Allow mutable derefs with feature gate

src/librustc_mir/transform/check_consts/ops.rs

@@ -216,7 +216,11 @@ impl NonConstOp for MutBorrow {
 
 #[derive(Debug)]
 pub struct MutDeref;
-impl NonConstOp for MutDeref {}
+impl NonConstOp for MutDeref {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_mut_refs)
+    }
+}
 
 #[derive(Debug)]
 pub struct Panic;

src/test/ui/consts/const-mut-refs/const_mut_refs.rs

@@ -7,7 +7,9 @@ struct Foo {
 }
 
 const fn bar(foo: &mut Foo) -> usize {
-    foo.x + 1
+    let x = &mut foo.x;
+    *x = 1;
+    *x
 }
 
 fn main() {