Rayon 0.7.1 is broken

[olivia-fl]

Rayon 0.7.1 compilation fails with the following error:

error[E0432]: unresolved import `rayon_core::spawn_async`
  --> /home/benjamin/.cargo/registry/src/g.yxqyang.asia-1ecc6299db9ec823/rayon-0.7.1/src/lib.rs:41:9
   |
41 | pub use rayon_core::spawn_async;
   |         ^^^^^^^^^^^^^^^^^^^^^^^ no `spawn_async` in the root

error[E0432]: unresolved import `rayon_core::spawn_future_async`
  --> /home/benjamin/.cargo/registry/src/g.yxqyang.asia-1ecc6299db9ec823/rayon-0.7.1/src/lib.rs:43:9
   |
43 | pub use rayon_core::spawn_future_async;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `spawn_future_async` in the root. Did you mean to use `spawn_future`?

I assume that this is a dependency specification issue, as rayon-core was recently updated to change the name of spawn_async.

[cuviper]

To be clear, stable rayon 0.7.1 still builds fine with the new rayon-core 1.1.

We don't have a really good story for how we change unstable features. We could make rayon pin a tilde requirement on rayon-core, and make sure we always bump minor versions for unstable changes, but that goes against our desire to make sure there's only one rayon-core everywhere. i.e. if there are multiple rayons in your project, we still want exactly one rayon-core.

If you can't upgrade, then you could also manually pin rayon-core = "~1.0" yourself. After all, you're the one relying on unstable features here. 😉

We should probably think about stabilizing those features, and avoid adding new features this way, because it won't fly well long term...

[olivia-fl]

Oh, that makes sense. The issue was actually not in my library, but in the specs library, so I might go open an pull request there to have them update instead.

[nikomatsakis]

Yes, Servo was also encountering a similar problem. This just serves to reinforce the reason that the Rust project has always drawn a big, bright line around unstable features. I guess we have to do the same.

[nikomatsakis]

Closing this issue, as I don't think this is a bug itself. But I opened https://github.com/nikomatsakis/rayon/issues/369, which is a proposal for a new way to handle unstable APIs.