diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index 89afe7057b17b..5b78aa632d17f 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -65,6 +65,7 @@ use self::Ordering::*;
 /// the same book if their ISBN matches, even if the formats differ:
 ///
 /// ```
+/// #[derive(PartialEq)]
 /// enum BookFormat {
 ///     Paperback,
 ///     Hardback,