libexpr: extract some global constants from EvalState

these do not rely on the GC being initialized. there's no reason for
them to not be statics, shared between all eval states in a process.

Change-Id: Ib9675f3945d3a0a7097d6c85096adcbd6f441d83

lix/libexpr/attr-set.cc

@@ -7,6 +7,7 @@
 
 namespace nix {
 
+Bindings Bindings::EMPTY{0};
 
 
 /* Allocate a new array of attributes for an attribute set with a specific
@@ -15,7 +16,7 @@ namespace nix {
 Bindings * EvalState::allocBindings(size_t capacity)
 {
     if (capacity == 0)
-        return &emptyBindings;
+        return &Bindings::EMPTY;
     if (capacity > std::numeric_limits<Bindings::size_t>::max())
         throw Error("attribute set of size %d is too big", capacity);
     nrAttrsets++;

lix/libexpr/attr-set.hh

@@ -51,6 +51,8 @@ public:
     typedef uint32_t size_t;
     PosIdx pos;
 
+    static Bindings EMPTY;
+
 private:
     size_t size_, capacity_;
     Attr attrs[0];

lix/libexpr/eval.cc

@@ -255,7 +255,6 @@ EvalState::EvalState(
         .overrides = symbols.create("__overrides"),
     }
     , repair(NoRepair)
-    , emptyBindings(0)
     , derivationInternal(rootPath(CanonPath("/builtin/derivation.nix")))
     , store(store)
     , buildStore(buildStore ? buildStore : store)
@@ -276,8 +275,6 @@ EvalState::EvalState(
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
 
-    vEmptyList.mkList(0);
-
     /* Initialise the Nix expression search path. */
     if (!evalSettings.pureEval) {
         for (auto & i : _searchPath.elements)
@@ -1200,7 +1197,7 @@ void ExprList::eval(EvalState & state, Env & env, Value & v)
 Value * ExprList::maybeThunk(EvalState & state, Env & env)
 {
     if (elems.empty()) {
-        return &state.vEmptyList;
+        return &Value::EMPTY_LIST;
     }
     return Expr::maybeThunk(state, env);
 }

lix/libexpr/eval.hh

@@ -187,13 +187,6 @@ public:
      */
     std::optional<PathSet> allowedPaths;
 
-    Bindings emptyBindings;
-
-    /**
-     * Empty list constant.
-     */
-    Value vEmptyList;
-
     const SourcePath derivationInternal;
 
     /**

lix/libexpr/primops.cc

@@ -1898,14 +1898,14 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg
 {
     state.forceValue(*args[0], pos);
     if (args[0]->isPrimOpApp() || args[0]->isPrimOp()) {
-        v.mkAttrs(&state.emptyBindings);
+        v.mkAttrs(&Bindings::EMPTY);
         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::EMPTY);
         return;
     }
 

lix/libexpr/value.cc

@@ -9,6 +9,8 @@
 namespace nix
 {
 
+Value Value::EMPTY_LIST{Value::list_t{}, {}};
+
 static void copyContextToValue(Value & v, const NixStringContext & context)
 {
     if (!context.empty()) {

lix/libexpr/value.hh

@@ -197,6 +197,11 @@ private:
 
 public:
 
+    /**
+     * Empty list constant.
+     */
+    static Value EMPTY_LIST;
+
     // Discount `using NewValueAs::*;`
 // NOLINTNEXTLINE(bugprone-macro-parentheses)
 #define USING_VALUETYPE(name) using name = NewValueAs::name