Auto merge of #89219 - nickkuk:str_split_once_get_unchecked, r=Mark-Simulacrum

Use get_unchecked in str::[r]split_once

This PR removes indices checking in `str::split_once` and `str::rsplit_once` methods.

Author: bors

Diff for: library/core/src/str/mod.rs

@@ -1536,7 +1536,8 @@ impl str {
     #[inline]
     pub fn split_once<'a, P: Pattern<'a>>(&'a self, delimiter: P) -> Option<(&'a str, &'a str)> {
         let (start, end) = delimiter.into_searcher(self).next_match()?;
-        Some((&self[..start], &self[end..]))
+        // SAFETY: `Searcher` is known to return valid indices.
+        unsafe { Some((self.get_unchecked(..start), self.get_unchecked(end..))) }
     }
 
     /// Splits the string on the last occurrence of the specified delimiter and
@@ -1556,7 +1557,8 @@ impl str {
         P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
     {
         let (start, end) = delimiter.into_searcher(self).next_match_back()?;
-        Some((&self[..start], &self[end..]))
+        // SAFETY: `Searcher` is known to return valid indices.
+        unsafe { Some((self.get_unchecked(..start), self.get_unchecked(end..))) }
     }
 
     /// An iterator over the disjoint matches of a pattern within the given string