Use Cargo features instead of cfg gating whole crates

[metajack]

Currently we add #[cfg(target_os=...)] lines to crates that are platform specific so they compile on every platform. We should instead use Cargo's new feature support.

[gilles-leblanc]

I see that there are #[cfg(target_os=...)] statements for extern crate definitions but also for different use and fn and other statements.

Is the scope of this issue only for the extern crates (like those present in lib.rs files)?

[metajack]

Currently cargo must compile every dependency so even if a crate is conditionally included in the rust source, it must be declared as a dependency and compiled. Therefore for rust-core-foundation, we have to cfg out the whole crate at the top level. Most of the time, this is a few annotations, but in the case of glfw-rs, it was quite a bit.

Cargo features allow us to not compile crates on some platforms, so all those cfgs can be removed in rust-core-foundation and we just tell cargo not to include that feature unless we're on macos.

Does that help?

[gilles-leblanc]

Yep, thanks. I'll start working on something and will probably push something to check if I'm on the right track.

[gilles-leblanc]

The pull request for the features feature has just landed in Cargo.

[mbrubeck]

I'm going to be reorganizing the code in components/compositing/platform, and will probably add feature gates to it in the process. Let me know (here or on IRC) if you've already made changes in that subdirectory, so we can avoid conflicts. I'll also link to this issue once I have a PR ready.

[gilles-leblanc]

@mbrubeck I had originally said I was looking into it but then found out the "features" feature wasn't implemented in Cargo yet so I started working on something else in the mean time and didn't get back to it.

So I haven't done anything yet.

I'll unassign myself from this for the mean time.

[mylainos]

It look like features is implemented now.

[frewsxcv]

This is possibly related to #5973

[faineance]

Anyone mind if I pick this up?

[frewsxcv]

Go for it!

[schuster]

A brief glance through some of the Cargo files suggests the problem was already fixed by making various dependencies target-specific. Is there anything left to do?

[mbrubeck]

No, I think this is done.