Solution

#[derive(Debug)]
/// An event in the elevator system that the controller must react to.
enum Event {
    /// A button was pressed.
    ButtonPressed(Button),
    /// The car has arrived at the given floor.
    CarArrived(Floor),
    /// The car's doors have opened.
    CarDoorOpened,
    /// The car's doors have closed.
    CarDoorClosed,
}
/// A floor is represented as an integer.
type Floor = i32;
/// A direction of travel.
#[derive(Debug)]
enum Direction {
    Up,
    Down,
}
/// A user-accessible button.
#[derive(Debug)]
enum Button {
    /// A button in the elevator lobby on the given floor.
    LobbyCall(Direction, Floor),
    /// A floor button within the car.
    CarFloor(Floor),
}
/// The car has arrived on the given floor.
fn car_arrived(floor: i32) -> Event {
    Event::CarArrived(floor)
}
/// The car doors have opened.
fn car_door_opened() -> Event {
    Event::CarDoorOpened
}
/// The car doors have closed.
fn car_door_closed() -> Event {
    Event::CarDoorClosed
}
/// A directional button was pressed in an elevator lobby on the given floor.
fn lobby_call_button_pressed(floor: i32, dir: Direction) -> Event {
    Event::ButtonPressed(Button::LobbyCall(dir, floor))
}
/// A floor button was pressed in the elevator car.
fn car_floor_button_pressed(floor: i32) -> Event {
    Event::ButtonPressed(Button::CarFloor(floor))
}
fn main() {
    println!(
        "A ground floor passenger has pressed the up button: {:?}",
        lobby_call_button_pressed(0, Direction::Up)
    );
    println!("The car has arrived on the ground floor: {:?}", car_arrived(0));
    println!("The car door opened: {:?}", car_door_opened());
    println!(
        "A passenger has pressed the 3rd floor button: {:?}",
        car_floor_button_pressed(3)
    );
    println!("The car door closed: {:?}", car_door_closed());
    println!("The car has arrived on the 3rd floor: {:?}", car_arrived(3));
}