Detect atomic support using target_has_atomic

Implements `TryFromBytes` and `FromZeros` for `AtomicPtr`; `FromBytes`
and `IntoBytes` are blocked by #170.

This is adapted from @josephlr's similar implementation in #1092.

Fixes #1086

Co-authored-by: Joe Richey <joerichey@google.com>

.github/workflows/ci.yml

@@ -46,7 +46,19 @@ 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",
+
+          # These are the names of specific Rust versions detected in
+          # `build.rs`. Each of these represents the minimum Rust version for
+          # which a particular feature is supported.
+          "zerocopy-generic-bounds-in-const-fn",
+          "zerocopy-target-has-atomics",
+          "zerocopy-aarch64-simd",
+          "zerocopy-panic-in-const"
+        ]
         target: [
           "i686-unknown-linux-gnu",
           "x86_64-unknown-linux-gnu",
@@ -57,6 +69,7 @@ jobs:
           "riscv64gc-unknown-linux-gnu",
           "s390x-unknown-linux-gnu",
           "x86_64-pc-windows-msvc",
+          "thumbv6m-none-eabi",
           "wasm32-wasi"
         ]
         features: [ "--no-default-features", "", "--features __internal_use_only_features_that_work_on_stable", "--all-features" ]
@@ -109,6 +122,8 @@ jobs:
             event_name: "pull_request"
           - target: "s390x-unknown-linux-gnu"
             event_name: "pull_request"
+          - target: "thumbv6m-none-eabi"
+            event_name: "pull_request"
           - target: "wasm32-wasi"
             event_name: "pull_request"
 

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 `cfg(target_has_atomics)`, which allows us to
+# detect whether a target supports particular sets of atomics.
+zerocopy-target-has-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/impls.rs

@@ -440,49 +440,137 @@ 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),* $(,)?) => {
-        $(
-            impl_for_transparent_wrapper!(=> TryFromBytes for $atomics);
-            impl_for_transparent_wrapper!(=> FromZeros for $atomics);
-            impl_for_transparent_wrapper!(=> FromBytes for $atomics);
-            impl_for_transparent_wrapper!(=> IntoBytes for $atomics);
-        )*
+#[cfg(zerocopy_target_has_atomics)]
+mod atomics {
+    use core::sync::atomic::{
+        AtomicBool, AtomicI16, AtomicI32, AtomicI64, AtomicI8, AtomicIsize, AtomicPtr, AtomicU16,
+        AtomicU32, AtomicU64, AtomicU8, AtomicUsize,
     };
-}
 
-#[rustfmt::skip]
-impl_traits_for_atomics!(
-    AtomicI16, AtomicI32, AtomicI8, AtomicIsize,
-    AtomicU16, AtomicU32, AtomicU8, AtomicUsize,
-);
+    use super::*;
 
-impl_for_transparent_wrapper!(=> TryFromBytes for AtomicBool);
-impl_for_transparent_wrapper!(=> FromZeros for AtomicBool);
-impl_for_transparent_wrapper!(=> IntoBytes for AtomicBool);
+    macro_rules! impl_traits_for_atomics {
+        ($($atomics:ident),* $(,)?) => {
+            $(
+                impl_known_layout!($atomics);
+                impl_for_transparent_wrapper!(=> TryFromBytes for $atomics);
+                impl_for_transparent_wrapper!(=> FromZeros for $atomics);
+                impl_for_transparent_wrapper!(=> FromBytes for $atomics);
+                impl_for_transparent_wrapper!(=> IntoBytes for $atomics);
+            )*
+        };
+    }
 
