Rollup merge of rust-lang#121943 - joshlf:patch-11, r=scottmcm

Clarify atomic bit validity

The previous definition used the phrase "representation", which is ambiguous given the current state of memory model nomenclature in Rust. For integer types and for `AtomicPtr<T>`, the new wording clarifies that size and bit validity are guaranteed to match the corresponding native integer type/`*mut T`. For `AtomicBool`, the new wording clarifies that size, alignment, and bit validity are guaranteed to match `bool`.

Note that we use the phrase "size and alignment" rather than "layout" since the latter term also implies that the field types are the same. This isn't true - `AtomicXxx` doesn't store an `xxx`, but rather an `UnsafeCell<xxx>`. This distinction is important for some `unsafe` code, which needs to reason about the presence or absence of interior mutability in order to ensure that their code is sound (see e.g. google/zerocopy#251).

Author: matthiaskrgr

library/core/src/sync/atomic.rs

@@ -243,7 +243,7 @@ const EMULATE_ATOMIC_BOOL: bool =
 
 /// A boolean type which can be safely shared between threads.
 ///
-/// This type has the same in-memory representation as a [`bool`].
+/// This type has the same size, alignment, and bit validity as a [`bool`].
 ///
 /// **Note**: This type is only available on platforms that support atomic
 /// loads and stores of `u8`.
@@ -272,7 +272,7 @@ unsafe impl Sync for AtomicBool {}
 
 /// A raw pointer type which can be safely shared between threads.
 ///
-/// This type has the same in-memory representation as a `*mut T`.
+/// This type has the same size and bit validity as a `*mut T`.
 ///
 /// **Note**: This type is only available on platforms that support atomic
 /// loads and stores of pointers. Its size depends on the target pointer's size.
@@ -2121,7 +2121,7 @@ macro_rules! atomic_int {
      $int_type:ident $atomic_type:ident) => {
         /// An integer type which can be safely shared between threads.
         ///
-        /// This type has the same in-memory representation as the underlying
+        /// This type has the same size and bit validity as the underlying
         /// integer type, [`
         #[doc = $s_int_type]
         /// `].