API for retrieving Weak's base pointer

I'd like to use rc::Weak as keys in a HashMap, such that clones of the same Weak map to the same value, but clones of a different Weak don't.

A cleanup routine would clean up dead Weaks every once in a while, so as to not leak memory.

This is useful because the HashMap would be inside the Rc, actually it would be inside all of the Rcs for which it has a Weak key. To write it any other way would require rewriting everything I've done so far, so it'd seriously break API compatibility.

For this to work, I need to be able to get access to the Weak's base pointer. It could even return an &MaybeUninit<T> or something, idk.

In fact it would be more useful if it did &MaybeUninit<T>, if the Rc also had a similar API. I have previously asked for the ability to compare an Rc and a Weak and this would provide that too.

No file or test is named in the issue. Start by reviewing the existing rc::Weak and Rc APIs and the HashMap use case described here, then determine the intended pointer-exposure and cleanup semantics. Done would require an agreed API design addressing cloned Weak identity and dead entries.