It's interesting that the intent discussed there doesn't match rustc's current behavior, which avoids niche optimization for UnsafeCell and RefCell, but still does it for Cell, RwLock, and Mutex.
Though I suppose for RwLock/Mutex it might be niching a different part of the structure, e.g. the Box for the system impl. So that could match the discussed plan + reënabling niching for Cell.