Attempting to use Atomics larger than max. atomic width

[utsavm9]

I am trying to compile crossbeam-utils = "0.8" for a non-unix target and I get this error:

error[E0432]: unresolved imports `core::sync::atomic::AtomicI64`, `core::sync::atomic::AtomicU64`
  --> /home/utsav/.cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/crossbeam-utils-0.8.5/src/lib.rs:79:49
   |
79 |             pub(crate) use core::sync::atomic::{AtomicI64, AtomicU64}; 
   |                                                 ^^^^^^^^^  ^^^^^^^^^ no `AtomicU64` in `sync::atomic`
   |                                                 |
   |                                                 no `AtomicI64` in `sync::atomic`
   |

The target I am compiling to has max_atomic_width: Some(32), set similar to what it is here:
https://github.com/rust-lang/rust/blob/e3231e35b6bbfabca1d9ceff9c4690af00328706/src/librustc_back/target/armv4t_unknown_linux_gnueabi.rs#L31

So, I think it makes sense why AtomicI64, AtomicU64 are not available from the standard library for my target. I see that you have a crossbeam_no_atomic_64 cfg available, but I could not set it like:

crossbeam-utils = { version = "0.8", features = ["crossbeam_no_atomic_64"]}
// errors with:
// the package `testdep` depends on `crossbeam-utils`, with features: `crossbeam_no_atomic_64` but `crossbeam-utils` does not have these features.

Do you know a way I can make it compile where maximum atomic width is 32 bits?

[taiki-e]

Currently, crossbeam's build script only handles built-in targets. For custom targets, it is recommended that set rustflags for now. See also #698, rust-lang/futures-rs#2400, rust-lang/futures-rs#2294.

So, I think it makes sense why AtomicI64, AtomicU64 are not available from the standard library for my target. I see that you have a crossbeam_no_atomic_64 cfg available, but I could not set it like:

To enable cfg, use RUSTFLAGS environment variable or rustflags config, instead of cargo feature.

[utsavm9]

This way of specifying rustflags in rust-project/.cargo/config.toml worked for me:

[target.my-custom-target]
rustflags = ["--cfg", "crossbeam_no_atomic_64"]

Thanks for helping!