Make cfg_has_atomic_x public

[Finomnis]

Quote from the library:

// Not public API. (please submit an issue if you want this to be public API)

Rationale

I have a library crate that wants to

impl MyTrait for AtomicU8 { /* ... */ }
impl MyTrait for AtomicU16 { /* ... */ }
impl MyTrait for AtomicU32 { /* ... */ }
impl MyTrait for AtomicU64 { /* ... */ }

The problem is that one of the target architectures does not support AtomicU64 by default, and as a lib I shouldn't enable any features of portable-atomic. So I get the compiler error that AtomicU64 does not exist.

I need a cfg detection of whether or not a specific atomic exists, so I can disable impls for unavailable atomic types.

[taiki-e]

Thanks for the request. I'm open to making them public, but I have one question.

one of the target architectures does not support AtomicU64 by default

At first glance, your use case seems to always require AtomicU64, why have you disabled the enabled by default fallback feature? If it is enabled, AtomicU64 should be provided except for one exception (mipsel-psx).

[Finomnis]

It's more in the line of : my function can receive any atomic integer. And I'm trying to model this by introducing a trait for it. So I don't "require all atomics", instead I would like to "support all existing atomics".
I did try the "fallback" feature, but AtomicU64 still wasn't available at thumbv6m-none-eabi. But again, I don't require it, I just want to support it if it's available without forcing flags on the user.

[taiki-e]

I did try the "fallback" feature, but AtomicU64 still wasn't available at thumbv6m-none-eabi.

Your code used fetch_add (which is not available in thumbv6m by default), so I was wondering if you enabled critical-section or single-core related features, otherwise AtomicU64 is indeed not provided even with fallback feature enabled.

[Finomnis]

Thanks <3

[taiki-e]

Published in 1.6.0.