You don't need to do all that much, you can use `Option` with `into_iter` paired with `flatten` instead: fn neighbors_with_send_capacity(&self, a: N, capac

You don't need to do all that much, you can use `Option` with `into_iter` paired with `flatten` instead: fn neighbors_with_send_capacity(&self, a: N, capacity: u128) -> impl Iterator<Item = &N> + '_ { self.nodes .get(&a) .map(|m| m.keys()) .into_iter() .flatten() .filter(move |b| self.get_send_capacity(&a, b) >= capacity) } The only downside is that I believe `size_hint` is not properly forwarded.