libexpr: Migrate EvalState::concatLists to return a Value

Part of #1136

Change-Id: Ie28ee8456191b3da20d53e95ea49e0ec6a6a6964

lix/libexpr/eval.cc

@@ -1400,9 +1400,7 @@ Value EvalState::updateAttrs(const Value & v1, const Value & v2)
     return v;
 }
 
-void EvalState::concatLists(
-    Value & v, std::span<Value> lists, const PosIdx pos, std::string_view errorCtx
-)
+Value EvalState::concatLists(std::span<Value> lists, const PosIdx pos, std::string_view errorCtx)
 {
     ctx.stats.nrListConcats++;
 
@@ -1418,12 +1416,10 @@ void EvalState::concatLists(
     }
 
     if (nonEmpty && len == nonEmpty->listSize()) {
-        v = *nonEmpty;
-        return;
+        return *nonEmpty;
     }
 
     auto list = ctx.mem.newList(len);
-    v = {NewValueAs::list, list};
     auto out = list->elems;
     for (size_t n = 0, pos = 0; n < lists.size(); ++n) {
         auto l = lists[n].listSize();
@@ -1432,6 +1428,7 @@ void EvalState::concatLists(
         }
         pos += l;
     }
+    return {NewValueAs::list, list};
 }
 
 // always force this to be separate, otherwise forceValue may inline it and take

lix/libexpr/eval.hh

@@ -827,8 +827,7 @@ public:
         Value & v);
 
     Value updateAttrs(const Value & v1, const Value & v2);
-    void
-    concatLists(Value & v, std::span<Value> lists, const PosIdx pos, std::string_view errorCtx);
+    Value concatLists(std::span<Value> lists, const PosIdx pos, std::string_view errorCtx);
 
 private:
 

lix/libexpr/primops.cc

@@ -2367,8 +2367,7 @@ static void prim_elem(EvalState & state, Value * * args, Value & v)
 static void prim_concatLists(EvalState & state, Value * * args, Value & v)
 {
     state.forceList(*args[0], noPos, "while evaluating the first argument passed to builtins.concatLists");
-    state.concatLists(
-        v,
+    v = state.concatLists(
         std::span{args[0]->listElems(), args[0]->listSize()},
         noPos,
         "while evaluating a value of the list passed to builtins.concatLists"