feat: Add extra-platforms feature for non-atomic targets

Cargo.toml

@@ -28,6 +28,11 @@ std = []
 # Enables async receiving via the AsyncReceiver receiver
 async = []
 
+[dependencies]
+# Use portable-atomic crate to support platforms without atomic CAS.
+# Enable require-cas feature to provide an error message if dependents require atomic CAS but the end user forgets to use single-core cfg or critical-section feature.
+extra-platforms = { package = "portable-atomic", version = "1.3", optional = true, default-features = false, features = ["require-cas"] }
+
 # Only used for internal correctness testing.
 # Downstream users of oneshot should never enable this feature. Enabling it does nothing.
 # To compile oneshot built against loom one must *also* set RUSTFLAGS="--cfg oneshot_loom"

src/atomic.rs

@@ -1,4 +1,6 @@
-#[cfg(not(oneshot_loom))]
+#[cfg(not(any(oneshot_loom, feature = "extra-platforms")))]
 pub use core::sync::atomic::{AtomicU8, Ordering, fence};
+#[cfg(feature = "extra-platforms")]
+pub use extra_platforms::{AtomicU8, Ordering, fence};
 #[cfg(oneshot_loom)]
 pub use loom::sync::atomic::{AtomicU8, Ordering, fence};