UI to unit test for those using Cell/RefCell/UnsafeCell

library/alloc/tests/boxed.rs

@@ -49,3 +49,12 @@ fn box_clone_from_ptr_stability() {
         assert_eq!(copy.as_ptr() as usize, copy_raw);
     }
 }
+
+#[test]
+fn box_deref_lval() {
+    use std::cell::Cell;
+
+    let x = Box::new(Cell::new(5));
+    x.set(1000);
+    assert_eq!(x.get(), 1000);
+}