#![allow(dead_code, deprecated, unused_variables, unused_mut)]
#![feature(hasher_prefixfree_extras)]
#![allow(non_local_definitions)]
struct MyCollection<T>(Option<T>);
impl<T> MyCollection<T> {
    fn len(&self) -> usize {
        todo!()
    }
}
impl<'a, T> IntoIterator for &'a MyCollection<T> {
    type Item = T;
    type IntoIter = std::iter::Empty<T>;
    fn into_iter(self) -> Self::IntoIter {
        todo!()
    }
}
use std::hash::{Hash, Hasher};
impl<T: Hash> Hash for MyCollection<T> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        state.write_length_prefix(self.len());
        for elt in self {
            elt.hash(state);
        }
    }
}
fn main() {}