state_machines2.rs

struct ThingA;

impl ThingA {
    fn do_action_a(self) -> ThingB {
        ThingB
    }
}

struct ThingB;

enum State {
    A(ThingA),
    B(ThingB),
}

struct StateMachine {
    state: Option<State>,
}

impl StateMachine {
    fn new() -> StateMachine {
        StateMachine {
            state: Some(State::A(ThingA)),
        }
    }
    
    fn nudge(&mut self) {
        let old_state = self.state.take().unwrap();
        self.state = match old_state {
            State::A(my_thing_a) => {
                println!("transitioning from A -> B");
                Some(State::B(my_thing_a.do_action_a()))
            },
            State::B(my_thing_b) => {
                println!("transitioning from B -> B");
                Some(State::B(my_thing_b))
            },
        }
    }
}

fn main() {
    let mut state_machine = StateMachine::new();
    state_machine.nudge();
    state_machine.nudge();
    
    println!("done.");
}