WIP - switch to std::future and async/await

[sd2k]

This PR moves the project over from futures 0.1 to the newly stabilized std::future, using tokio 0.2 (alpha) and futures-preview 0.3. It won't compile on stable until Rust 1.39 at the earliest since it uses async/await, so it'll obviously bump up the minimum Rust version too. It may be possible to remove the async/await which I think would allow it to compile on 1.36.0.

Tests are passing and examples working on nightly.

I guess this won't want to be merged until things stabilize a little more, but thought it might be useful when that time comes!

[Softsapiens]

hi, first of all thanks for you work @sd2k !
I would like to know if there is any plan on merge this PR?
Perhaps with rust 1.39?

thanks in advance

[benesch]

Thanks for all the work here, @sd2k! I'll get you a detailed review once we're a bit closer to Rust 1.39 shipping.

@Softsapiens, let's tentatively say we can merge this post the release of Rust 1.39. I'd like to release at least one more version of rust-rdkafka before then with an older MSRV.

[jerry73204]

Can you rebase this PR to master branch? I would like to give std::future a try along with update-to-date patches.

[sd2k]

@jerry73204 Done. Let me know if you have any issues.

[dbcfd]

@sd2k Given that this is taking a while to get in, do you want me to do a PR against your branch for #166 to have that functionality as part of this branch?

[sd2k]

@dbcfd Sure, sounds good! I'll take a look at it once it's in.

[jakubtrnka]

What's the plan now? I'd really like to have std::future in this crate.
Can I help somehow?

[benesch]

Hi @jakubtrnka, I apologize for the delay! I don't have permission to push versions of this crate to crates.io, so I'm waiting on @fede1024. I'd like to release 0.22 as the last version to support the tokio 0.1/futures 0.1 ecosystem—as soon as that's out I'll work on getting this merged and released as 0.23.

[sd2k]

On my part I'm a bit unsure how to proceed with this PR and #166 (which is based off mine). @dbcfd has made a PR to merge #166 into my branch, but it makes a lot of changes that I wouldn't know how to review. I could either keep my branch as is and have the PRs handled separately, or merge #166 in and just have a single large PR - let me know which you'd prefer when you have time, @benesch!

[benesch]

Thanks very much for your work on this, @sd2k! I reworked this to solve the merge conflicts and merged as #187. I'll handle #167 in the discussion there, but I do believe this PR is done!

[jakubtrnka]

Thanks @benesch and @sd2k. I appreciate your work.