Gate Atomic type behind the corresponding #[cfg(target_has_atomic)]

Signed-off-by: Joe Richey <[email protected]>

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
       matrix:
         # See `INTERNAL.md` for an explanation of these pinned toolchain
         # versions.
-        toolchain: [ "msrv", "stable", "nightly",  "zerocopy-generic-bounds-in-const-fn", "zerocopy-aarch64-simd", "zerocopy-panic-in-const", ]
+        toolchain: [ "msrv", "stable", "nightly",  "zerocopy-generic-bounds-in-const-fn", "zerocopy-atomics", "zerocopy-aarch64-simd", "zerocopy-panic-in-const", ]
         target: [
           "i686-unknown-linux-gnu",
           "x86_64-unknown-linux-gnu",
@@ -61,6 +61,8 @@ jobs:
             features: "--all-features"
           - toolchain: "zerocopy-generic-bounds-in-const-fn"
             features: "--all-features"
+          - toolchain: "zerocopy-atomics"
+            features: "--all-features"
           - toolchain: "zerocopy-aarch64-simd"
             features: "--all-features"
           - toolchain: "zerocopy-panic-in-const"
@@ -80,6 +82,8 @@ jobs:
           # zerocopy-derive doesn't behave different on these toolchains.
           - crate: "zerocopy-derive"
             toolchain: "zerocopy-generic-bounds-in-const-fn"
+          - crate: "zerocopy-derive"
+            toolchain: "zerocopy-atomics"
           - crate: "zerocopy-derive"
             toolchain: "zerocopy-aarch64-simd"
           - crate: "zerocopy-derive"

Cargo.toml

@@ -35,6 +35,10 @@ exclude = [".*"]
 # From 1.61.0, Rust supports generic types with trait bounds in `const fn`.
 zerocopy-generic-bounds-in-const-fn = "1.61.0"
 
+# From 1.60.0, Rust supports detecting if a target supports atomics, so we can
+# reliably implement traits on the Atomic* types.
+zerocopy-atomics = "1.60.0"
+
 # When the "simd" feature is enabled, include SIMD types from the
 # `core::arch::aarch64` module, which was stabilized in 1.59.0. On earlier Rust
 # versions, these types require the "simd-nightly" feature.

src/lib.rs

@@ -292,6 +292,7 @@ pub mod layout;
 pub mod macro_util;
 #[doc(hidden)]
 pub mod pointer;
+#[macro_use]
 mod util;
 // TODO(#252): If we make this pub, come up with a better name.
 mod wrappers;
@@ -314,12 +315,11 @@ use core::{
     ops::{Deref, DerefMut},
     ptr::{self, NonNull},
     slice,
-    sync::atomic::{
-        AtomicBool, AtomicI16, AtomicI32, AtomicI8, AtomicIsize, AtomicPtr, AtomicU16, AtomicU32,
-        AtomicU8, AtomicUsize,
-    },
 };
 
+#[cfg(zerocopy_atomics)]
+use core::sync::atomic::*;
+
 use crate::pointer::invariant;
 
 #[cfg(any(feature = "alloc", test))]
@@ -665,9 +665,7 @@ impl_known_layout!(
     u8, i8, u16, i16, u32, i32, u64, i64, u128, i128, usize, isize, f32, f64,
     bool, char,
     NonZeroU8, NonZeroI8, NonZeroU16, NonZeroI16, NonZeroU32, NonZeroI32,
-    NonZeroU64, NonZeroI64, NonZeroU128, NonZeroI128, NonZeroUsize, NonZeroIsize,
-    AtomicBool, AtomicI16, AtomicI32, AtomicI8, AtomicIsize, AtomicU16, AtomicU32,
-    AtomicU8, AtomicUsize
+    NonZeroU64, NonZeroI64, NonZeroU128, NonZeroI128, NonZeroUsize, NonZeroIsize
 );
 #[rustfmt::skip]
 impl_known_layout!(
@@ -676,8 +674,7 @@ impl_known_layout!(
     T         => Wrapping<T>,
     T         => MaybeUninit<T>,
     T: ?Sized => *const T,
-    T: ?Sized => *mut T,
-    T         => AtomicPtr<T>
+    T: ?Sized => *mut T
 );
 impl_known_layout!(const N: usize, T => [T; N]);
 
@@ -3797,46 +3794,96 @@ safety_comment! {
     unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_extern_c_fn!(...));
 }
 
