`Pin` is unsound due to rogue Deref/DerefMut implementations

[RalfJung]

@comex found a soundness bug in Pin. This issue is to track that.

Discussion is happening on IRLO currently, let's try not to fork that.

[eddyb]

I don't think people are necessarily seeing this, pinging all labelled teams:
cc @rust-lang/compiler @rust-lang/lang @rust-lang/libs

[est31]

PR with a proposed (partial, thanks @RalfJung) fix: #67039

[RalfJung]

Well, a proposed partial fix.

[sugar700]

I still think the partial fix is worth it. I don't think we want to resolve this particular issue in any other way. Meanwhile, other soundness issues are more complicated, and can be solved separately from PartialEq issue (my preferred solution would be to prevent DerefMut implementations for &T and Clone implementations for &mut T, but I think this is still being discussed).

[RalfJung]

I still think the partial fix is worth it.

Fully agreed, and thanks for preparing that PR! I was just pointing out that the PR will not be sufficient to consider the problem solved.