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

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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. https://github.com/google/zerocopy/issues/251).
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Matthias Krüger 2024-03-27 23:27:22 +01:00 • committed by GitHub
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1 changed files with 3 additions and 3 deletions
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6
library/core/src/sync/atomic.rs
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@ -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]
/// `].