-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.
-    ///
-    /// [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);
+    #[cfg(target_has_atomic = "8")]
+    #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "8")))]
+    mod atomic_8 {
+        use super::*;
+
+        impl_traits_for_atomics!(AtomicU8, AtomicI8);
+
+        impl_known_layout!(AtomicBool);
+
+        impl_for_transparent_wrapper!(=> TryFromBytes for AtomicBool);
+        impl_for_transparent_wrapper!(=> FromZeros for AtomicBool);
+        impl_for_transparent_wrapper!(=> IntoBytes for AtomicBool);
+
+        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.
+            ///
+            /// [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);
+
+            /// SAFETY:
+            /// All of these pass an atomic type and that type's native equivalent, as
+            /// required by the macro safety preconditions.
+            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU8 [u8], AtomicI8 [i8], AtomicBool [bool]);
+        }
+    }
+
+    #[cfg(target_has_atomic = "16")]
+    #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "16")))]
+    mod atomic_16 {
+        use super::*;
+
+        impl_traits_for_atomics!(AtomicU16, AtomicI16);
+
+        safety_comment! {
+            /// SAFETY:
+            /// All of these pass an atomic type and that type's native equivalent, as
+            /// required by the macro safety preconditions.
+            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU16 [u16], AtomicI16 [i16]);
+        }
+    }
+
+    #[cfg(target_has_atomic = "32")]
+    #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "32")))]
+    mod atomic_32 {
+        use super::*;
+
+        impl_traits_for_atomics!(AtomicU32, AtomicI32);
+
+        safety_comment! {
+            /// SAFETY:
+            /// All of these pass an atomic type and that type's native equivalent, as
+            /// required by the macro safety preconditions.
+            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU32 [u32], AtomicI32 [i32]);
+        }
+    }
+
+    #[cfg(target_has_atomic = "64")]
+    #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "64")))]
+    mod atomic_64 {
+        use super::*;
+
+        impl_traits_for_atomics!(AtomicU64, AtomicI64);
+
+        safety_comment! {
+            /// SAFETY:
+            /// All of these pass an atomic type and that type's native equivalent, as
+            /// required by the macro safety preconditions.
+            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU64 [u64], AtomicI64 [i64]);
+        }
+    }
+
+    #[cfg(target_has_atomic = "ptr")]
+    #[cfg_attr(doc_cfg, doc(cfg(target_has_atomic = "ptr")))]
+    mod atomic_ptr {
+        use super::*;
+
+        impl_traits_for_atomics!(AtomicUsize, AtomicIsize);
+
+        impl_known_layout!(T => AtomicPtr<T>);
+
+        // TODO(#170): Implement `FromBytes` and `IntoBytes` once we implement
+        // those traits for `*mut T`.
+        impl_for_transparent_wrapper!(T => TryFromBytes for AtomicPtr<T>);
+        impl_for_transparent_wrapper!(T => FromZeros for AtomicPtr<T>);
+
+        safety_comment! {
+            /// SAFETY:
+            /// This passes an atomic type and that type's native equivalent, as
+            /// required by the macro safety preconditions.
+            unsafe_impl_transparent_wrapper_for_atomic!(AtomicUsize [usize], AtomicIsize [isize]);
+            unsafe_impl_transparent_wrapper_for_atomic!(T => AtomicPtr<T> [*mut T]);
+        }
+    }
 }
 
 safety_comment! {

src/lib.rs

@@ -296,6 +296,8 @@ extern crate self as zerocopy;
 
 #[macro_use]
 mod macros;
+#[macro_use]
+mod util;
 
 pub mod byte_slice;
 pub mod byteorder;
@@ -313,7 +315,6 @@ pub mod macro_util;
 #[doc(hidden)]
 pub mod pointer;
 mod r#ref;
-mod util;
 // TODO(#252): If we make this pub, come up with a better name.
 mod wrappers;
 
@@ -337,10 +338,6 @@ use core::{
     ops::{Deref, DerefMut},
     ptr::{self, NonNull},
     slice,
-    sync::atomic::{
-        AtomicBool, AtomicI16, AtomicI32, AtomicI8, AtomicIsize, AtomicPtr, AtomicU16, AtomicU32,
-        AtomicU8, AtomicUsize,
-    },
 };
 
 use crate::pointer::{invariant, BecauseExclusive, BecauseImmutable};
@@ -819,9 +816,7 @@ 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!(
@@ -830,8 +825,7 @@ 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]);
 

src/util.rs

@@ -12,10 +12,6 @@ use core::{
     mem::{self, ManuallyDrop, MaybeUninit},
     num::{NonZeroUsize, Wrapping},
     ptr::NonNull,
-    sync::atomic::{
-        AtomicBool, AtomicI16, AtomicI32, AtomicI8, AtomicIsize, AtomicPtr, AtomicU16, AtomicU32,
-        AtomicU8, AtomicUsize,
-    },
 };
 
