mir_build: Don't use a mixture of THIR pattern kinds for pin-patterns
While looking for improvements to match-lowering, I had been trying to figure out why pin-patterns sometimes use `PatKind::Deref` and sometimes use `PatKind::DerefPattern`, which seemed confusing to me. In particular, both pattern kinds are being used with an inner type of `&` or `&mut`, which is very surprising.
The conclusion I came to was that `DerefPattern` (normally associated with calls to Deref/DerefMut) was only being used for *implicit* pin-patterns produced by match-ergonomics adjustment, whereas `Deref` was being used for explicit pin-patterns.
The inconsistency seems like a mistake to me, so this PR removes all uses of `DerefPattern` for pin-patterns, and consistently uses `PatKind::Deref` for pin-patterns instead. I'm not entirely happy with that outcome, because I think pin-patterns should probably have their own `thir::PatKind` variant, but this change will at least make that easier to achieve in a later PR.
r? Nadrieril
Stuart Cook
0e64ced2f7