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

library/core/src/slice/ascii.rs

@@ -172,6 +172,7 @@ impl_fn_for_zst! {
 /// documentation for more information.
 #[stable(feature = "inherent_ascii_escape", since = "1.60.0")]
 #[derive(Clone)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct EscapeAscii<'a> {
     inner: iter::FlatMap<super::Iter<'a, u8>, ascii::EscapeDefault, EscapeByte>,
 }