symbol-table: reference entries instead of allocating Values

src/libexpr/eval.cc

@@ -90,6 +90,11 @@ std::string printValue(EvalState & state, Value & v)
     return out.str();
 }
 
+Value * Value::toPtr(SymbolStr str) noexcept
+{
+    return const_cast<Value *>(str.valuePtr());
+}
+
 void Value::print(EvalState & state, std::ostream & str, PrintOptions options)
 {
     printValue(state, str, *this, options);
@@ -919,11 +924,6 @@ void Value::mkStringMove(const char * s, const NixStringContext & context)
     mkString(s, encodeContext(context));
 }
 
-void Value::mkString(const SymbolStr & s)
-{
-    mkString(s.c_str(), nullptr);
-}
-
 void Value::mkPath(const SourcePath & path)
 {
     mkPath(&*path.accessor, makeImmutableString(path.path.abs()));

src/libexpr/include/nix/expr/symbol-table.hh

@@ -145,6 +145,12 @@ public:
         return s->size_;
     }
 
+    [[gnu::always_inline]]
+    const Value * valuePtr() const noexcept
+    {
+        return s;
+    }
+
     explicit operator Symbol() const noexcept
     {
         return Symbol{s->idx + 1};

src/libexpr/include/nix/expr/value.hh

@@ -172,6 +172,11 @@ private:
 
 public:
 
+    /**
+     * Never modify the backing `Value` object!
+     */
+    static Value * toPtr(SymbolStr str) noexcept;
+
     void print(EvalState &state, std::ostream &str, PrintOptions options = PrintOptions {});
 
     // Functions needed to distinguish the type
@@ -331,8 +336,6 @@ public:
 
     void mkStringMove(const char * s, const NixStringContext & context);
 
-    void mkString(const SymbolStr & s);
-
     void mkPath(const SourcePath & path);
     void mkPath(std::string_view path);
 

src/libexpr/primops.cc

@@ -2710,7 +2710,7 @@ static void prim_attrNames(EvalState & state, const PosIdx pos, Value * * args,
     auto list = state.buildList(args[0]->attrs()->size());
 
     for (const auto & [n, i] : enumerate(*args[0]->attrs()))
-        (list[n] = state.allocValue())->mkString(state.symbols[i.name]);
+        list[n] = Value::toPtr(state.symbols[i.name]);
 
     std::sort(list.begin(), list.end(),
               [](Value * v1, Value * v2) { return strcmp(v1->c_str(), v2->c_str()) < 0; });
@@ -3170,9 +3170,8 @@ static void prim_mapAttrs(EvalState & state, const PosIdx pos, Value * * args, V
     auto attrs = state.buildBindings(args[1]->attrs()->size());
 
     for (auto & i : *args[1]->attrs()) {
-        Value * vName = state.allocValue();
+        Value * vName = Value::toPtr(state.symbols[i.name]);
         Value * vFun2 = state.allocValue();
-        vName->mkString(state.symbols[i.name]);
         vFun2->mkApp(args[0], vName);
         attrs.alloc(i.name).mkApp(vFun2, i.value);
     }
@@ -3236,8 +3235,7 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
     auto attrs = state.buildBindings(attrsSeen.size());
 
     for (auto & [sym, elem] : attrsSeen) {
-        auto name = state.allocValue();
+        auto name = Value::toPtr(state.symbols[sym]);
-        name->mkString(state.symbols[sym]);
         auto call1 = state.allocValue();
         call1->mkApp(args[0], name);
         auto call2 = state.allocValue();