missing data structure helper -- didn't you already just name-check that though, since that's basically RefCell .. or if you're willing to roll the dice... UnsafeCell (aka "trust me I know what I'm doing")?
trait Allocator {
fn allocate<'a, T>(&'a self, init: T) -> Handle<'a, T>;
fn deallocate<'a, T>(&'a self, handle: Handle<'a, T>);
fn read<T>(&self, handle: Handle<'_, T>) -> impl Deref<T>;
fn write<T>(&self, handle: Handle<'_, T>) -> impl DerefMut<T>;
}
Effectively, handles are like weak pointers in that you can detect when the underlying object has been freed, but unlike weak pointers, there's no need for a reference counter to know when to deallocate the object--the object is freed when the allocator itself dies, or it can manually be freed earlier. It is possible to write code that will attempt to use the freed object, and the compiler will be happy, but the runtime will detect that it has been freed and panic instead. (RefCell does something similar, except it only detects violations of multiple readers xor one writer requirement, not overall lifteime). You can also add other wrappers around Handles to automatically free those Handles on scope exit, but the point is you can now have multiple references to an object that can be upgraded to a mutable reference if you desire.