Auto merge of #136316 - GrigorenkoPV:generic_atomic, r=Mark-Simulacrum

 Create `Atomic<T>` type alias (rebase)

Rebase of #130543.

Additional changes:
- Switch from `allow` to `expect` for `private_bounds` on `AtomicPrimitive`
- Unhide `AtomicPrimitive::AtomicInner` from docs, because rustdoc shows the definition `pub type Atomic<T> = <T as AtomicPrimitive>::AtomicInner;` and generated links for it.
  - `NonZero` did not have this issue, because they kept the new alias private before the direction was changed.
- Use `Atomic<_>` in more places, including inside `Once`'s `Futex`. This is possible thanks to https://github.com/rust-lang/rust-clippy/pull/14125

The rest will either get moved back to #130543 or #130543 will be closed in favor of this instead.

---

* ACP: https://github.com/rust-lang/libs-team/issues/443#event-14293381061
* Tracking issue: #130539

1 files changed, 94 insertions, 0 deletions

diff --git a/library/core/src/sync/atomic.rs b/library/core/src/sync/atomic.rs
index 9b1b13e7129..84c7f1aafe1 100644
--- a/library/core/src/sync/atomic.rs
+++ b/library/core/src/sync/atomic.rs
@@ -247,6 +247,100 @@ use crate::cell::UnsafeCell;
 use crate::hint::spin_loop;
 use crate::{fmt, intrinsics};
 
+trait Sealed {}
+
+/// A marker trait for primitive types which can be modified atomically.
+///
+/// This is an implementation detail for <code>[Atomic]\<T></code> which may disappear or be replaced at any time.
+///
+/// # Safety
+///
+/// Types implementing this trait must be primitives that can be modified atomically.
+///
+/// The associated `Self::AtomicInner` type must have the same size and bit validity as `Self`,
+/// but may have a higher alignment requirement, so the following `transmute`s are sound:
+///
+/// - `&mut Self::AtomicInner` as `&mut Self`
+/// - `Self` as `Self::AtomicInner` or the reverse
+#[unstable(
+    feature = "atomic_internals",
+    reason = "implementation detail which may disappear or be replaced at any time",
+    issue = "none"
+)]
+#[expect(private_bounds)]
+pub unsafe trait AtomicPrimitive: Sized + Copy + Sealed {
+    /// Temporary implementation detail.
+    type AtomicInner: Sized;
+}
+
+macro impl_atomic_primitive(
+    $Atom:ident $(<$T:ident>)? ($Primitive:ty),
+    size($size:literal),
+    align($align:literal) $(,)?
+) {
+    impl $(<$T>)? Sealed for $Primitive {}
+
+    #[unstable(
+        feature = "atomic_internals",
+        reason = "implementation detail which may disappear or be replaced at any time",
+        issue = "none"
+    )]
+    #[cfg(target_has_atomic_load_store = $size)]
+    unsafe impl $(<$T>)? AtomicPrimitive for $Primitive {
+        type AtomicInner = $Atom $(<$T>)?;
+    }
+}
+
+impl_atomic_primitive!(AtomicBool(bool), size("8"), align(1));
+impl_atomic_primitive!(AtomicI8(i8), size("8"), align(1));
+impl_atomic_primitive!(AtomicU8(u8), size("8"), align(1));
+impl_atomic_primitive!(AtomicI16(i16), size("16"), align(2));
+impl_atomic_primitive!(AtomicU16(u16), size("16"), align(2));
+impl_atomic_primitive!(AtomicI32(i32), size("32"), align(4));
+impl_atomic_primitive!(AtomicU32(u32), size("32"), align(4));
+impl_atomic_primitive!(AtomicI64(i64), size("64"), align(8));
+impl_atomic_primitive!(AtomicU64(u64), size("64"), align(8));
+impl_atomic_primitive!(AtomicI128(i128), size("128"), align(16));
+impl_atomic_primitive!(AtomicU128(u128), size("128"), align(16));
+
+#[cfg(target_pointer_width = "16")]
+impl_atomic_primitive!(AtomicIsize(isize), size("ptr"), align(2));
+#[cfg(target_pointer_width = "32")]
+impl_atomic_primitive!(AtomicIsize(isize), size("ptr"), align(4));
+#[cfg(target_pointer_width = "64")]
+impl_atomic_primitive!(AtomicIsize(isize), size("ptr"), align(8));
+
+#[cfg(target_pointer_width = "16")]
+impl_atomic_primitive!(AtomicUsize(usize), size("ptr"), align(2));
+#[cfg(target_pointer_width = "32")]
+impl_atomic_primitive!(AtomicUsize(usize), size("ptr"), align(4));
+#[cfg(target_pointer_width = "64")]
+impl_atomic_primitive!(AtomicUsize(usize), size("ptr"), align(8));
+
+#[cfg(target_pointer_width = "16")]
+impl_atomic_primitive!(AtomicPtr<T>(*mut T), size("ptr"), align(2));
+#[cfg(target_pointer_width = "32")]
+impl_atomic_primitive!(AtomicPtr<T>(*mut T), size("ptr"), align(4));
+#[cfg(target_pointer_width = "64")]
+impl_atomic_primitive!(AtomicPtr<T>(*mut T), size("ptr"), align(8));
+
+/// A memory location which can be safely modified from multiple threads.
+///
+/// This has the same size and bit validity as the underlying type `T`. However,
+/// the alignment of this type is always equal to its size, even on targets where
+/// `T` has alignment less than its size.
+///
+/// For more about the differences between atomic types and non-atomic types as
+/// well as information about the portability of this type, please see the
+/// [module-level documentation].
+///
+/// **Note:** This type is only available on platforms that support atomic loads
+/// and stores of `T`.
+///
+/// [module-level documentation]: crate::sync::atomic
+#[unstable(feature = "generic_atomic", issue = "130539")]
+pub type Atomic<T> = <T as AtomicPrimitive>::AtomicInner;
+
 // Some architectures don't have byte-sized atomics, which results in LLVM
 // emulating them using a LL/SC loop. However for AtomicBool we can take
 // advantage of the fact that it only ever contains 0 or 1 and use atomic OR/AND