Make path values lazy

This fixes the double copy problem and improves performance
for expressions that don't force the whole source to be added to the
store.

Rules for fast expressions:

- Use path literals where possible
   - import ./foo.nix
- Use + operator with slash in string
   - src = fetchTree foo + "/src";
- Use source filtering, lib.fileset

- AVOID toString
- If possible, AVOID interpolations ("${./.}")
- If possible, move slashes into the interpolation to add less to the store
   - "${./src}/foo" -> "${./src/foo}"

toString may be improved later as part of lazy-trees, so these
recommendations are a snapshot. Path values are quite nice though.

src/libexpr/eval.cc

@@ -1978,6 +1978,7 @@ void EvalState::concatLists(Value & v, size_t nrLists, Value * const * lists, co
     v.mkList(list);
 }
 
+// FIXME limit recursion
 Value * resolveOutPath(EvalState & state, Value * v, const PosIdx pos)
 {
     state.forceValue(*v, pos);
@@ -2335,6 +2336,8 @@ BackedStringView EvalState::coerceToString(
               v.payload.path.path
             : copyToStore
             ? store->printStorePath(copyPathToStore(context, v.path()))
+            : v.path().accessor->toStringReturnsStorePath()
+            ? store->printStorePath(copyPathToStore(context, SourcePath(v.path().accessor, CanonPath::root))) + v.path().path.absOrEmpty()
             : std::string(v.path().path.abs());
     }
 
@@ -2447,10 +2450,14 @@ SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext
     if (v.type() == nPath)
         return v.path();
 
-    /* Similarly, handle __toString where the result may be a path
+    /* Similarly, handle outPath and __toString where the result may be a path
        value. */
     if (v.type() == nAttrs) {
-        auto i = v.attrs()->find(sToString);
+        auto i = v.attrs()->find(sOutPath);
+        if (i != v.attrs()->end()) {
+            return coerceToPath(pos, *i->value, context, errorCtx);
+        }
+        i = v.attrs()->find(sToString);
         if (i != v.attrs()->end()) {
             Value v1;
             callFunction(*i->value, v, v1, pos);
@@ -3115,6 +3122,8 @@ std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Pat
                 store,
                 fetchSettings,
                 EvalSettings::resolvePseudoUrl(value));
+            // Traditional search path lookups use the absolute path space for
+            // historical consistency.
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
             res.emplace(rootPath(CanonPath(store->toRealPath(storePath))));
         } catch (Error & e) {

src/libexpr/nixexpr.hh

@@ -136,7 +136,10 @@ struct ExprPath : Expr
     ExprPath(SourcePath && path)
         : path(path)
     {
-        v.mkPath(&*path.accessor, strdup(path.path.abs().c_str()));
+        v.mkPath(
+            &*path.accessor,
+            // TODO: GC_STRDUP
+            strdup(path.path.abs().c_str()));
     }
     Value * maybeThunk(EvalState & state, Env & env) override;
     COMMON_METHODS

src/libexpr/primops/fetchTree.cc

@@ -92,7 +92,7 @@ struct FetchTreeParams {
     bool emptyRevFallback = false;
     bool allowNameArgument = false;
     bool isFetchGit = false;
-    bool returnPath = true; // whether to return a SourcePath or a StorePath
+    bool returnPath = true; // whether to return a SourcePath instead of a StorePath
 };
 
 static void fetchTree(
@@ -212,18 +212,31 @@ static void fetchTree(
 
     state.checkURI(input.toURLString());
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    if (params.returnPath) {
+        // Clang16+: change to `auto [accessor, input2] =`
+        auto pair = input.getAccessor(state.store);
+        auto & accessor = pair.first;
+        auto & input2 = pair.second;
 
-    state.allowPath(storePath);
+        emitTreeAttrs(state, input2, v,
+            [&](Value & vOutPath) {
+                state.registerAccessor(accessor);
+                vOutPath.mkPath(SourcePath { accessor, CanonPath::root });
+            },
+            params.emptyRevFallback, false);
+    } else {
+        auto pair = input.fetchToStore(state.store);
+        auto & storePath = pair.first;
+        auto & input2 = pair.second;
 
-    emitTreeAttrs(state, input2, v,
-        [&](Value & vOutPath) {
-            if (params.returnPath)
-                vOutPath.mkPath(state.rootPath(state.store->toRealPath(storePath)));
-            else
+        state.allowPath(storePath);
+
+        emitTreeAttrs(state, input2, v,
+            [&](Value & vOutPath) {
                 state.mkStorePathString(storePath, vOutPath);
-        },
-        params.emptyRevFallback, false);
+            },
+            params.emptyRevFallback, false);
+    }
 }
 
 static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)

src/libutil/posix-source-accessor.cc

@@ -195,4 +195,9 @@ ref<SourceAccessor> makeFSSourceAccessor(std::filesystem::path root)
 {
     return make_ref<PosixSourceAccessor>(std::move(root));
 }
+
+bool PosixSourceAccessor::toStringReturnsStorePath() const {
+    return false;
+}
+
 }

src/libutil/posix-source-accessor.hh

@@ -57,6 +57,8 @@ struct PosixSourceAccessor : virtual SourceAccessor
      */
     static SourcePath createAtRoot(const std::filesystem::path & path);
 
+    virtual bool toStringReturnsStorePath() const override;
+
 private:
 
     /**

src/libutil/source-accessor.cc

@@ -105,4 +105,8 @@ CanonPath SourceAccessor::resolveSymlinks(
     return res;
 }
 
+bool SourceAccessor::toStringReturnsStorePath() const {
+    return true;
+}
+
 }

src/libutil/source-accessor.hh

@@ -161,6 +161,15 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
 
     virtual std::string showPath(const CanonPath & path);
 
+    /**
+     * System paths: `toString /foo/bar = "/foo/bar"`
+     * Virtual paths: fetched to the store
+     *
+     * In both cases, the returned string functionally identifies the path,
+     * and can still be read.
+     */
+    virtual bool toStringReturnsStorePath() const;
+
     /**
      * Resolve any symlinks in `path` according to the given
      * resolution mode.