diff --git a/src/libcore/mem/manually_drop.rs b/src/libcore/mem/manually_drop.rs
index 3ad1223e331ec..bb35399323628 100644
--- a/src/libcore/mem/manually_drop.rs
+++ b/src/libcore/mem/manually_drop.rs
@@ -119,6 +119,8 @@ impl<T: ?Sized> ManuallyDrop<T> {
     /// This function runs the destructor of the contained value and thus the wrapped value
     /// now represents uninitialized data. It is up to the user of this method to ensure the
     /// uninitialized data is not actually used.
+    /// In particular, this function can only be called called at most once
+    /// for a given instance of `ManuallyDrop<T>`.
     ///
     /// [`ManuallyDrop::into_inner`]: #method.into_inner
     #[stable(feature = "manually_drop", since = "1.20.0")]