disable a doctests that does not work yet

library/core/src/mem/maybe_dangling.rs

diff --git a/library/core/src/mem/maybe_dangling.rs b/library/core/src/mem/maybe_dangling.rs
index 3a8af8c..3c54377 100644
--- a/library/core/src/mem/maybe_dangling.rs
+++ b/library/core/src/mem/maybe_dangling.rs
@@ -36,7 +36,8 @@
 ///
 /// To fix this we could use `MaybeDangling`:
 ///
-/// ```rust
+// FIXME: remove `no_run` once the semantics are actually implemented
+/// ```rust,no_run
 /// #![feature(maybe_dangling, box_as_ptr)]
 /// # use std::alloc::{dealloc, Layout};
 /// # use std::mem::{self, MaybeDangling};