The Pin unsoundness and `CoerceUnsized`

[comex]

@HeroicKatora

I would argue that this coercion was broken, and not what was achieved with the dyn Dubious<'a> afterwards and fixing it properly might fix all exploits and not even require a negative impl for Clone on mutable references!

It doesn't obviate the need for negative impls. To quote myself from the original thread:

@comex

Here is a version that does the coercion before pinning. It goes from Box<Wrapper> to Box<dyn Dubious> to Pin<Box<dyn Dubious>> to Pin<&mut dyn Dubious> , so there's no coercion happening within the pin.

I'd like to make a version that doesn't use trait objects at all, but the lifetimes don't work out; having a type contain a mutable reference to itself makes the borrow checker act rather weird. I may have been wrong when I said trait objects weren't the only possibility.