add #[must_use] to functions of slice and its iterators.

Author: jendrikw

library/core/tests/slice.rs

@@ -2344,31 +2344,31 @@ fn slice_rsplit_array_mut() {
 fn slice_split_array_ref_out_of_bounds() {
     let v = &[1, 2, 3, 4, 5, 6][..];
 
-    v.split_array_ref::<7>();
+    let _ = v.split_array_ref::<7>();
 }
 
 #[should_panic]
 #[test]
 fn slice_split_array_mut_out_of_bounds() {
     let v = &mut [1, 2, 3, 4, 5, 6][..];
 
-    v.split_array_mut::<7>();
+    let _ = v.split_array_mut::<7>();
 }
 
 #[should_panic]
 #[test]
 fn slice_rsplit_array_ref_out_of_bounds() {
     let v = &[1, 2, 3, 4, 5, 6][..];
 
-    v.rsplit_array_ref::<7>();
+    let _ = v.rsplit_array_ref::<7>();
 }
 
 #[should_panic]
 #[test]
 fn slice_rsplit_array_mut_out_of_bounds() {
     let v = &mut [1, 2, 3, 4, 5, 6][..];
 
-    v.rsplit_array_mut::<7>();
+    let _ = v.rsplit_array_mut::<7>();
 }
 
 macro_rules! take_tests {