Function update_state

tests/binary/traits/src/main.rs:80–82

Takes a mutable trait object reference

(calc: &mut dyn Calculator, val: i32)

impl DynSource for I32Source {
    type Item = i32;

    fn next_value(&mut self) -> Option<Self::Item> {
        let value = self.next;
        self.next += 1;
        Some(value)
    }