a.rs

//use std::time::Duration;
use std::task::Poll;
use std::future::{poll_fn, Future };
use tokio::process::Command;

fn main() {
//    async fn foo() {
//        println!("1");
//        println!("{:?}", Command::new("date").output().await.unwrap().stdout);
//        println!("{:?}", Command::new("date").output().await.unwrap().stdout);
//        println!("2");
//    }
//    let mut future = Box::pin(foo());

    let mut state = 0;
    let mut inner_future = None;
    let mut future = Box::pin(poll_fn(move |context| {
        loop {
            match state {
                0 => {
                    println!("1");
                    inner_future = Some(Box::pin(Command::new("date").output()));
                    state = 1;
                },
                1 => {
                    match inner_future.as_mut().unwrap().as_mut().poll(context) {
                        Poll::Ready(v) => {
                            println!("{:?}", v);
                            inner_future = Some(Box::pin(Command::new("date").output()));
                            state = 2;
                        },
                        Poll::Pending => return Poll::Pending,
                    }
                }
                2 => {
                    match inner_future.as_mut().unwrap().as_mut().poll(context) {
                        Poll::Ready(v) => {
                            println!("{:?}", v);
                            println!("2");
                            return Poll::Ready(());
                        }
                        Poll::Pending => return Poll::Pending,
                    }
                }
                _ => panic!("invalid state {state}")
            }
        }
    }));
    let poller = poll_fn(move |context| {
        println!("[main poll]");
        let tmp = future.as_mut().poll(context); //.as_mut().unwrap().as_mut().poll(context);
        dbg!(&tmp);
        tmp
    });

    tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .unwrap()
        .block_on(poller);
}