Mark Value pointers in Value::elems as const

This catches modification of finalized values (e.g. in prim_sort).

src/libexpr/eval.cc

@@ -1946,7 +1946,7 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
 }
 
 
-void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx)
+void EvalState::concatLists(Value & v, size_t nrLists, Value * const * lists, const PosIdx pos, std::string_view errorCtx)
 {
     nrListConcats++;
 

src/libexpr/eval.hh

@@ -671,7 +671,7 @@ public:
         const SingleDerivedPath & p,
         Value & v);
 
-    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
+    void concatLists(Value & v, size_t nrLists, Value * const * lists, const PosIdx pos, std::string_view errorCtx);
 
     /**
      * Print statistics, if enabled.

src/libexpr/primops.cc

@@ -3372,7 +3372,6 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
     auto list = state.buildList(len);
     for (const auto & [n, v] : enumerate(list))
         state.forceValue(*(v = args[1]->listElems()[n]), pos);
-    v.mkList(list);
 
     auto comparator = [&](Value * a, Value * b) {
         /* Optimization: if the comparator is lessThan, bypass
@@ -3391,7 +3390,9 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
     /* FIXME: std::sort can segfault if the comparator is not a strict
        weak ordering. What to do? std::stable_sort() seems more
        resilient, but no guarantees... */
-    std::stable_sort(v.listElems(), v.listElems() + len, comparator);
+    std::stable_sort(list.begin(), list.end(), comparator);
+
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_sort({

src/libexpr/value.hh

@@ -246,7 +246,7 @@ public:
         Bindings * attrs;
         struct {
             size_t size;
-            Value * * elems;
+            Value * const * elems;
         } bigList;
         Value * smallList[2];
         ClosureThunk thunk;
@@ -425,7 +425,7 @@ public:
         return internalType == tList1 || internalType == tList2 || internalType == tListN;
     }
 
-    Value * * listElems()
+    Value * const * listElems()
     {
         return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
     }