de4d5ce11e
Make `PolyTraitRef::self_ty` return `Binder<Ty>` This came up during review of #71618. The current implementation is the same as a call to `skip_binder` but harder to audit. Make it preserve binding levels and add a call to `skip_binder` at all use sites so they can be audited as part of #72507.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.