add similar attributes to slice::split_at_mut_unchecked

Author: KGrewal1

library/core/src/str/mod.rs

@@ -772,9 +772,18 @@ impl str {
     ///
     /// The caller must ensure that `mid` is a valid byte offset from the start
     /// of the string and falls on the boundary of a UTF-8 code point.
+    #[inline]
+    #[must_use]
     unsafe const fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
         let len = self.len();
         let ptr = self.as_mut_ptr();
+
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "slice::split_at_mut_unchecked requires the index to be within the slice",
+            (mid: usize = mid, len: usize = len) => mid <= len,
+        );
+
         // SAFETY: caller guarantees `mid` is on a char boundary.
         unsafe {
             (