Derive Trace, Finalize triggers non-local impl lint warning on nightly

[TheBB]

You need almost nothing to trigger this:

use gc::{Trace, Finalize, Gc};

#[derive(Trace, Finalize)]
struct Test();

fn main() {
    let _x = Gc::new(Test());
}

Warning:

warning: non-local `impl` definition, `impl` blocks should be written at the same level as their item
 --> src/main.rs:3:17
  |
3 | #[derive(Trace, Finalize)]
  |                 ^-------
  |                 |
  |                 `Finalize` is not local
  |                 move the `impl` block outside of this constant `_DERIVE_gc_Finalize_FOR_Test`
4 | struct Test();
  |        ---- `Test` is not local
  |

This is probably due to an implementation detail in the derive macro.

It seems this may become an error in the future, so this should be fixed.