libexpr: Migrate EvalState::mkOutputString to return a Value

part of #1136 and a small step toward resolving #744

Change-Id: I4e1602b37c40517a92cbab8fe4074d7e6a6a6964

lix/libexpr/eval.cc

@@ -882,13 +882,9 @@ std::string EvalState::mkOutputStringRaw(
     return ctx.store->printStorePath(staticOutputPath);
 }
 
-
-void EvalState::mkOutputString(
-    Value & value,
-    const SingleDerivedPath::Built & b,
-    const StorePath & staticOutputPath)
+Value EvalState::mkOutputString(const SingleDerivedPath::Built & b, const StorePath & staticOutputPath)
 {
-    value = {NewValueAs::string, mkOutputStringRaw(staticOutputPath), NixStringContext{b}};
+    return {NewValueAs::string, mkOutputStringRaw(staticOutputPath), NixStringContext{b}};
 }
 
 

lix/libexpr/eval.hh

@@ -809,18 +809,13 @@ public:
      * single `NixStringContextElem::Built` element of the drv path and
      * output name.
      *
-     * @param value Value we are settings
-     *
      * @param b the drv whose output we are making a string for, and the
      * output
      *
      * @param staticOutputPath Output path for that string.
      * Will be printed to form string.
      */
-    void mkOutputString(
-        Value & value,
-        const SingleDerivedPath::Built & b,
-        const StorePath & staticOutputPath);
+    Value mkOutputString(const SingleDerivedPath::Built & b, const StorePath & staticOutputPath);
 
     /**
      * Create a string representing a `SingleDerivedPath`.

lix/libexpr/primops.cc

@@ -158,13 +158,13 @@ static void mkOutputString(
     const StorePath & drvPath,
     const std::pair<std::string, DerivationOutput> & o)
 {
-    state.mkOutputString(
-        attrs.alloc(o.first),
-        SingleDerivedPath::Built {
+    attrs.alloc(o.first) = state.mkOutputString(
+        SingleDerivedPath::Built{
             .drvPath = makeConstantStorePath(drvPath),
             .output = o.first,
         },
-        o.second.path(*state.ctx.store, Derivation::nameFromPath(drvPath), o.first));
+        o.second.path(*state.ctx.store, Derivation::nameFromPath(drvPath), o.first)
+    );
 }
 
 /* Load and evaluate an expression from path specified by the

tests/unit/libexpr/derived-path.cc

@@ -41,8 +41,7 @@ RC_GTEST_FIXTURE_PROP(
     prop_derived_path_built_out_path_round_trip,
     (const SingleDerivedPath::Built & b, const StorePath & outPath))
 {
-    Value v;
-    state.mkOutputString(v, b, outPath);
+    Value v = state.mkOutputString(b, outPath);
     auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, v, "");
     RC_ASSERT(SingleDerivedPath { b } == d);
 }