Why Unpin bound for Select?

[tisonkun]

Hi @taiki-e!

I found futures-rs Select has an unpin bound:

pub struct Select<A, B> {
    inner: Option<(A, B)>,
}

Seems it is for returning the unfinished future as part of the result, but I'm not quite sure for its real-world user scenarios.

Instead, in my crate fastimer I implement a Select struct without the Unpin bound:

#[derive(Debug, Clone)]
pub enum Either<A, B> {
    Left(A),
    Right(B),
}

#[must_use = "futures do nothing unless you `.await` or poll them"]
#[derive(Debug)]
#[pin_project::pin_project]
pub struct Select<A, B> {
    #[pin]
    a: A,
    #[pin]
    b: B,
}

impl<A, B> Future for Select<A, B>
where
    A: Future,
    B: Future,
{
    type Output = Either<A::Output, B::Output>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.project();
        if let Poll::Ready(a) = this.a.poll(cx) {
            return Poll::Ready(Either::Left(a));
        }
        if let Poll::Ready(b) = this.b.poll(cx) {
            return Poll::Ready(Either::Right(b));
        }
        Poll::Pending
    }
}

I'd like to know the design background and understand if one is better than the other.

Possibly related commit - 151d68c

[tisonkun]

I found:

https://github.com/smol-rs/futures-lite/blob/445f3fc922bea7993713f0f80b9efa96b152f7ce/FEATURES.md?plain=1#L187

.. that describes a similar combinator in futures-lite.

How should I set expectations between futures-rs and futures-lite? Especially given that (1) futures-rs is under the rust-lang org (2) they seems both maintained by @taiki-e.