Rollup merge of #78083 - ChaiTRex:master, r=m-ou-se

Stabilize or_insert_with_key

Stabilizes the `or_insert_with_key` feature from rust-lang/rust#71024. This allows inserting key-derived values when a `HashMap`/`BTreeMap` entry is vacant.

The difference between this and  `.or_insert_with(|| ... )` is that this provides a reference to the key to the closure after it is moved with `.entry(key_being_moved)`, avoiding the need to copy or clone the key.

collections/btree/map/entry.rs

@@ -116,15 +116,16 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
         }
     }
 
-    #[unstable(feature = "or_insert_with_key", issue = "71024")]
-    /// Ensures a value is in the entry by inserting, if empty, the result of the default function,
-    /// which takes the key as its argument, and returns a mutable reference to the value in the
-    /// entry.
+    /// Ensures a value is in the entry by inserting, if empty, the result of the default function.
+    /// This method allows for generating key-derived values for insertion by providing the default
+    /// function a reference to the key that was moved during the `.entry(key)` method call.
+    ///
+    /// The reference to the moved key is provided so that cloning or copying the key is
+    /// unnecessary, unlike with `.or_insert_with(|| ... )`.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(or_insert_with_key)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
@@ -134,6 +135,7 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
     /// assert_eq!(map["poneyland"], 9);
     /// ```
     #[inline]
+    #[stable(feature = "or_insert_with_key", since = "1.50.0")]
     pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V {
         match self {
             Occupied(entry) => entry.into_mut(),