Rollup merge of #58074 - scottmcm:stabilize-sort_by_cached_key, r=SimonSapin

Stabilize slice_sort_by_cached_key

I was going to ask on the tracking issue (https://github.com/rust-lang/rust/issues/34447), but decided to just send this and hope for an FCP here.  The method was added last March by https://github.com/rust-lang/rust/pull/48639.

Signature: https://doc.rust-lang.org/std/primitive.slice.html#method.sort_by_cached_key
```rust
impl [T] {
    pub fn sort_by_cached_key<K, F>(&mut self, f: F)
        where F: FnMut(&T) -> K, K: Ord;
}
```

That's an identical signature to the existing `sort_by_key`, so I think the questions are just naming, implementation, and the usual "do we want this?".

The implementation seems to have proven its use in rustc at least, which many uses: https://github.com/rust-lang/rust/search?l=Rust&q=sort_by_cached_key

(I'm asking because it's exactly what I just needed the other day:
```rust
    all_positions.sort_by_cached_key(|&n|
        data::CITIES.iter()
            .map(|x| *metric_closure.get_edge(n, x.pos).unwrap())
            .sum::<usize>()
    );
```
since caching that key is a pretty obviously good idea.)

Closes #34447

src/liballoc/benches/lib.rs

@@ -1,5 +1,4 @@
 #![feature(repr_simd)]
-#![feature(slice_sort_by_cached_key)]
 #![feature(test)]
 
 extern crate rand;

src/liballoc/slice.rs

@@ -257,6 +257,10 @@ impl<T> [T] {
     /// This sort is stable (i.e., does not reorder equal elements) and `O(m n log(m n))`
     /// worst-case, where the key function is `O(m)`.
     ///
+    /// For expensive key functions (e.g. functions that are not simple property accesses or
+    /// basic operations), [`sort_by_cached_key`](#method.sort_by_cached_key) is likely to be
+    /// significantly faster, as it does not recompute element keys.
+    ///
     /// When applicable, unstable sorting is preferred because it is generally faster than stable
     /// sorting and it doesn't allocate auxiliary memory.
     /// See [`sort_unstable_by_key`](#method.sort_unstable_by_key).
@@ -312,7 +316,6 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
-    /// #![feature(slice_sort_by_cached_key)]
     /// let mut v = [-5i32, 4, 32, -3, 2];
     ///
     /// v.sort_by_cached_key(|k| k.to_string());
@@ -320,7 +323,7 @@ impl<T> [T] {
     /// ```
     ///
     /// [pdqsort]: https://github.com/orlp/pdqsort
-    #[unstable(feature = "slice_sort_by_cached_key", issue = "34447")]
+    #[stable(feature = "slice_sort_by_cached_key", since = "1.34.0")]
     #[inline]
     pub fn sort_by_cached_key<K, F>(&mut self, f: F)
         where F: FnMut(&T) -> K, K: Ord

src/liballoc/tests/lib.rs

@@ -4,7 +4,6 @@
 #![feature(exact_size_is_empty)]
 #![feature(pattern)]
 #![feature(repeat_generic_slice)]
-#![feature(slice_sort_by_cached_key)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(vecdeque_rotate)]