third runtime with tokio

main.rs

// from https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=fbb7daec1be993d3a4cd6f6181f332a0

use std::future::Future;
use std::pin::Pin;
use std::thread;

use futures::future::{pending, poll_fn};
use lazy_static::lazy_static;
use tokio::runtime::{Handle, Runtime};

lazy_static! {
    static ref HANDLE: Handle = {
        let mut rt = Runtime::new().unwrap();
        let handle = rt.handle().clone();
        thread::spawn(move || rt.block_on(pending::<()>()));
        handle
    };
}

pub async fn enter_tokio<T>(mut f: impl Future<Output = T>) -> T {
    poll_fn(|context| {
        HANDLE.enter(|| {
            // Safety: pinned on stack, and we are in an async fn
            // WARN: DO NOT use f in other places
            let f = unsafe { Pin::new_unchecked(&mut f) };
            f.poll(context)
        })
    })
    .await
}

fn main() {
    futures::executor::block_on(enter_tokio(async {
        tokio::fs::write("x", b"xxxxxxxxx").await.unwrap();
        dbg!(tokio::fs::read_to_string("x").await.unwrap());
    }));
}