Rollup merge of rust-lang#70194 - kornelski:must_split, r=joshtriplett

#[must_use] on split_off()

I've noticed this function used for truncation in the wild. `must_use` will clear that up.

src/liballoc/vec.rs

@@ -1377,6 +1377,7 @@ impl<T> Vec<T> {
     /// assert_eq!(vec2, [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 {
         assert!(at <= self.len(), "`at` out of bounds");