use std::{any::Any, future::Future, pin::Pin};

trait TaskFn {
    fn call_task(&self) -> Pin<Box<dyn Future<Output = ()> + Send>>;
}

type Node = dyn Any;

impl<F: Fn() -> Fut, Fut: Future<Output = ()> + Send + 'static> TaskFn for F {
    fn call_task(&self) -> Pin<Box<dyn Future<Output = ()> + Send>> {
        Box::pin((self)())
    }
}

trait Visit {
    type EdgesFuture: Send;
}

impl<VisitFn: Fn(&Node) -> EdgesFuture, EdgesFuture: Send> Visit for VisitFn {
    type EdgesFuture = EdgesFuture;
}

fn visit<V: Visit>(_: V) -> impl Future<Output = Option<V::EdgesFuture>> + Send {
    futures::future::ready(None::<V::EdgesFuture>)
}

fn main() {
    futures::executor::block_on(
        (Box::new(|| async {
            visit(|_: &Node| async {}).await;
        }) as Box<dyn TaskFn>)
            .call_task(),
    )
}