libexpr: Migrate Evaluator::evalLazily to return a Value

Change-Id: I9f778407d454e141cb9b7959bbc02ed26a6a6964

lix/libcmd/common-eval-args.cc

@@ -185,9 +185,7 @@ Bindings * MixEvalArgs::getAutoArgs(Evaluator & state)
     for (auto & i : autoArgs) {
         Value v;
         if (i.second[0] == 'E')
-            state.evalLazily(
-                state.parseExprFromString(i.second.substr(1), CanonPath::fromCwd()), v
-            );
+            v = state.evalLazily(state.parseExprFromString(i.second.substr(1), CanonPath::fromCwd()));
         else
             v = {NewValueAs::string, ((std::string_view) i.second).substr(1)};
         res.insert(state.symbols.create(i.first), v);

lix/libexpr/eval.cc

@@ -844,14 +844,12 @@ Value::List * EvalMemory::newList(size_t size)
     return list;
 }
 
-
-void Evaluator::evalLazily(Expr & e, Value & v)
+Value Evaluator::evalLazily(Expr & e)
 {
-    v = {NewValueAs::thunk, mem, builtins.env, e};
     stats.nrThunks++;
+    return {NewValueAs::thunk, mem, builtins.env, e};
 }
 
-
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto origin = ctx.positions.originOf(p);

lix/libexpr/eval.hh

@@ -554,7 +554,7 @@ public:
     /**
      * Creates a thunk that will evaluate the given expression when forced.
      */
-    void evalLazily(Expr & e, Value & v);
+    Value evalLazily(Expr & e);
 
     /** If debugging is enabled, returns the next trace. Otherwise, std::nullopt. */
     std::optional<DebugTrace const *> nextDebugTrace() const;