Macro on_receive_notification 

macro_rules! on_receive_notification {
    () => { ... };
}

This macro is used for the value of the to_future_hack parameter of Builder::on_receive_notification and Builder::on_receive_notification_from.

It expands to |f, notif, cx| Box::pin(f(notif, cx)).

This is needed until return-type notation is stabilized.