Auto merge of rust-lang#87870 - WaffleLapkin:pub_split_at_unchecked, r=dtolnay

Make `<[T]>::split_at_unchecked` and `<[T]>::split_at_mut_unchecked` public

The methods were originally added in rust-lang#75936 (sdroege@30dc32b), but for some reason as private. Nevertheless, the methods have documentation and even a [tracking issue](rust-lang#76014).

It's very weird to have a tracking issue for private methods and these methods may be useful outside of the standard library. As such, this PR makes the methods public.

Author: bors

library/core/src/slice/mod.rs

@@ -1556,7 +1556,7 @@ impl<T> [T] {
     ///
     /// # Examples
     ///
-    /// ```compile_fail
+    /// ```
     /// #![feature(slice_split_at_unchecked)]
     ///
     /// let v = [1, 2, 3, 4, 5, 6];
@@ -1581,7 +1581,7 @@ impl<T> [T] {
     /// ```
     #[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")]
     #[inline]
-    unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
+    pub unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
         // SAFETY: Caller has to check that `0 <= mid <= self.len()`
         unsafe { (self.get_unchecked(..mid), self.get_unchecked(mid..)) }
     }
@@ -1605,7 +1605,7 @@ impl<T> [T] {
     ///
     /// # Examples
     ///
-    /// ```compile_fail
+    /// ```
     /// #![feature(slice_split_at_unchecked)]
     ///
     /// let mut v = [1, 0, 3, 0, 5, 6];
@@ -1621,7 +1621,7 @@ impl<T> [T] {
     /// ```
     #[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")]
     #[inline]
-    unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
+    pub unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
         let len = self.len();
         let ptr = self.as_mut_ptr();