Pin-projecting in-line tuples

[yoshuawuyts]

The following currently works:

use pin_project::pin_project;
use core::future::Future;

#[pin_project]
struct Pinned<A: Future, B: Future>(#[pin] A, #[pin] B);

But this doesn't:

use pin_project::pin_project;
use core::future::Future;

#[pin_project]
struct Pinned<A: Future, B: Future> {
    futures: (#[pin] A, #[pin] B),
}

So instead we now have to do:

use pin_project::pin_project;
use core::future::Future;

#[pin_project]
struct Futures<A: Future, B: Future> (#[pin] A, #[pin] B);

#[pin_project]
struct Pinned<A: Future, B: Future> {
    futures: Futures<A, B>,
}

We ran into some issues with this in yoshuawuyts/futures-concurrency#74. I'm not sure how feasible this would be to provide, but being able to pin-project into individual enum fields would save us from having to generate an intermediate struct. So I figured I'd raise it here; hope that's alright!

[taiki-e]

Thanks for the suggestion! At first glance, this seems reasonable to me.

[taiki-e]

Another approach is providing a trait for pin projection and implementing it for tuples. This allows the removal of tuple-struct workaround. However, unfortunately, this approach cannot support arbitrarily sized tuples, though.
As mentioned in 1.0.0 tracking issue, this approach needs recently stabilized GAT. Something like the following:

trait PinProject {
    type Projection<'a>;
    fn project<'a>(self: Pin<&'a mut Self>) -> Self::Projection<'a>;
}

On the other hand, this probably has the advantage of being able to support slices and arrays. (Self::Projection will probably be the iterator instead of slice/array, though.)

(We can probably provide both.)