Enable integer_atomics feature for Atomic*128

[kcking]

Proposing this change as it fixed the build of rust-cuda for me on windows with rustc channel = "nightly-2024-07-21", though I am not sure if there is a more correct way to solve this.

[juntyr]

Huh, this is curious. Your change seems to require 128b atomics to be enabled for any atomic implementations to work. I don't think that's what we want - would you be able to share the error you got when building rust-cuda? Perhaps I can identify the root issue then :)

[kcking]

I believe the root cause is that the integer_atomics feature must be enabled to use AtomicU/I128. AtomicU/I128 isn't present on some platforms, so I am guessing that is why no one has run into this yet. This PR shouldn't limit functionality on any platforms as the integer_atomics feature isn't enabled in any scenarios on main, this PR just enables it if the platform has 128b atomics. A simpler change might be to just always enable the integer_atomics feature, even on platforms where it isn't needed.

error[E0658]: use of unstable library feature 'integer_atomics'
  --> C:\Users\kevin\.cargo\registry\src\index.crates.io-6f17d22bba15001f\const-type-layout-0.3.2\src\impls\core\sync\atomic.rs:9:36
   |
9  |           unsafe impl TypeLayout for core::sync::atomic::$at {
   |                                      ^
...
39 | / impl_atomic_int_layout! {
40 | |     AtomicBool (1:"8") => u8 => false,
41 | |     AtomicI8 (1:"8") => i8 => 0, AtomicI16 (2:"16") => i16 => 0,
42 | |     AtomicI32 (4:"32") => i32 => 0, AtomicI64 (8:"64") => i64 => 0,
...  |
45 | |     AtomicI128 (16:"128") => i128 => 0, AtomicU128 (16:"128") => u128 => 0
46 | | }
   | |_- in this macro invocation
   |
   = note: see issue #99069 <https://github.com/rust-lang/rust/issues/99069> for more information
   = help: add `#![feature(integer_atomics)]` to the crate attributes to enable
   = note: this compiler was built on 2024-07-20; consider upgrading it if it is out of date
   = note: this error originates in the macro `impl_atomic_int_layout` (in Nightly builds, run with -Z macro-backtrace for more info)

[juntyr]

Thanks! I think I'm slowly understanding what's happening - the feature(integer_atomics) is required for Atomic[U|I]128, but because CI doesn't support 128b atomics at all, we never caught this issue before.

There are two possibilities now.

a) Your code clearly fixes the issue by enabling the feature if needed (I misread the change earlier)
b) We could add an additional crate feature, e.g. imps-atomics-128, to gate the Atomic[U|I]128 behind so that one has to additionally opt into using the unstable Atomic[U|I]128 types

[juntyr]

@kcking Could you check that this version now works for you (also if you actually use an Atomic[I|U]128 type)?

[kcking]

I am actually only using this crate transitively through rust-cuda, in my test case I'm using AtomicU64 and windows. Replacing rust-cuda's version of this crate with cc94bc8 works for me! (I can't test AtomicU128 with rust-cuda as I don't think rust ptx has Atomic*128 support).