libexpr: Make emptyBindings a global constant

This object is always constant and will never get modified.
Having it as a global (constant) static is much easier and
unclutters the EvalState.

Same idea as in f017f9ddd3.

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

src/libexpr/attr-path.cc

@@ -110,8 +110,8 @@ std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v
 {
     Value * v2;
     try {
-        auto dummyArgs = state.allocBindings(0);
-        v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v).first;
+        auto & dummyArgs = Bindings::emptyBindings;
+        v2 = findAlongAttrPath(state, "meta.position", dummyArgs, v).first;
     } catch (Error &) {
         throw NoPositionInfo("package '%s' has no source location information", what);
     }

src/libexpr/attr-set.cc

@@ -5,13 +5,15 @@
 
 namespace nix {
 
+Bindings Bindings::emptyBindings;
+
 /* Allocate a new array of attributes for an attribute set with a specific
    capacity. The space is implicitly reserved after the Bindings
    structure. */
 Bindings * EvalState::allocBindings(size_t capacity)
 {
     if (capacity == 0)
-        return &emptyBindings;
+        return &Bindings::emptyBindings;
     if (capacity > std::numeric_limits<Bindings::size_t>::max())
         throw Error("attribute set of size %d is too big", capacity);
     nrAttrsets++;

src/libexpr/eval.cc

@@ -202,7 +202,6 @@ EvalState::EvalState(
     , settings{settings}
     , symbols(StaticEvalSymbols::staticSymbolTable())
     , repair(NoRepair)
-    , emptyBindings(Bindings())
     , storeFS(makeMountedSourceAccessor({
           {CanonPath::root, makeEmptySourceAccessor()},
           /* In the pure eval case, we can simply require

src/libexpr/include/nix/expr/attr-set.hh

@@ -54,6 +54,12 @@ public:
     typedef uint32_t size_t;
     PosIdx pos;
 
+    /**
+     * An instance of bindings objects with 0 attributes.
+     * This object must never be modified.
+     */
+    static Bindings emptyBindings;
+
 private:
     size_t size_ = 0;
     Attr attrs[0];

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

@@ -313,8 +313,6 @@ public:
      */
     RepairFlag repair;
 
-    Bindings emptyBindings;
-
     /**
      * Empty list constant.
      */

src/libexpr/primops.cc

@@ -3326,14 +3326,14 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value ** args
 {
     state.forceValue(*args[0], pos);
     if (args[0]->isPrimOpApp() || args[0]->isPrimOp()) {
-        v.mkAttrs(&state.emptyBindings);
+        v.mkAttrs(&Bindings::emptyBindings);
         return;
     }
     if (!args[0]->isLambda())
         state.error<TypeError>("'functionArgs' requires a function").atPos(pos).debugThrow();
 
     if (!args[0]->lambda().fun->hasFormals()) {
-        v.mkAttrs(&state.emptyBindings);
+        v.mkAttrs(&Bindings::emptyBindings);
         return;
     }