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 rust-lang/rust-clippy#14125

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

---

* ACP: rust-lang/libs-team#443 (comment)
* Tracking issue: #130539

Author: bors

library/alloc/src/lib.rs

@@ -122,6 +122,7 @@
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
 #![feature(formatting_options)]
+#![feature(generic_atomic)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(inplace_iteration)]
 #![feature(iter_advance_by)]

library/alloc/src/sync.rs

@@ -26,8 +26,8 @@ use core::pin::{Pin, PinCoerceUnsized};
 use core::ptr::{self, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
-use core::sync::atomic;
 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
+use core::sync::atomic::{self, Atomic};
 use core::{borrow, fmt, hint};
 
 #[cfg(not(no_global_oom_handling))]
@@ -369,12 +369,12 @@ impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
 // inner types.
 #[repr(C)]
 struct ArcInner<T: ?Sized> {
-    strong: atomic::AtomicUsize,
+    strong: Atomic<usize>,
 
     // the value usize::MAX acts as a sentinel for temporarily "locking" the
     // ability to upgrade weak pointers or downgrade strong ones; this is used
     // to avoid races in `make_mut` and `get_mut`.
-    weak: atomic::AtomicUsize,
+    weak: Atomic<usize>,
 
     data: T,
 }
@@ -2760,8 +2760,8 @@ impl<T, A: Allocator> Weak<T, A> {
 /// Helper type to allow accessing the reference counts without
 /// making any assertions about the data field.
 struct WeakInner<'a> {
-    weak: &'a atomic::AtomicUsize,
-    strong: &'a atomic::AtomicUsize,
+    weak: &'a Atomic<usize>,
+    strong: &'a Atomic<usize>,
 }
 
 impl<T: ?Sized> Weak<T> {

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

library/std/src/alloc.rs

@@ -57,7 +57,7 @@
 #![stable(feature = "alloc_module", since = "1.28.0")]
 
 use core::ptr::NonNull;
-use core::sync::atomic::{AtomicPtr, Ordering};
+use core::sync::atomic::{Atomic, AtomicPtr, Ordering};
 use core::{hint, mem, ptr};
 
 #[stable(feature = "alloc_module", since = "1.28.0")]
@@ -287,7 +287,7 @@ unsafe impl Allocator for System {
     }
 }
 
-static HOOK: AtomicPtr<()> = AtomicPtr::new(ptr::null_mut());
+static HOOK: Atomic<*mut ()> = AtomicPtr::new(ptr::null_mut());
 
 /// Registers a custom allocation error hook, replacing any that was previously registered.
 ///

library/std/src/backtrace.rs

@@ -92,8 +92,8 @@ use crate::backtrace_rs::{self, BytesOrWideString};
 use crate::ffi::c_void;
 use crate::panic::UnwindSafe;
 use crate::sync::LazyLock;
-use crate::sync::atomic::AtomicU8;
 use crate::sync::atomic::Ordering::Relaxed;
+use crate::sync::atomic::{Atomic, AtomicU8};
 use crate::sys::backtrace::{lock, output_filename, set_image_base};
 use crate::{env, fmt};
 
@@ -254,7 +254,7 @@ impl Backtrace {
         // Cache the result of reading the environment variables to make
         // backtrace captures speedy, because otherwise reading environment
         // variables every time can be somewhat slow.
-        static ENABLED: AtomicU8 = AtomicU8::new(0);
+        static ENABLED: Atomic<u8> = AtomicU8::new(0);
         match ENABLED.load(Relaxed) {
             0 => {}
             1 => return false,

