Tracking Issue for atomic volatile operations (`atomic_volatile`)

Feature gate: #![feature(atomic_volatile)]

This is a tracking issue for atomic volatile operations

• ACP: https://github.com/rust-lang/libs-team/issues/801
• t-opsem approval for the general principle: https://github.com/rust-lang/unsafe-code-guidelines/issues/615

Public API

// Not actually a generic impl, but you get the idea.
impl Atomic<T> {
  /// Convenient method to create a raw pointer to `Atomic<T>` suitable for volatile accesses.  
  pub fn from_ptr_raw(ptr: *mut T) -> *const Self;

  /// Performs a volatile atomic load of the given `order`.
  pub unsafe fn load_volatile(self: *const Self, order: Ordering) -> T;
  /// Performs a volatile atomic store of the given `order`.
  pub unsafe fn store_volatile(self: *const Self, val: T, order: Ordering);
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• ACP: https://github.com/rust-lang/libs-team/issues/801
  • Since then, I renamed volatile_load to load_volatile to be consistent with the existing read_volatile that also makes volatile a suffix rather than a prefix (and same for stores)
• Implementation
  • https://github.com/rust-lang/rust/pull/160058
  • https://github.com/rust-lang/rust/pull/161301
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Currently the API uses arbitrary_self_types_pointers. Are we okay with that or would it be bad to expose that on stable?
• On LLVM 22 and older, we don't correctly handle atomic volatile loads that trap. Adding the same black_box we add for non-atomic volatile loads is tricky because of how the code is structured in the backend. Maybe we should just wait until the minimum LLVM version is 23 before stabilizing this...

Start with the atomic_volatile feature-gate API in this issue, then read ACP 801 and implementation PRs #160058 and #161301. Review the two unresolved questions about arbitrary_self_types_pointers and LLVM 22-or-older trap handling. Done requires the final comment period and a stabilization PR.