Rollup merge of #158282 - ArhanChaudhary:unchecked-subslice, r=tgross35,jhpratt

slice_split_once: bounds check optimization note

Tracking issue: https://github.com/rust-lang/rust/issues/112811

~~Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.~~
Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

library/core/src/slice/mod.rs

diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index 9b077f1..7ab476e 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -2530,6 +2530,7 @@ pub fn split_once<F>(&self, pred: F) -> Option<(&[T], &[T])>
         F: FnMut(&T) -> bool,
     {
         let index = self.iter().position(pred)?;
+        // Slice bounds checks optimized are away (as of June 2026)
         Some((&self[..index], &self[index + 1..]))
     }
 
@@ -2558,6 +2559,7 @@ pub fn rsplit_once<F>(&self, pred: F) -> Option<(&[T], &[T])>
         F: FnMut(&T) -> bool,
     {
         let index = self.iter().rposition(pred)?;
+        // Slice bounds checks optimized are away (as of June 2026)
         Some((&self[..index], &self[index + 1..]))
     }