Feature: Stream impl for Dequeue

[khionu]

Implementing Iterator/Stream gives a lot of convenience, it would expand a lot of ergonomics to implement Stream on Dequeue.

[faiface]

@khionu Thanks! I now implemented a Dequeue::into_stream1 method that transforms a Dequeue<T, ()> (so without a continuation) into a futures::Stream of T.

An implementation for a Dequeue with a continuation would be possible, but I'm not sure about the design yet. I see two possible designs:

1. Have dequeue.into_stream(|recv_cont: Recv<S>| ...) pass the continuation via the provided Recv. It needs to be provided into a closure to maintain deadlock prevention.
2. Make dequeue.into_stream() produce a Stream of a sum type of either a T or an S. Unfortunately, there is no standard "either" type in Rust, so it would have to be a custom sum type.

I'm leaning towards the first option, what do you think?

[khionu]

I'm not sure how the first would function without reducing ergonomics? It wouldn't be able to be consumed within the iterator chain or loop blocks because you wouldn't be able to enforce a break of the loop.

A custom Either is a very common approach, and given the nature of Par I think it would be very appropriate for it to have one.

[faiface]

That's true, thanks!

The first approach introduces a scope separation where it's not appropriate.

I'll implement the second approach.

[faiface]

Implemented: https://docs.rs/par/0.3.9/par/queue/struct.Dequeue.html#method.into_stream

@khionu Let me know if anything is missing, or if this issue is now resolved!