libexpr: Migrate mkStorePathString to return a Value

part of #1136

Change-Id: I32ba0f8ef369a82ceb8f75816a2ce6dc6a6a6964

lix/legacy/user-env.cc

@@ -111,7 +111,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     /* Construct a Nix expression that calls the user environment
        builder with the manifest as argument. */
     auto attrs = state.ctx.buildBindings(3);
-    state.ctx.paths.mkStorePathString(manifestFile, attrs.alloc("manifest"));
+    attrs.alloc("manifest") = state.ctx.paths.mkStorePathString(manifestFile);
     attrs.insert(state.ctx.symbols.create("derivations"), vManifest);
     Value args = {NewValueAs::attrs, attrs};
 

lix/libexpr/eval.cc

@@ -438,7 +438,7 @@ void EvalPaths::allowAndSetStorePathString(const StorePath & storePath, Value &
 {
     allowPath(storePath);
 
-    mkStorePathString(storePath, v);
+    v = mkStorePathString(storePath);
 }
 
 CheckedSourcePath EvalPaths::checkSourcePath(const SourcePath & path_)
@@ -865,10 +865,9 @@ void EvalState::mkPos(Value & v, PosIdx p)
         v.mkNull();
 }
 
-
-void EvalPaths::mkStorePathString(const StorePath & p, Value & v)
+Value EvalPaths::mkStorePathString(const StorePath & p)
 {
-    v = {
+    return {
         NewValueAs::string,
         store->printStorePath(p),
         NixStringContext{

lix/libexpr/eval.hh

@@ -449,7 +449,7 @@ public:
      * The string is the printed store path with a context containing a
      * single `NixStringContextElem::Opaque` element of that store path.
      */
-    void mkStorePathString(const StorePath & storePath, Value & v);
+    Value mkStorePathString(const StorePath & storePath);
 };
 
 struct EvalStatistics

lix/libexpr/primops/fetchMercurial.cc

@@ -88,7 +88,7 @@ void prim_fetchMercurial(EvalState & state, Value ** args, Value & v)
     auto [tree, input2] = state.aio.blockOn(input.fetch(state.ctx.store));
 
     auto attrs2 = state.ctx.buildBindings(8);
-    state.ctx.paths.mkStorePathString(tree.storePath, attrs2.alloc(state.ctx.symbols.sym_outPath));
+    attrs2.alloc(state.ctx.symbols.sym_outPath) = state.ctx.paths.mkStorePathString(tree.storePath);
     if (input2.getRef())
         attrs2.alloc("branch") = {NewValueAs::string, *input2.getRef()};
     // Backward compatibility: set 'rev' to

lix/libexpr/primops/fetchTree.cc

@@ -26,7 +26,7 @@ Value emitTreeAttrs(
 
     auto attrs = state.buildBindings(10);
 
-    state.paths.mkStorePathString(tree.storePath, attrs.alloc(state.symbols.sym_outPath));
+    attrs.alloc(state.symbols.sym_outPath) = state.paths.mkStorePathString(tree.storePath);
 
     // FIXME: support arbitrary input attributes.
 

tests/unit/libexpr/derived-path.cc

@@ -27,8 +27,7 @@ RC_GTEST_FIXTURE_PROP(
     prop_opaque_path_round_trip,
     (const SingleDerivedPath::Opaque & o))
 {
-    Value v;
-    evaluator.paths.mkStorePathString(o.path, v);
+    Value v = evaluator.paths.mkStorePathString(o.path);
     auto d = state.coerceToSingleDerivedPath(noPos, v, "");
     RC_ASSERT(SingleDerivedPath { o } == d);
 }