Use subcrates

[jeehoonkang]

This is a long-waited architectural change that makes crossbeam use subcrates we've been developing so far: crossbeam-epoch, crossbeam-utils, and crossbeam-deque.

It closes #168, closes #157, closes #70, closes #166, closes #165, closes #159, closes #137, closes #116, closes #113, closes #112, closes #60, closes #37, and closes #13. If any of them seems still relevant to the new version, please leave a message here.

It contains a huge API breaking change, so I bumped the version to 0.4.0. Before publishing a new version, I think we need to discuss how to organize this crate and how to document things, esp. those in the subcrates. Maybe we need an RFC for making a consensus.

[Vtec234]

Could also include crossbeam-utils/nightly and crossbeam-channel/nightly.

[Vtec234]

Same here, crossbeam-utils/use_std.

[Vtec234]

Do we not want to expose channel for now?

[jeehoonkang]

Eventually, crossbeam-channel should replace the two queue implementations in sync. I don't know what's the best way to do it. I omitted this migration in this PR because I think it's @stjepang's call to make.

[jeehoonkang]

@Vtec234 I adjusted [features] in Cargo.toml as you said. Thanks!

[ghost]

LGTM, at least as a first step in the big refactoring. :)

Just one more thing: let's add crossbeam-channel as a dependency and re-export it as:

#[macro_use]
pub extern crate crossbeam_channel as channel;

In the future, I'd like to see the stack and queues moved into crossbeam-stack and crossbeam-queue, but I guess it's okay if leaving them for now. Also, we should move ArcCell somewhere else (perhaps into crossbeam-atomic?) and clean up crossbeam-utils (it still has an unsound implementation of AtomicOption).

[jeehoonkang]

@stjepang hmm, do you want to reexport the macros defined in channel? I think it's not stabilized yet: rust-lang/rust#29638

When I just copy-and-pasted your code snippet in sync/mod.rs, the compiler says:

error[E0468]: an `extern crate` loading macros must be at the crate root
  --> src/sync/mod.rs:11:1
   |
11 | pub extern crate crossbeam_channel as channel;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When I copy-and-pasted your code snippet in lib.rs, the compiler says:

warning: unused `#[macro_use]` import
  --> src/lib.rs:30:1
   |
30 | #[macro_use]
   | ^^^^^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default

[ghost]

Does this solution work?

Also, I think it'd be better to re-export channel inside lib.rs rather than sync/mod.rs.

[jeehoonkang]

@stjepang It works, but as a side effect, all the members of crossbeam_channel is now visible in crossbeam, e.g. we have crossbeam::bounded(). I don't think there's a better workaround..

[jeehoonkang]

@stjepang The side-effect of visibility (e.g. crossbeam::bounded()) seems too big to my eyes. How about merging this PR without incorporating crossbeam-channel, and then later making another PR for crossbeam-channel?

[ghost]

Yes, I'm okay with leaving crossbeam-channel out.