libexpr: Make constant Values global constants, move out of EvalState

These constant Values have no business being in the EvalState in the
first place. The ultimate goal is to get rid of the ugly `getBuiltins`
and its relience (in `createBaseEnv`) on these global constants is getting in the way.

Same idea as in f017f9ddd3.

Co-authored-by: eldritch horrors <pennae@lix.systems>

src/libexpr/eval.cc

@@ -284,10 +284,6 @@ EvalState::EvalState(
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
 
-    vEmptyList.mkList(buildList(0));
-    vNull.mkNull();
-    vTrue.mkBool(true);
-    vFalse.mkBool(false);
     vStringRegular.mkStringNoCopy("regular");
     vStringDirectory.mkStringNoCopy("directory");
     vStringSymlink.mkStringNoCopy("symlink");
@@ -894,7 +890,7 @@ ListBuilder::ListBuilder(EvalState & state, size_t size)
 
 Value * EvalState::getBool(bool b)
 {
-    return b ? &vTrue : &vFalse;
+    return b ? &Value::vTrue : &Value::vFalse;
 }
 
 unsigned long nrThunks = 0;
@@ -1300,7 +1296,7 @@ void ExprList::eval(EvalState & state, Env & env, Value & v)
 Value * ExprList::maybeThunk(EvalState & state, Env & env)
 {
     if (elems.empty()) {
-        return &state.vEmptyList;
+        return &Value::vEmptyList;
     }
     return Expr::maybeThunk(state, env);
 }

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

@@ -313,32 +313,6 @@ public:
      */
     RepairFlag repair;
 
-    /**
-     * Empty list constant.
-     */
-    Value vEmptyList;
-
-    /**
-     * `null` constant.
-     *
-     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
-     */
-    Value vNull;
-
-    /**
-     * `true` constant.
-     *
-     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
-     */
-    Value vTrue;
-
-    /**
-     * `true` constant.
-     *
-     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
-     */
-    Value vFalse;
-
     /** `"regular"` */
     Value vStringRegular;
     /** `"directory"` */

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

@@ -833,6 +833,34 @@ struct Value : public ValueStorage<sizeof(void *)>
 {
     friend std::string showType(const Value & v);
 
+    /**
+     * Empty list constant.
+     *
+     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
+     */
+    static Value vEmptyList;
+
+    /**
+     * `null` constant.
+     *
+     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
+     */
+    static Value vNull;
+
+    /**
+     * `true` constant.
+     *
+     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
+     */
+    static Value vTrue;
+
+    /**
+     * `true` constant.
+     *
+     * This is _not_ a singleton. Pointer equality is _not_ sufficient.
+     */
+    static Value vFalse;
+
 private:
     template<InternalType... discriminator>
     bool isa() const noexcept

src/libexpr/meson.build

@@ -163,6 +163,7 @@ sources = files(
   'search-path.cc',
   'value-to-json.cc',
   'value-to-xml.cc',
+  'value.cc',
   'value/context.cc',
 )
 

src/libexpr/primops.cc

@@ -1075,11 +1075,11 @@ static void prim_tryEval(EvalState & state, const PosIdx pos, Value ** args, Val
     try {
         state.forceValue(*args[0], pos);
         attrs.insert(state.s.value, args[0]);
-        attrs.insert(state.symbols.create("success"), &state.vTrue);
+        attrs.insert(state.symbols.create("success"), &Value::vTrue);
     } catch (AssertionError & e) {
         // `value = false;` is unfortunate but removing it is a breaking change.
-        attrs.insert(state.s.value, &state.vFalse);
-        attrs.insert(state.symbols.create("success"), &state.vFalse);
+        attrs.insert(state.s.value, &Value::vFalse);
+        attrs.insert(state.symbols.create("success"), &Value::vFalse);
     }
 
     // restore the debugRepl pointer if we saved it earlier.
@@ -4613,7 +4613,7 @@ void prim_match(EvalState & state, const PosIdx pos, Value ** args, Value & v)
         auto list = state.buildList(match.size() - 1);
         for (const auto & [i, v2] : enumerate(list))
             if (!match[i + 1].matched)
-                v2 = &state.vNull;
+                v2 = &Value::vNull;
             else
                 v2 = mkString(state, match[i + 1]);
         v.mkList(list);
@@ -4705,7 +4705,7 @@ void prim_split(EvalState & state, const PosIdx pos, Value ** args, Value & v)
             auto list2 = state.buildList(slen);
             for (const auto & [si, v2] : enumerate(list2)) {
                 if (!match[si + 1].matched)
-                    v2 = &state.vNull;
+                    v2 = &Value::vNull;
                 else
                     v2 = mkString(state, match[si + 1]);
             }
@@ -5059,7 +5059,7 @@ void EvalState::createBaseEnv(const EvalSettings & evalSettings)
 
     addConstant(
         "null",
-        &vNull,
+        &Value::vNull,
         {
             .type = nNull,
             .doc = R"(

src/libexpr/value.cc

@@ -0,0 +1,29 @@
+#include "nix/expr/value.hh"
+
+namespace nix {
+
+Value Value::vEmptyList = []() {
+    Value res;
+    res.setStorage(List{.size = 0, .elems = nullptr});
+    return res;
+}();
+
+Value Value::vNull = []() {
+    Value res;
+    res.mkNull();
+    return res;
+}();
+
+Value Value::vTrue = []() {
+    Value res;
+    res.mkBool(true);
+    return res;
+}();
+
+Value Value::vFalse = []() {
+    Value res;
+    res.mkBool(false);
+    return res;
+}();
+
+} // namespace nix

src/nix/nix-env/nix-env.cc

@@ -158,7 +158,7 @@ static void loadSourceExpr(EvalState & state, const SourcePath & path, Value & v
        directory). */
     else if (st.type == SourceAccessor::tDirectory) {
         auto attrs = state.buildBindings(maxAttrs);
-        attrs.insert(state.symbols.create("_combineChannels"), &state.vEmptyList);
+        attrs.insert(state.symbols.create("_combineChannels"), &Value::vEmptyList);
         StringSet seen;
         getAllExprs(state, path, seen, attrs);
         v.mkAttrs(attrs);