IIUC lacking 64-bit CAS is an example of a place where on the surface the Rust situation looks worse than C++ but that's misleading
In this scenario C++ will provide "atomic" types that are just locking - they work but they don't have the behaviour you presumably wanted - and Rust will say OK, there's no 64-bit CAS so you can't have these features.
If all you actually needed was a type that has apparently atomic behaviour then I guess the C++ feature is perfect, but I'm assuming almost nobody asks for an atomic type if they would have accepted a locked type instead and so for them Rust saying "No, sorry" is actually a good heads up that they need to think again for this target.
Interesting perspective. In our case, we accepted a patch to effectively use a lock around a 64-bit integer as the workaround for these platforms, however.
In this scenario C++ will provide "atomic" types that are just locking - they work but they don't have the behaviour you presumably wanted - and Rust will say OK, there's no 64-bit CAS so you can't have these features.
If all you actually needed was a type that has apparently atomic behaviour then I guess the C++ feature is perfect, but I'm assuming almost nobody asks for an atomic type if they would have accepted a locked type instead and so for them Rust saying "No, sorry" is actually a good heads up that they need to think again for this target.