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.