Make sure to enforce ~const DerefMut on mutability

Author: compiler-errors

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -274,7 +274,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Adjust::Pointer(_pointer_coercion) => {
                     // FIXME(effects): We should probably enforce these.
                 }
-                Adjust::ReborrowPin(_region, _mutability) => {
+                Adjust::ReborrowPin(_mutability) => {
                     // FIXME(effects): We could enforce these; they correspond to
                     // `&mut T: DerefMut` tho, so it's kinda moot.
                 }

compiler/rustc_hir_typeck/src/place_op.rs

@@ -289,13 +289,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         )
                     {
                         let method = self.register_infer_ok_obligations(ok);
-                        let ty::Ref(_, _, mutbl) = *method.sig.output().kind() else {
-                            span_bug!(
-                                self.tcx.def_span(method.def_id),
-                                "expected DerefMut to return a &mut"
-                            );
-                        };
-                        *deref = OverloadedDeref { mutbl, span: deref.span };
+                        if let ty::Ref(_, _, mutbl) = *method.sig.output().kind() {
+                            *deref = OverloadedDeref { mutbl, span: deref.span };
+                        }
+                        self.enforce_context_effects(expr.span, method.def_id, method.args);
                         // If this is a union field, also throw an error for `DerefMut` of `ManuallyDrop` (see RFC 2514).
                         // This helps avoid accidental drops.
                         if inside_union