add #[must_use] to array::repeat

Author: joboet

library/core/src/array/mod.rs

@@ -48,6 +48,7 @@ pub use iter::IntoIter;
 /// assert_eq!(strings, ["Hello there!", "Hello there!"]);
 /// ```
 #[inline]
+#[must_use = "cloning is often expensive and is not expected to have side effects"]
 #[stable(feature = "array_repeat", since = "CURRENT_RUSTC_VERSION")]
 pub fn repeat<T: Clone, const N: usize>(val: T) -> [T; N] {
     from_trusted_iterator(repeat_n(val, N))