Implement "&<pat> everywhere"

The original proposal allows reference patterns
with "compatible" mutability, however it's not clear
what that means so for now we require an exact match.

I don't know the type system code well, so if something
seems to not make sense it's probably because I made a
mistake

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -65,9 +65,16 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
         // we wrap the unadjusted pattern in `PatKind::Deref` repeatedly, consuming the
         // adjustments in *reverse order* (last-in-first-out, so that the last `Deref` inserted
         // gets the least-dereferenced type).
-        let unadjusted_pat = self.lower_pattern_unadjusted(pat);
+        let unadjusted = if self.typeck_results.ref_pats_that_dont_deref().contains(pat.hir_id) {
+            match pat.kind {
+                hir::PatKind::Ref(inner, _) => self.lower_pattern_unadjusted(inner),
+                _ => span_bug!(pat.span, "non ref pattern marked as non-deref ref pattern"),
+            }
+        } else {
+            self.lower_pattern_unadjusted(pat)
+        };
         self.typeck_results.pat_adjustments().get(pat.hir_id).unwrap_or(&vec![]).iter().rev().fold(
-            unadjusted_pat,
+            unadjusted,
             |pat: Box<_>, ref_ty| {
                 debug!("{:?}: wrapping pattern with type {:?}", pat, ref_ty);
                 Box::new(Pat {