/// A listener for events emmited on a wl_registry object.
trait wayland::client::RegistryListener {
    fn global(&mut self, registry: wayland::client::RegistryListener, name: u32, interface: &str, version: u32);
    fn global_remove(&mut self, registry: wayland::client::RegistryListener, name: u32);
}

impl wayland::client::Registry {
    fn add_listener(&mut self, listener: impl RegistryListener) { ... }
    fn bind(...) { ... }
}

struct MySeatData {
    name: Option<String>, // None if not yet known
    seat: wayland::client::Seat,
}

struct MyAwesomeRegistryListener {
    // Whatever user data you want goes here.
    // You get to decide whether it's Send/Sync.
    seats: Vec<MySeatData>,
}

impl MyAwesomeRegistryListener {
    fn new() -> Self {
        Self { seats: vec![] }
    }

    fn find_seat_data_mut(&mut self, seat: wayland::client::Seat) -> Option<&mut MySeatData> {
        self.seats.iter_mut()
            .find(|&seat_data| seat_data.seat == seat)
    }
}

impl wayland::client::RegistryListener for MyAwesomeRegistryListener {
    fn global(&mut self, registry: wayland::client::RegistryListener, name: u32, interface: &str, version: u32) {
        match interface {
            wayland::client::Seat::interface::name /* "wl_seat" */ =>  {
                let seat = registry.bind(name, ...);
                self.seats.push(SeatData { name: None, seat });
            }
        }
    }
}

fn main() -> Result<(), Error> {
    let display = wayland::client::Display::connect()?;
    let registry = display.get_registry();
    registry.add_listener(MyAwesomeRegistryListener::new());
    loop {
        display.roundtrip()?;
    }
}