Tracking Issue for generic `Atomic`

Feature gate: #![feature(generic_atomic)]

This is a tracking issue for replacing the distinct Atomic* types with a generic Atomic<T> type. This allows using Atomic with FFI type aliases and helps clean up some API surface. Only types with existing AtomicT are usable in Atomic<T>.

Public API

// core::sync::atomic

pub struct Atomic<T: AtomicPrimitive>(/* private fields*/);

pub type AtomicI32 = Atomic<i32>; // etc

Steps / History

• ACP: https://github.com/rust-lang/libs-team/issues/443
• Implement Atomic<T> as an alias to AtomicT: https://github.com/rust-lang/rust/pull/136316
• Flip alias so AtomicT is an alias to Atomic<T>: #153015
• Move generic functionality from AtomicT to Atomic<_>
  • new/into_inner/from_ptr/as_ptr/Default/From<T>: #153407
• Pseudo-prerequisite: Stabilize 128-bit atomics: https://github.com/rust-lang/rust/issues/99069
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Atomic<T> is soft-blocked on 128-bit atomics, because since trait implementations cannot be unstable, gating Atomic<i128> separately from Atomic<i32> isn't possible.
  • If necessary, AtomicI128 could instead name Atomic<Wrapper<i128>> for some unstable name Wrapper until 128-bit atomics are stable, to prevent Atomic<i128> from being usable from stable earlier than intended.

Start in the core::sync::atomic API described here and review the remaining work tracked by #153407, including the generic methods and conversions. Check the 128-bit atomic prerequisite in #99069 and the stabilization process documentation. Done means the remaining checklist items are resolved, unresolved questions are settled, and the feature reaches stabilization.