Rollup merge of rust-lang#48628 - strake:reverse, r=sfackler

Fixes rust-lang#47311.
r? @nrc

Author: Manishearth

src/libcore/cmp.rs

@@ -343,7 +343,7 @@ impl Ordering {
 /// v.sort_by_key(|&num| (num > 3, Reverse(num)));
 /// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
 /// ```
-#[derive(PartialEq, Eq, Debug, Copy, Clone)]
+#[derive(PartialEq, Eq, Debug, Copy, Clone, Default, Hash)]
 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 pub struct Reverse<T>(#[stable(feature = "reverse_cmp_key", since = "1.19.0")] pub T);