string> source; };
     struct String { ref<std::string> source; };
 
-    typedef std::variant<none_tag, Stdin, String, Path> Origin;
+    typedef std::variant<none_tag, Stdin, String, SourcePath> Origin;
 
     Origin origin;
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 8b67665db..4d981712a 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -700,7 +700,7 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta
     auto buffer = path.readFile();
     // readFile hopefully have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), path.path.abs(), path.parent(), staticEnv);
+    return parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index c77d45bd3..7c34e93ad 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -4136,7 +4136,6 @@ void EvalState::createBaseEnv()
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
-    sDerivationNix = symbols.create(derivationNixPath);
     auto vDerivation = allocValue();
     addConstant("derivation", vDerivation);
 
@@ -4153,7 +4152,7 @@ void EvalState::createBaseEnv()
         // the parser needs two NUL bytes as terminators; one of them
         // is implied by being a C string.
         "\0";
-    eval(parse(code, sizeof(code), derivationNixPath, {CanonPath::root}, staticBaseEnv), *vDerivation);
+    eval(parse(code, sizeof(code), derivationInternal, {CanonPath::root}, staticBaseEnv), *vDerivation);
 }
 
 
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index a9fcb803e..4e87ac4b6 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -24,8 +24,8 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
 static void posToXML(EvalState & state, XMLAttrs & xmlAttrs, const Pos & pos)
 {
-    if (auto path = std::get_if<Path>(&pos.origin))
-        xmlAttrs["path"] = *path;
+    if (auto path = std::get_if<SourcePath>(&pos.origin))
+        xmlAttrs["path"] = path->path.abs();
     xmlAttrs["line"] = fmt("%1%", pos.line);
     xmlAttrs["column"] = fmt("%1%", pos.column);
 }

From ad57cff9bc38a4a97f4ecccc7655e2dfd73fc75c Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 24 Apr 2023 13:34:46 +0200
Subject: [PATCH 3/4] Document tMisc

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
---
 src/libfetchers/input-accessor.hh | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
index 23c510d4d..1cb233c51 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libfetchers/input-accessor.hh
@@ -13,7 +13,17 @@ class Store;
 
 struct InputAccessor
 {
-    enum Type { tRegular, tSymlink, tDirectory, tMisc };
+    enum Type {
+      tRegular, tSymlink, tDirectory,
+      /**
+        Any other node types that may be encountered on the file system, such as device nodes, sockets, named pipe, and possibly even more exotic things.
+
+        Responsible for `"unknown"` from `builtins.readFileType "/dev/null"`.
+
+        Unlike `DT_UNKNOWN`, this must not be used for deferring the lookup of types.
+      */
+      tMisc
+    };
 
     struct Stat
     {

From 5d3f6dbf59554fb4701cb678a4ef29918db97767 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 24 Apr 2023 13:37:51 +0200
Subject: [PATCH 4/4] Add some more SourcePath docs

---
 src/libfetchers/input-accessor.hh | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
index 1cb233c51..5a2f17f62 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libfetchers/input-accessor.hh
@@ -125,9 +125,17 @@ struct SourcePath
     std::string to_string() const
     { return path.abs(); }
 
+    /**
+     * Append a `CanonPath` to this path.
+     */
     SourcePath operator + (const CanonPath & x) const
     { return {path + x}; }
 
+    /**
+     * Append a single component `c` to this path. `c` must not
+     * contain a slash. A slash is implicitly added between this path
+     * and `c`.
+     */
     SourcePath operator + (std::string_view c) const
     {  return {path + c}; }