Rollup merge of #84121 - workingjubilee:stabilize-btree-retain, r=dto…

…lnay

Stabilize BTree{Map,Set}::retain

Closes #79025.
FCP concluded here: #79025 (comment)

This is an approved feature on BTree{Map,Set} to mirror a functionality in Hash{Map,Set}, which has had some adequate testing since its introduction in #79026 and doesn't seem to have caused any problems since.

library/alloc/src/collections/btree/map.rs

@@ -940,7 +940,6 @@ impl<K, V> BTreeMap<K, V> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(btree_retain)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x*10)).collect();
@@ -949,7 +948,7 @@ impl<K, V> BTreeMap<K, V> {
     /// assert!(map.into_iter().eq(vec![(0, 0), (2, 20), (4, 40), (6, 60)]));
     /// ```
     #[inline]
-    #[unstable(feature = "btree_retain", issue = "79025")]
+    #[stable(feature = "btree_retain", since = "1.53.0")]
     pub fn retain<F>(&mut self, mut f: F)
     where
         K: Ord,

library/alloc/src/collections/btree/set.rs

@@ -851,7 +851,6 @@ impl<T> BTreeSet<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(btree_retain)]
     /// use std::collections::BTreeSet;
     ///
     /// let xs = [1, 2, 3, 4, 5, 6];
@@ -860,7 +859,7 @@ impl<T> BTreeSet<T> {
     /// set.retain(|&k| k % 2 == 0);
     /// assert!(set.iter().eq([2, 4, 6].iter()));
     /// ```
-    #[unstable(feature = "btree_retain", issue = "79025")]
+    #[stable(feature = "btree_retain", since = "1.53.0")]
     pub fn retain<F>(&mut self, mut f: F)
     where
         T: Ord,