Add missed use_std  attribute
@@ -507,6 +507,7 @@ pub trait Itertools : Iterator {
/// assert_eq!(lookup[&2], vec![12, 42]);
/// assert_eq!(lookup[&3], vec![13, 33]);
/// ```
+ #[cfg(feature = "use_std")]
fn to_lookup<K, F>(self, get_key: F) -> HashMap<K, Vec<Self::Item>>
where Self: Iterator + Sized,
K: Hash + Eq,
