Add an atomic-polyfill optional dependancy

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## 1.9
+
+- Added an `atomic-polyfill` optional dependency to compile `race` on platforms without atomics
+
 ## 1.8.0
 
 - Add `try_insert` API -- a version of `set` that returns a reference.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "once_cell"
-version = "1.8.0"
+version = "1.9.0"
 authors = ["Aleksey Kladov <aleksey.kladov@gmail.com>"]
 license = "MIT OR Apache-2.0"
 edition = "2018"
@@ -24,6 +24,13 @@ members = ["xtask"]
 # for up to 16 bytes smaller, depending on the size of the T.
 parking_lot = { version = "0.11", optional = true, default_features = false }
 
+# To be used in order to enable the race feature on targets
+# that do not have atomics
+# *Warning:* This can be unsound. Please read the README of
+# [atomic-polyfill](https://github.com/embassy-rs/atomic-polyfill)
+# and make sure you understand all the implications
+atomic-polyfill = { version = "0.1", optional = true }
+
 [dev-dependencies]
 lazy_static = "1.0.0"
 crossbeam-utils = "0.7.2"

src/race.rs

@@ -6,10 +6,13 @@
 //!
 //! This module does not require `std` feature.
 
-use core::{
-    num::NonZeroUsize,
-    sync::atomic::{AtomicUsize, Ordering},
-};
+#[cfg(feature = "atomic-polyfill")]
+use atomic_polyfill as atomic;
+#[cfg(not(feature = "atomic-polyfill"))]
+use core::sync::atomic;
+
+use atomic::{AtomicUsize, Ordering};
+use core::num::NonZeroUsize;
 
 /// A thread-safe cell which can be written to only once.
 #[derive(Default, Debug)]
@@ -160,21 +163,23 @@ pub use self::once_box::OnceBox;
 
 #[cfg(feature = "alloc")]
 mod once_box {
-    use core::{
-        marker::PhantomData,
-        ptr,
-        sync::atomic::{AtomicPtr, Ordering},
-    };
+    use super::atomic::{AtomicPtr, Ordering};
+    use core::{marker::PhantomData, ptr};
 
     use alloc::boxed::Box;
 
     /// A thread-safe cell which can be written to only once.
-    #[derive(Debug)]
     pub struct OnceBox<T> {
         inner: AtomicPtr<T>,
         ghost: PhantomData<Option<Box<T>>>,
     }
 
+    impl<T> core::fmt::Debug for OnceBox<T> {
+        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+            write!(f, "OnceBox({:?})", self.inner.load(Ordering::Relaxed))
+        }
+    }
+
     impl<T> Default for OnceBox<T> {
         fn default() -> Self {
             Self::new()