-macro_rules! impl_traits_for_atomics {
-    ($($atomics:ident [$inners:ident]),* $(,)?) => {
-        $(
-            impl_for_transparent_wrapper!(TryFromBytes for $atomics [UnsafeCell<$inners>]);
-            impl_for_transparent_wrapper!(FromZeros for $atomics [UnsafeCell<$inners>]);
-            impl_for_transparent_wrapper!(FromBytes for $atomics [UnsafeCell<$inners>]);
-            impl_for_transparent_wrapper!(IntoBytes for $atomics [UnsafeCell<$inners>]);
-        )*
-    };
+/// Implements the zerocopy traits for atomic integer types.
+///
+/// Note that we only implement traits for atomics types if the corresponding
+/// [`#[cfg(target_has_atomic)]`][target_has_atomic] configuration is true.
+///
+/// Note: On targets like `thumbv6m-none-eabi` (which has an [`AtomicU32`]
+/// type but `#[cfg(target_has_atomic = "32")]` is false), we don't implement
+/// the zerocopy traits. We will be able to handle this when Rust stabilizes
+/// [`#[cfg(target_has_atomic_load_store)]`][target_has_atomic_load_store].
+///
+/// [target_has_atomic]: https://doc.rust-lang.org/reference/conditional-compilation.html#target_has_atomic
+/// [target_has_atomic_load_store]: https://www.github.com/rust-lang/rust/issues/94039
+///
+/// # Safety
+///
+/// `$atomic` must be an atomic type with corresponding native type `$native`.
+#[cfg(zerocopy_atomics)]
+macro_rules! unsafe_impl_traits_for_atomics {
+    ($($atomic:ident [$native:ident]),*) => {$(
+        // SAFETY: `$atomic` is an atomic type with native type `$native`.
+        unsafe impl util::Atomic for $atomic { type Native = $native; }
+
+        impl_for_transparent_wrapper!(TryFromBytes for $atomic);
+        impl_for_transparent_wrapper!(FromZeros for $atomic);
+        unsafe_impl_traits_for_atomics!(@from_bytes $atomic);
+        impl_for_transparent_wrapper!(IntoBytes for $atomic);
+        impl_known_layout!($atomic);
+    )*};
+    (@from_bytes AtomicBool) => {};
+    (@from_bytes $atomic:ty) => { impl_for_transparent_wrapper!(FromBytes for $atomic); };
 }
 
-#[rustfmt::skip]
-impl_traits_for_atomics!(
-    AtomicBool [bool],
-    AtomicI16 [i16], AtomicI32 [i32], AtomicI8 [i8], AtomicIsize [isize],
-    AtomicU16 [u16], AtomicU32 [u32], AtomicU8 [u8], AtomicUsize [usize],
-);
+#[cfg(zerocopy_atomics)]
+#[cfg(target_has_atomic = "8")]
+#[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "8")))]
+mod atomic_8 {
+    use super::*;
+    unsafe_impl_traits_for_atomics!(AtomicBool[bool], AtomicU8[u8], AtomicI8[i8]);
+    safety_comment! {
+        /// SAFETY:
+        /// Per [1], `AtomicBool`, `AtomicU8`, and `AtomicI8` have an alignment of 1.
+        ///
+        /// [1] Per https://doc.rust-lang.org/nightly/core/sync/atomic/struct.AtomicBool.html:
+        ///
+        ///     This type has the same size, alignment, and bit validity as [the native type].
+        unsafe_impl!(AtomicBool: Unaligned);
+        unsafe_impl!(AtomicU8: Unaligned);
+        unsafe_impl!(AtomicI8: Unaligned);
+    }
+    assert_unaligned!(AtomicBool, AtomicU8, AtomicI8);
+}
+#[cfg(zerocopy_atomics)]
+#[cfg(target_has_atomic = "16")]
+#[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "16")))]
+mod atomic_16 {
+    use super::*;
+    unsafe_impl_traits_for_atomics!(AtomicU16[u16], AtomicI16[i16]);
+}
+#[cfg(zerocopy_atomics)]
+#[cfg(target_has_atomic = "32")]
+#[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "32")))]
+mod atomic_32 {
+    use super::*;
+    unsafe_impl_traits_for_atomics!(AtomicU32[u32], AtomicI32[i32]);
+}
+#[cfg(zerocopy_atomics)]
+#[cfg(target_has_atomic = "64")]
+#[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "64")))]
+mod atomic_64 {
+    use super::*;
+    unsafe_impl_traits_for_atomics!(AtomicU64[u64], AtomicI64[i64]);
+}
+#[cfg(zerocopy_atomics)]
+#[cfg(target_has_atomic = "ptr")]
+#[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "ptr")))]
+mod atomic_ptr {
+    use super::*;
+    unsafe_impl_traits_for_atomics!(AtomicUsize[usize], AtomicIsize[isize]);
 
-safety_comment! {
     /// SAFETY:
-    /// Per [1], `AtomicBool`, `AtomicU8`, and `AtomicI8` have the same size as
-    /// `bool`, `u8`, and `i8` respectively. Since a type's alignment cannot be
-    /// smaller than 1 [2], and since its alignment cannot be greater than its
-    /// size [3], the only possible value for the alignment is 1. Thus, it is
-    /// sound to implement `Unaligned`.
-    ///
-    /// [1] TODO(#896), TODO(https://github.com/rust-lang/rust/pull/121943):
-    ///     Cite docs once they've landed.
+    /// `AtomicPtr<T>`` is garunteed to wrap a `*mut T`. Just like *mut T, we
+    /// don't implement FromBytes/IntoBytes.
     ///
-    /// [2] Per https://doc.rust-lang.org/reference/type-layout.html#size-and-alignment:
-    ///
-    ///     Alignment is measured in bytes, and must be at least 1.
-    ///
-    /// [3] Per https://doc.rust-lang.org/reference/type-layout.html#size-and-alignment:
-    ///
-    ///     The size of a value is always a multiple of its alignment.
-    unsafe_impl!(AtomicBool: Unaligned);
-    unsafe_impl!(AtomicU8: Unaligned);
-    unsafe_impl!(AtomicI8: Unaligned);
-    assert_unaligned!(AtomicBool, AtomicU8, AtomicI8);
+    /// See the comments on the `*mut T` impls for more information.
+    unsafe impl<T> util::Atomic for AtomicPtr<T> {
+        type Native = *mut T;
+    }
+    impl_for_transparent_wrapper!(T => TryFromBytes for AtomicPtr<T>);
+    impl_for_transparent_wrapper!(T => FromZeros for AtomicPtr<T>);
+    impl_known_layout!(T => AtomicPtr<T>);
 }
 
 safety_comment! {