Allow shortening lifetime in CoerceUnsized for &mut

[theemathas]

This modifies the &mut -> &mut CoerceUnsized impl so that, it can shorten the lifetime.

Note that there are already two impls that allow shortening the lifetime like this (the &mut T -> &U and the &T -> &U impls). So this change makes the impls consistent with each other.

I initially tried to also do the same to the CoerceUnsized impl for core::cell::{Ref, RefMut}. However, this can't be done because Ref and RefMut "store" the lifetime and the data in different fields, and CoerceUnsized can only coerce one field.

I don't know if there is a visible effect in stable code or not.

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[theemathas]

I don't quite understand the errors here...

[zachs18]

IIUC, the errors are because when implementing CoerceUnsized for ADTs, the non-coerced fields (i.e. the fields other than the pointer field) must be exactly the same type between the source and destination instantiations in the current implementation.

https://github.com/rust-lang/rust/blob/94b49fd998d6723e0a9240a7cff5f9df37b84dd8/compiler/rustc_hir_analysis/src/coherence/builtin.rs#L485-502 (the error is produced on line 557, lines 485-492 describe the check)

Since BorrowRefMut<'a> is not exactly the same type as BorrowRefMut<'b> (as per Ty::eq), this isn't allowed. The comment starting on line 552 (just before the equality check) is also probably relevant

rust/compiler/rustc_hir_analysis/src/coherence/builtin.rs

Lines 522 to 533 in 94b49fd

	// Ignore fields that aren't changed; it may
	// be that we could get away with subtyping or
	// something more accepting, but we use
	// equality because we want to be able to
	// perform this check without computing
	// variance or constraining opaque types' hidden types.
	// (This is because we may have to evaluate constraint
	// expressions in the course of execution.)
	// See e.g., #41936.
	if a == b {
	return None;
	}

[theemathas]

I changed the PR to just edit the impl for &mut, and leave the impls for Ref and RefMut alone. I don't know if this is the right way to go though...