use std::collections::HashSet;
use std::iter;

#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
struct Id(pub usize);

type View = Vec<Id>;
type IdSet = HashSet<Id>;

fn find_stuff(view: View, ids: &IdSet) -> impl Iterator<Item=Id> {
    let mut ids = ids.iter();
    iter::from_fn(move || {
        while let Some(id) = ids.next() {
            if view.contains(id) {
                return Some(*id);
            }
        }
        None
    })
}

fn set_from_seed(id: Id) -> IdSet {
    let mut result = IdSet::with_capacity(1);
    result.insert(id);
    result
}

fn main() {
    let view = vec![Id(0), Id(1), Id(2), Id(3)];
    for x in find_stuff(view, &set_from_seed(Id(2))) {
        println!("{:?}", x);
    }
}