Allow shortening reborrows

Generating a call to `as_mut()` let to more restrictive borrows than
what reborrowing usually gives us. Instead, we change the desugaring to
reborrow the pin internals directly which makes things more expressive.

compiler/rustc_middle/src/ty/adjustment.rs

@@ -105,9 +105,8 @@ pub enum Adjust<'tcx> {
     /// Cast into a dyn* object.
     DynStar,
 
-    /// Take a `Pin<Ptr>` and call either `as_mut()` or `as_ref()` to get a `Pin<&mut T>` or
-    /// `Pin<&T>`.
-    ReborrowPin(AutoBorrow<'tcx>),
+    /// Take a pinned reference and reborrow as a `Pin<&mut T>` or `Pin<&T>`.
+    ReborrowPin(ty::Region<'tcx>, hir::Mutability),
 }
 
 /// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`