ICE with (hopefully) illegal CoerceUnsized impl

[nikomatsakis]

I was experimenting with CoerceUnsized and I encountered an ICE with this example:

#[derive(Clone)]
struct Redirectable<'a, T: ?Sized> {
    data: Rc<Cell<&'a T>>
}

impl<U, T: CoerceUnsized<U>> CoerceUnsized<Redirectable<'_, U>> for Redirectable<'_, T> {}

This is hopefully an illegal CoerceUnsized impl, as it would permit us to do illegal mutation (see the full playground link above for the context). Possible duplicate of #44861.

Note that the ICE requires a feature gate.

[eddyb]

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `&mut T: std::marker::Unsize`
#1 [coerce_unsized_info] processing `<Redirectable<'_, T> as std::ops::CoerceUnsized<Redirectable<'_, U>>>`
#2 [coherent_trait] coherence checking all impls of trait `std::ops::CoerceUnsized`
#3 [analysis] running analysis passes on this crate
end of query stack

Judging by this, I'd say this isn't that scary, since it ICEs while trying to check the validity of the CoerceUnsized impl.

But also, &T never implements Unsize, so this could never compile even if the checking was buggy, is Rc<Cell<&T>> the intended example, or is that one too many indirections?

[Alexendoo]

Fixed by #72194