fix(3p/nix): Use a proper pointer in Env to carry with-attrs

This eliminates the value-smuggling that would trip up the GC.

Change-Id: I8057df78cf0bf6bea9faf1b44233aa9820ae44f5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1504
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/nix/src/libexpr/eval.cc

@@ -600,10 +600,13 @@ inline Value* EvalState::lookupVar(Env* env, const ExprVar& var, bool noEval) {
       if (noEval) {
         return nullptr;
       }
+      if (!env->withAttrsExpr) {
+        CHECK(false) << "HasWithExpr evaluated twice";
+      }
       Value* v = allocValue();
-      // TODO(kanepyork): Here's the other end of the cast smuggle.
-      evalAttrs(*env->up, reinterpret_cast<Expr*>(env->values[0]), *v);
+      evalAttrs(*env->up, env->withAttrsExpr, *v);
       env->values[0] = v;
+      env->withAttrsExpr = nullptr;
       env->type = Env::HasWithAttrs;
     }
     Bindings::iterator j = env->values[0]->attrs->find(var.name);
@@ -1179,9 +1182,9 @@ void ExprWith::eval(EvalState& state, Env& env, Value& v) {
   env2.up = &env;
   env2.prevWith = prevWith;
   env2.type = Env::HasWithExpr;
-  // TODO(kanepyork): Figure out what's going on here. `Expr* attrs` is not
-  // layout-compatible with Value*.
-  env2.values[0] = reinterpret_cast<Value*>(attrs);
+  /* placeholder for result of attrs */
+  env2.values[0] = nullptr;
+  env2.withAttrsExpr = this->attrs;
 
   body->eval(state, env2, v);
 }

third_party/nix/src/libexpr/eval.hh

@@ -42,6 +42,7 @@ struct Env : public gc {
   unsigned short prevWith : 14;  // nr of levels up to next `with' environment
   enum { Plain = 0, HasWithExpr, HasWithAttrs } type : 2;
   std::vector<Value*, traceable_allocator<Value*>> values;
+  Expr* withAttrsExpr = nullptr;
 };
 
 Value& mkString(Value& v, const std::string& s,