There is another alternative which may be significantly more efficient as it means the caller always gets back a direct iterator instead of an enum it must bran

There is another alternative which may be significantly more efficient as it means the caller always gets back a direct iterator instead of an enum it must branch on: make a dummy empty hashmap with a static lifetime you can refer to. Then the implementation becomes static DUMMY: LazyLock<HashMap<N, (u128, u128)>> = LazyLock::new(HashMap::new()); fn neighbors_with_send_capacity_option(&self, a: N, capacity: u128) -> impl Iterator<Item=&N> { let node = self.nodes.get(&a).unwrap_or_else(|| &*DUMMY); node.keys().filter(move |b| self.get_send_capacity(&a, b) >= capacity) } Unfortunately we have to use a LazyLock for the static because HashMap::new isn't const. It only adds a single always-correctly-predicted branch that checks for initialization on access though, so it's fine.