What is the Context that is used by JoinSet::poll_join_next()?

[DanielT]

I was just reading the documentation for tokio::JoinSet for the first time.
It contains the functions poll_join_next() and poll_join_next_with_id().

poll_join_next() looks like it would be useful to me, but the two functions are unlike all the others in JoinSet: they need a Context. The context is not created by any function I currently have in my code, neither its origin nor its use are explained in the documentation, and I have no idea how to get one.

Can someone please explain this to me here?

[Darksonn]

The Future trait is defined like this:

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}

documentation

So when you manually implement the Future trait, the context is available to you. It's not available when using the async/await syntax.

If all you want is a way try to get the next future if one is immediately available, then you shouldn't use the poll methods. Instead, use now_or_never on the non-poll method. You may also want to file a feature request for a try_join_next. (Similar to how mpsc::Receiver has recv, try_recv, and poll_recv.)

[DanielT]

Thanks!
I hadn't looked "under the hood" of futures yet.