library/std/src/io/stdio.rs

@@ -11,7 +11,7 @@ use crate::io::{
     self, BorrowedCursor, BufReader, IoSlice, IoSliceMut, LineWriter, Lines, SpecReadByte,
 };
 use crate::panic::{RefUnwindSafe, UnwindSafe};
-use crate::sync::atomic::{AtomicBool, Ordering};
+use crate::sync::atomic::{Atomic, AtomicBool, Ordering};
 use crate::sync::{Arc, Mutex, MutexGuard, OnceLock, ReentrantLock, ReentrantLockGuard};
 use crate::sys::stdio;
 use crate::thread::AccessError;
@@ -37,7 +37,7 @@ thread_local! {
 /// have a consistent order between set_output_capture and print_to *within
 /// the same thread*. Within the same thread, things always have a perfectly
 /// consistent order. So Ordering::Relaxed is fine.
-static OUTPUT_CAPTURE_USED: AtomicBool = AtomicBool::new(false);
+static OUTPUT_CAPTURE_USED: Atomic<bool> = AtomicBool::new(false);
 
 /// A handle to a raw instance of the standard input stream of this process.
 ///

library/std/src/lib.rs

@@ -344,6 +344,7 @@
 #![feature(float_gamma)]
 #![feature(float_minimum_maximum)]
 #![feature(fmt_internals)]
+#![feature(generic_atomic)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(hashmap_internals)]
 #![feature(hint_must_use)]

library/std/src/os/uefi/env.rs

@@ -4,13 +4,13 @@
 
 use crate::ffi::c_void;
 use crate::ptr::NonNull;
-use crate::sync::atomic::{AtomicBool, AtomicPtr, Ordering};
+use crate::sync::atomic::{Atomic, AtomicBool, AtomicPtr, Ordering};
 
-static SYSTEM_TABLE: AtomicPtr<c_void> = AtomicPtr::new(crate::ptr::null_mut());
-static IMAGE_HANDLE: AtomicPtr<c_void> = AtomicPtr::new(crate::ptr::null_mut());
+static SYSTEM_TABLE: Atomic<*mut c_void> = AtomicPtr::new(crate::ptr::null_mut());
+static IMAGE_HANDLE: Atomic<*mut c_void> = AtomicPtr::new(crate::ptr::null_mut());
 // Flag to check if BootServices are still valid.
 // Start with assuming that they are not available
-static BOOT_SERVICES_FLAG: AtomicBool = AtomicBool::new(false);
+static BOOT_SERVICES_FLAG: Atomic<bool> = AtomicBool::new(false);
 
 /// Initializes the global System Table and Image Handle pointers.
 ///

library/std/src/os/xous/services.rs

@@ -1,4 +1,4 @@
-use core::sync::atomic::{AtomicU32, Ordering};
+use core::sync::atomic::{Atomic, AtomicU32, Ordering};
 
 use crate::os::xous::ffi::Connection;
 
@@ -106,7 +106,7 @@ pub fn try_connect(name: &str) -> Option<Connection> {
     ns::try_connect_with_name(name)
 }
 
-static NAME_SERVER_CONNECTION: AtomicU32 = AtomicU32::new(0);
+static NAME_SERVER_CONNECTION: Atomic<u32> = AtomicU32::new(0);
 
 /// Returns a `Connection` to the name server. If the name server has not been started,
 /// then this call will block until the name server has been started. The `Connection`

library/std/src/os/xous/services/dns.rs

@@ -1,4 +1,4 @@
-use core::sync::atomic::{AtomicU32, Ordering};
+use core::sync::atomic::{Atomic, AtomicU32, Ordering};
 
 use crate::os::xous::ffi::Connection;
 use crate::os::xous::services::connect;
@@ -17,7 +17,7 @@ impl Into<usize> for DnsLendMut {
 /// Returns a `Connection` to the DNS lookup server. This server is used for
 /// querying domain name values.
 pub(crate) fn dns_server() -> Connection {
-    static DNS_CONNECTION: AtomicU32 = AtomicU32::new(0);
+    static DNS_CONNECTION: Atomic<u32> = AtomicU32::new(0);
     let cid = DNS_CONNECTION.load(Ordering::Relaxed);
     if cid != 0 {
         return cid.into();