Make sure to enforce ~const DerefMut on mutability

Author: compiler-errors

Diff for: compiler/rustc_hir_typeck/src/place_op.rs

@@ -292,6 +292,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         if let ty::Ref(region, _, mutbl) = *method.sig.output().kind() {
                             *deref = OverloadedDeref { region, 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