diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index 1eec4def1bbff..9efac3e8d4cb2 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -1024,9 +1024,6 @@ impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
     }
 }
 
-#[unstable(feature = "coerce_unsized", issue = "27732")]
-impl<T: CoerceUnsized<U>, U> CoerceUnsized<ManuallyDrop<U>> for ManuallyDrop<T> {}
-
 /// A pinned reference.
 ///
 /// A pinned reference is a lot like a mutable reference, except that it is not