Add core::stream::pending

[ibraheemdev]

This PR adds core::stream::{pending, Pending}, analogous to core::future::{pending, Pending}.

Prior art: https://docs.rs/futures/latest/futures/stream/fn.pending.html

[rust-highfive]

r? @m-ou-se

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[dtolnay]

Do you think it's important that this be a distinct type, or would it be fine as impl Stream for the existing core::future::Pending struct?

[ibraheemdev]

Using the same type would work, but would it live in core::future or core::stream then?

[taiki-e]

would it be fine as impl Stream for the existing core::future::Pending struct?

I guess that if we implement both Future and Stream, the FutureExt (futures, futures-lite, async-std) and StreamExt (futures, tokio, futures-lite, async-std) methods will conflict.

[taiki-e]

cc @rust-lang/wg-async-foundations

[ibraheemdev]

the FutureExt (futures, futures-lite, async-std) and StreamExt (futures, tokio, futures-lite, async-std) methods will conflict.

That is a good point, I think this does have to be separate then.

[yoshuawuyts]

Adding this makes sense to me. There is precedence for this in async_std::stream::pending as well, and from experience it's convenient to be able to use stream::pending when authoring examples.

Note though that this should not be stabilized before the working group has been able to complete a feasibility study of async overloading, as that may mean that we may no longer have an std::stream submodule, and instead would likely fold the functionality of std::stream into std::iter, likely exposing this as std::iter::pending instead.

[eholk]

Looks good to me. It looks like there are a few implementations in the wild, so pulling this into std seems worthwhile.

[crlf0710]

Will merge conflict with #93613.

[bors]

☔ The latest upstream changes (presumably #94121) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@ibraheemdev this has some conflicts that if you can resolve we can push this forward

[JohnCSimon]

@ibraheemdev
Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this. Thanks for your contribution.

@rustbot label: +S-inactive