libexpr: Migrate EvalState::mkPos to return a Value

Part of #1136 and progress towards #744

Change-Id: I31d0169077954ea82c9c7d341afdccd76a6a6964

lix/libexpr/eval-expr.cc

@@ -527,9 +527,7 @@ Value ExprConcatStrings::eval(EvalState & state, Env & env)
 
 Value ExprPos::eval(EvalState & state, Env & env)
 {
-    Value v;
-    state.mkPos(v, pos);
-    return v;
+    return state.mkPos(pos);
 }
 
 Value ExprBlackHole::eval(EvalState & state, Env & env)

lix/libexpr/eval.cc

@@ -851,7 +851,7 @@ Value Evaluator::evalLazily(Expr & e)
     return {NewValueAs::thunk, mem, builtins.env, e};
 }
 
-void EvalState::mkPos(Value & v, PosIdx p)
+Value EvalState::mkPos(PosIdx p)
 {
     auto origin = ctx.positions.originOf(p);
     if (auto path = std::get_if<CheckedSourcePath>(&origin)) {
@@ -860,9 +860,9 @@ void EvalState::mkPos(Value & v, PosIdx p)
         auto [line, col] = makePositionThunks(*this, p);
         attrs.insert(ctx.symbols.sym_line, line);
         attrs.insert(ctx.symbols.sym_column, col);
-        v = {NewValueAs::attrs, attrs};
+        return {NewValueAs::attrs, attrs};
     } else
-        v = Value::VNULL;
+        return Value::VNULL;
 }
 
 Value EvalPaths::mkStorePathString(const StorePath & p)

lix/libexpr/eval.hh

@@ -800,7 +800,7 @@ public:
      */
     Value autoCallFunction(Bindings & args, Value & fun, PosIdx pos);
 
-    void mkPos(Value & v, PosIdx pos);
+    Value mkPos(PosIdx pos);
 
     /**
      * Create a string representing a `SingleDerivedPath::Built`.

lix/libexpr/primops.cc

@@ -1906,7 +1906,7 @@ static void prim_unsafeGetAttrPos(EvalState & state, Value * * args, Value & v)
     if (!i) {
         v = Value::VNULL;
     } else {
-        state.mkPos(v, i->pos);
+        v = state.mkPos(i->pos);
     }
 }