diff --git a/src/libcore/iter/sources.rs b/src/libcore/iter/sources.rs
index 103d02da22f0..59220b320918 100644
--- a/src/libcore/iter/sources.rs
+++ b/src/libcore/iter/sources.rs
@@ -443,6 +443,8 @@ unsafe impl<A, F: FnOnce() -> A> TrustedLen for OnceWith<F> {}
 /// Basic usage:
 ///
 /// ```
+/// #![feature(iter_once_with)]
+///
 /// use std::iter;
 ///
 /// // one is the loneliest number