 use crate::{
@@ -332,12 +328,13 @@ unsafe impl<T, I: Invariants> TransparentWrapper<I> for Unalign<T> {
 ///
 /// The caller promises that `$atomic` is an atomic type whose natie equivalent
 /// is `$native`.
+#[cfg(zerocopy_target_has_atomics)]
 macro_rules! unsafe_impl_transparent_wrapper_for_atomic {
     ($(#[$attr:meta])* $(,)?) => {};
     ($(#[$attr:meta])* $atomic:ty [$native:ty], $($atomics:ty [$natives:ty]),* $(,)?) => {
         $(#[$attr])*
         // SAFETY: See safety comment in next match arm.
-        unsafe impl<I: Invariants> TransparentWrapper<I> for $atomic {
+        unsafe impl<I: crate::invariant::Invariants> crate::util::TransparentWrapper<I> for $atomic {
             unsafe_impl_transparent_wrapper_for_atomic!(@inner $atomic [$native]);
         }
         unsafe_impl_transparent_wrapper_for_atomic!($(#[$attr])* $($atomics [$natives],)*);
@@ -351,7 +348,7 @@ macro_rules! unsafe_impl_transparent_wrapper_for_atomic {
         // [1] TODO(#896), TODO(https://github.com/rust-lang/rust/pull/121943):
         //     Cite docs once they've landed.
         $(#[$attr])*
-        unsafe impl<$tyvar, I: Invariants> TransparentWrapper<I> for $atomic {
+        unsafe impl<$tyvar, I: crate::invariant::Invariants> crate::util::TransparentWrapper<I> for $atomic {
             unsafe_impl_transparent_wrapper_for_atomic!(@inner $atomic [$native]);
         }
     };
@@ -381,11 +378,11 @@ macro_rules! unsafe_impl_transparent_wrapper_for_atomic {
         //
         // [1] TODO(#896), TODO(https://github.com/rust-lang/rust/pull/121943):
         //     Cite docs once they've landed.
-        type UnsafeCellVariance = Covariant;
+        type UnsafeCellVariance = crate::util::Covariant;
 
         // SAFETY: No safety justification is required for an invariant
         // variance.
-        type AlignmentVariance = Invariant;
+        type AlignmentVariance = crate::util::Invariant;
 
         // SAFETY: Per [1], all atomic types have the same bit validity as their
         // native counterparts. The caller has promised that `$atomic` and
@@ -394,7 +391,7 @@ macro_rules! unsafe_impl_transparent_wrapper_for_atomic {
         //
         // [1] TODO(#896), TODO(https://github.com/rust-lang/rust/pull/121943):
         //     Cite docs once they've landed.
-        type ValidityVariance = Covariant;
+        type ValidityVariance = crate::util::Covariant;
 
         fn cast_into_inner(ptr: *mut $atomic) -> *mut UnsafeCell<$native> {
             // SAFETY: Per [1] (from comment on impl block), `$atomic` has the
@@ -414,24 +411,6 @@ macro_rules! unsafe_impl_transparent_wrapper_for_atomic {
     };
 }
 
-safety_comment! {
-    /// SAFETY:
-    /// All of these pass an atomic type and that type's native equivalent, as
-    /// required by the macro safety preconditions.
-    unsafe_impl_transparent_wrapper_for_atomic!(T => AtomicPtr<T> [*mut T]);
-    unsafe_impl_transparent_wrapper_for_atomic!(
-        AtomicBool [bool],
-        AtomicI16 [i16], AtomicI32 [i32], AtomicI8 [i8], AtomicIsize [isize],
-        AtomicU16 [u16], AtomicU32 [u32], AtomicU8 [u8], AtomicUsize [usize],
-    );
-    #[cfg(not(target_arch = "powerpc"))]
-    unsafe_impl_transparent_wrapper_for_atomic!(
-        #[cfg_attr(doc_cfg, doc(cfg(not(target_arch = "powerpc"))))]
-        core::sync::atomic::AtomicI64 [i64],
-        core::sync::atomic::AtomicU64 [u64],
-    );
-}
-
 /// Like [`PhantomData`], but [`Send`] and [`Sync`] regardless of whether the
 /// wrapped `T` is.
 pub(crate) struct SendSyncPhantomData<T: ?Sized>(PhantomData<T>);

tests/ui-nightly/include_value_not_from_bytes.stderr

@@ -10,12 +10,12 @@ error[E0277]: the trait bound `NotZerocopy<u32>: zerocopy::FromBytes` is not sat
    = help: the following other types implement trait `zerocopy::FromBytes`:
              ()
              AU16
-             AtomicI16
-             AtomicI32
-             AtomicI8
-             AtomicIsize
-             AtomicU16
-             AtomicU32
+             F32<O>
+             F64<O>
+             I128<O>
+             I16<O>
+             I32<O>
+             I64<O>
            and $N others
 note: required by a bound in `AssertIsFromBytes`
   --> tests/ui-nightly/include_value_not_from_bytes.rs:15:42

tests/ui-nightly/transmute-dst-not-frombytes.stderr

@@ -10,12 +10,12 @@ error[E0277]: the trait bound `NotZerocopy: zerocopy::FromBytes` is not satisfie
    = help: the following other types implement trait `zerocopy::FromBytes`:
              ()
              AU16
-             AtomicI16
-             AtomicI32
-             AtomicI8
-             AtomicIsize
-             AtomicU16
-             AtomicU32
+             F32<O>
+             F64<O>
+             I128<O>
+             I16<O>
+             I32<O>
+             I64<O>
            and $N others
 note: required by a bound in `AssertIsFromBytes`
   --> tests/ui-nightly/transmute-dst-not-frombytes.rs:19:41