diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index 6d64d91312e0f..6836f8622b0ee 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -959,6 +959,7 @@ impl<T> ManuallyDrop<T> {
     /// ManuallyDrop::new(Box::new(()));
     /// ```
     #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_manually_drop_new")]
     #[inline]
     pub const fn new(value: T) -> ManuallyDrop<T> {
         ManuallyDrop { value: value }