Suggest using a lock for `*Cell: Sync` bounds I mostly did this for `OnceCell<T>` at first because users will be confused to see that the `OnceCell<T>` in `std` isn't `Sync` but then extended it to `Cell<T>` and `RefCell<T>` as well.
rustc_on_unimplemented
Sync
bindings_with_variant_name