Stabilize cmp::Reverse

Closes #40893

src/libcore/cmp.rs

@@ -335,18 +335,17 @@ impl Ordering {
 /// Example usage:
 ///
 /// ```
-/// #![feature(reverse_cmp_key)]
 /// use std::cmp::Reverse;
 ///
 /// let mut v = vec![1, 2, 3, 4, 5, 6];
 /// v.sort_by_key(|&num| (num > 3, Reverse(num)));
 /// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
 /// ```
 #[derive(PartialEq, Eq, Debug)]
-#[unstable(feature = "reverse_cmp_key", issue = "40893")]
+#[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 pub struct Reverse<T>(pub T);
 
-#[unstable(feature = "reverse_cmp_key", issue = "40893")]
+#[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 impl<T: PartialOrd> PartialOrd for Reverse<T> {
     #[inline]
     fn partial_cmp(&self, other: &Reverse<T>) -> Option<Ordering> {
@@ -363,7 +362,7 @@ impl<T: PartialOrd> PartialOrd for Reverse<T> {
     fn gt(&self, other: &Self) -> bool { other.0 > self.0 }
 }
 
-#[unstable(feature = "reverse_cmp_key", issue = "40893")]
+#[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 impl<T: Ord> Ord for Reverse<T> {
     #[inline]
     fn cmp(&self, other: &Reverse<T>) -> Ordering {