Code shared from the Rust Playground

playground.rs

#![feature(async_await)]

use std::{cell::Cell, future::Future, io, net::SocketAddr, pin::Pin};

pub trait TcpStream {}

impl<T: TcpStream + ?Sized> TcpStream for Pin<Box<T>> {}

pub trait Runtime: Send + Sync + 'static {
    type TcpStream: TcpStream;
    type ConnectTcpStream: Future<Output = io::Result<Self::TcpStream>>;

    fn connect_tcp_stream(&self, addr: &SocketAddr) -> Self::ConnectTcpStream;
}

impl<R: Runtime> Runtime for Box<R> {
    type TcpStream = Pin<Box<dyn TcpStream>>;
    type ConnectTcpStream = Pin<Box<dyn Future<Output = io::Result<Self::TcpStream>>>>;

    fn connect_tcp_stream(&self, addr: &SocketAddr) -> Self::ConnectTcpStream {
        let fut = R::connect_tcp_stream(&**self, addr);
        Box::pin(async { Ok(Box::pin(fut.await?) as _) })
    }
}

type DynRuntime = dyn Runtime<
    TcpStream = Pin<Box<dyn TcpStream>>,
    ConnectTcpStream = Pin<Box<dyn Future<Output = io::Result<Pin<Box<dyn TcpStream>>>>>>,
>;

thread_local! {
    static RUNTIME: Cell<Option<&'static DynRuntime>> = Cell::new(None);
}

#[inline]
pub fn current_runtime() -> &'static DynRuntime {
    RUNTIME.with(|r| r.get().expect("the runtime has not been set"))
}