callPathFilter(): Pass a lazy store path

Fixes 'nix build --json --dry-run https://api.flakehub.com/f/pinned/9glenda/ocaml-flake/0.2.2/018b9a82-26e4-7990-8aa8-65d2b7c28df1/source.tar.gz#devShells.x86_64-linux.default'.

src/libexpr/eval.cc

@@ -957,7 +957,7 @@ void EvalState::mkPos(Value & v, PosIdx p)
     auto origin = positions.originOf(p);
     if (auto path = std::get_if<SourcePath>(&origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(encodePath(*path));
+        attrs.alloc(sFile).mkString(encodePath(*path)); // FIXME
         makePositionThunks(*this, p, attrs.alloc(sLine), attrs.alloc(sColumn));
         v.mkAttrs(attrs);
     } else
@@ -977,12 +977,8 @@ void EvalState::mkStorePathString(const StorePath & p, Value & v)
 
 void EvalState::mkPathString(Value & v, const SourcePath & path)
 {
-    assert(path.path.isRoot());
-
-    auto s = encodePath(path);
-
     v.mkString(
-        s,
+        encodePath(path),
         NixStringContext {
             NixStringContextElem::InputAccessor { .accessor = path.accessor->number },
         });

src/libexpr/eval.hh

@@ -778,7 +778,6 @@ public:
     bool callPathFilter(
         Value * filterFun,
         const SourcePath & path,
-        std::string_view pathArg,
         PosIdx pos);
 
 private:

src/libexpr/primops.cc

@@ -2269,15 +2269,20 @@ static RegisterPrimOp primop_toFile({
 bool EvalState::callPathFilter(
     Value * filterFun,
     const SourcePath & path,
-    std::string_view pathArg,
     PosIdx pos)
 {
     auto st = path.lstat();
 
     /* Call the filter function.  The first argument is the path, the
        second is a string indicating the type of the file. */
     Value arg1;
-    arg1.mkString(pathArg);
+    if (path.accessor == rootFS)
+        arg1.mkString(path.path.abs());
+    else
+        /* Backwards compatibility: encode the path as a lazy store
+           path string with context so that e.g. `dirOf path ==
+           "/nix/store"`. */
+        mkPathString(arg1, path);
 
     // assert that type is not "unknown"
     Value * args []{&arg1, fileTypeToString(*this, st.type)};
@@ -2320,7 +2325,7 @@ static void addPath(
         if (filterFun)
             filter = std::make_unique<PathFilter>([&](const Path & p) {
                 auto p2 = CanonPath(p);
-                return state.callPathFilter(filterFun, {path.accessor, p2}, p2.abs(), pos);
+                return state.callPathFilter(filterFun, {path.accessor, p2}, pos);
             });
 
         std::optional<StorePath> expectedStorePath;

src/libexpr/primops/filterPath.cc

@@ -23,7 +23,7 @@ struct FilterPathInputAccessor : CachingFilteringInputAccessor
             return false;
         // Note that unlike 'builtins.{path,filterSource}', we don't
         // pass the prefix to the filter function.
-        return state.callPathFilter(filterFun, {next, prefix / path}, path.abs(), pos);
+        return state.callPathFilter(filterFun, {next, prefix / path}, pos);
     }
 };