once_cell::race does not compile on ARM

[tkaitchuck]

These four lines: 1, 2, 3, 4 fail to compile on ARM because on ARM the AtomicUsize lacks a compare_exchange method.
It may be possible to work around this with a load and a store and a loop.

[matklad]

Do you know which cfg is used to detect if it is available for a particular target?

[tkaitchuck]

The only way I am aware of is by using target_arch = "arm". I don't know if there is a list somewhere of which architectures have which features.

[taiki-e]

FYI:

• futures, crossbeam, heapless, defmt, etc. detect these targets using the TARGET environment variables provided by cargo for the build script and a list of targets that do not have atomic CAS. See Remove uses of unstable feature(cfg_target_has_atomic) rust-lang/futures-rs#2400 for more.
• Another way that is available in stable rust is to use crates like autocfg. See Use autocfg instead of unstable feature(cfg_target_has_atomic) rust-lang/futures-rs#2294 for more.