How tokio join macro works

tokio's join macro is pretty amazing

https://docs.rs/tokio/0.2.20/tokio/macro.join.html

To find out how this join! macro works exactly,  you can do blow to expand the macro to see exact code generated.

rustup toolchain install nightly
cargo install cargo-expand

then in your project

cargo expand

You will find the expanded code like below

	// Macro expaned from
	// let (first, second) = tokio::join!(do_stuff1(), do_stuff2());
	let (first, second) = {
	use ::tokio::macros::support::{maybe_done, poll_fn, Future, Pin};
	use ::tokio::macros::support::Poll::{Ready, Pending};
	let mut futures = (maybe_done(do_stuff1()), maybe_done(do_stuff2()));
	poll_fn(move |cx| {
	let mut is_pending = false;
	let (fut, ..) = &mut futures;
	let mut fut = unsafe { Pin::new_unchecked(fut) };
	if fut.poll(cx).is_pending() {
	is_pending = true;
	}
	let (_, fut, ..) = &mut futures;
	let mut fut = unsafe { Pin::new_unchecked(fut) };
	if fut.poll(cx).is_pending() {
	is_pending = true;
	}
	if is_pending {
	Pending
	} else {
	Ready((
	{
	let (fut, ..) = &mut futures;
	let mut fut = unsafe { Pin::new_unchecked(fut) };
	fut.take_output().expect("expected completed future")
	},
	{
	let (_, fut, ..) = &mut futures;
	let mut fut = unsafe { Pin::new_unchecked(fut) };
	fut.take_output().expect("expected completed future")
	},
	))
	}
	})
	.await
	};

view raw tokio_join_expand.rs hosted with ❤ by GitHub