Conditionally compile in Arc

[CryZe]

Apparently the latest release of bytemuck doesn't compile on some of the targets that it used to because it uses Arc. Arc is not a type that exists on every target, because not every target supports atomics.

Error message with 1.12.2:

error[E0432]: unresolved import `alloc::sync`
  --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/bytemuck-1.12.2/src/allocation.rs:17:3
   |
17 |   sync::Arc,
   |   ^^^^ could not find `sync` in `alloc`

[notgull]

Correct me if I'm wrong, but wasn't target_has_atomic introduced in a Rust version after 1.34? This would break the MSRV.

[CryZe]

The extern_crate_alloc feature does not follow the MSRV. At least that's how I'm reading the README (and how the CI is set up). Alternatively arc could be a feature (that internally also does the check) and 1.12.2 gets yanked.

Opt-in features of the crate are not held to the same standard, and may work only on the latest Stable or latest Nightly.

[Lokathor]

That's accurate, the CI isn't that precise with the MSRV. Only the "basic" build sticks exactly to 1.34. Everything else is only tested on latest stable.

I'd rather not create even more features for people to have to manage, if it's reasonable to just keep two features together. In the case of target_has_atomic, it seems to be the "intended' cfg to check for this sort of thing. According to rust-lang/rust#32976 it's been stable since 1.60 back in Feb, which seems "long enough" that we can generally assume people are using at least that version. That's new enough that it's not in Debian Stable, but it is in Debian Testing, which should become the stable version sometime in 2023.

I'm inclined to accept this as is, unless @notgull you really think we should break it up?

[notgull]

I'm fine with this! However, I do think that an MSRV for the alloc feature (and others) should be defined to prevent surprises.

[Lokathor]

I agree, but I'm not sure I have the energy for that much CI fiddling

[CryZe]

Can we have a bugfix release for this? My CI blocks me otherwise xD

[Lokathor]

Yeah, I had enough time to hit the button before the bus to work but can't sit down at a publish capable thing until some time later today.

[CryZe]

Ah no worries then :D

[Lokathor]

1.12.3 should be available now

[CryZe]

Nice, everything works now 👍
Thank you :)