Rollup merge of rust-lang#70359 - kornelski:mustsplit, r=Dylan-DPC

must_use on split_off

A couple more for rust-lang#70194

src/liballoc/collections/vec_deque.rs

@@ -1876,6 +1876,7 @@ impl<T> VecDeque<T> {
     /// assert_eq!(buf2, [2, 3]);
     /// ```
     #[inline]
+    #[must_use = "use `.truncate()` if you don't need the other half"]
     #[stable(feature = "split_off", since = "1.4.0")]
     pub fn split_off(&mut self, at: usize) -> Self {
         let len = self.len();

src/liballoc/string.rs

@@ -1461,6 +1461,7 @@ impl String {
     /// ```
     #[inline]
     #[stable(feature = "string_split_off", since = "1.16.0")]
+    #[must_use = "use `.truncate()` if you don't need the other half"]
     pub fn split_off(&mut self, at: usize) -> String {
         assert!(self.is_char_boundary(at));
         let other = self.vec.split_off(at);

src/liballoc/tests/string.rs

@@ -266,14 +266,14 @@ fn test_split_off_empty() {
 fn test_split_off_past_end() {
     let orig = "Hello, world!";
     let mut split = String::from(orig);
-    split.split_off(orig.len() + 1);
+    let _ = split.split_off(orig.len() + 1);
 }
 
 #[test]
 #[should_panic]
 fn test_split_off_mid_char() {
     let mut orig = String::from("山");
-    orig.split_off(1);
+    let _ = orig.split_off(1);
 }
 
 #[test]