revert split_at_mut

Author: KGrewal1

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

@@ -679,9 +679,12 @@ impl str {
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
     pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
-        match self.split_at_mut_checked(mid) {
-            None => slice_error_fail(self, 0, mid),
-            Some(pair) => pair,
+        // is_char_boundary checks that the index is in [0, .len()]
+        if self.is_char_boundary(mid) {
+            // SAFETY: just checked that `mid` is on a char boundary.
+            unsafe { self.split_at_mut_unchecked(mid) }
+        } else {
+            slice_error_fail(self, 0, mid)
         }
     }