Remove impl AsObjectArg<T> for &mut Gd<T>

`&mut obj` args can only be passed once, since &mut T: !Copy. This means
that passing it to a Godot API works once, and then causes moved-out
errors. This comes with a few issues:

* It either needs the `&*` pattern or not, depending on the order of
  calls. Reordering breaks them.

* It's inconsistent with `AsArg`, which is only implemented for `&T` in
  case of by-ref builtins.

* Consuming `&mut T` should probably be allowed iff consuming `T` is
  allowed.

That said, we'll need to check the ergonomic impact and see how
arguments are being passed in practice.

Author: Bromeon

godot-core/src/obj/object_arg.rs

@@ -25,7 +25,6 @@ use std::ptr;
 /// The GDExtension API does not inform about nullability of its function parameters. It is up to you to verify that the arguments you pass
 /// are only null when this is allowed. Doing this wrong should be safe, but can lead to the function call failing.
 /// </div>
-
 #[diagnostic::on_unimplemented(
     message = "Argument of type `{Self}` cannot be passed to an `impl AsObjectArg<{T}>` parameter",
     note = "If you pass by value, consider borrowing instead.",
@@ -80,22 +79,6 @@ where
     }
 }
 
-impl<T, U> AsObjectArg<T> for &mut Gd<U>
-where
-    T: GodotClass + Bounds<Declarer = bounds::DeclEngine>,
-    U: Inherits<T>,
-{
-    // Delegate to &Gd impl.
-
-    fn as_object_arg(&self) -> ObjectArg<T> {
-        <&Gd<U>>::as_object_arg(&&**self)
-    }
-
-    fn consume_arg(self) -> ObjectCow<T> {
-        <&Gd<U>>::consume_arg(&*self)
-    }
-}
-
 impl<T, U> AsObjectArg<T> for Option<U>
 where
     T: GodotClass + Bounds<Declarer = bounds::DeclEngine>,

itest/rust/src/object_tests/object_arg_test.rs

@@ -39,8 +39,12 @@ fn object_arg_borrowed() {
 fn object_arg_borrowed_mut() {
     with_objects(|mut manual, mut refc| {
         let db = ClassDb::singleton();
-        let a = db.class_set_property(&mut manual, "name", &Variant::from("hello"));
-        let b = db.class_set_property(&mut refc, "value", &Variant::from(-123));
+
+        let manual_ref = &mut manual;
+        let refc_ref = &mut refc;
+
+        let a = db.class_set_property(&*manual_ref, "name", &Variant::from("hello"));
+        let b = db.class_set_property(&*refc_ref, "value", &Variant::from(-123));
         (a, b)
     });
 }
@@ -69,10 +73,16 @@ fn object_arg_option_borrowed() {
 
 #[itest]
 fn object_arg_option_borrowed_mut() {
+    // If you have an Option<&mut Gd<T>>, you can use as_deref() to get Option<&Gd<T>>.
+
     with_objects(|mut manual, mut refc| {
         let db = ClassDb::singleton();
-        let a = db.class_set_property(Some(&mut manual), "name", &Variant::from("hello"));
-        let b = db.class_set_property(Some(&mut refc), "value", &Variant::from(-123));
+
+        let manual_opt: Option<&mut Gd<Node>> = Some(&mut manual);
+        let refc_opt: Option<&mut Gd<RefcPayload>> = Some(&mut refc);
+
+        let a = db.class_set_property(manual_opt.as_deref(), "name", &Variant::from("hello"));
+        let b = db.class_set_property(refc_opt.as_deref(), "value", &Variant::from(-123));
         (a, b)
     });
 }