Allow usage on no_std targets and targets using portable-atomic

Cargo.toml

@@ -19,5 +19,9 @@ keywords = ["waker", "notify", "wake", "futures", "async"]
 categories = ["asynchronous", "concurrency"]
 exclude = ["/.*"]
 
+[dependencies]
+# Uses portable-atomic polyfill atomics on targets without them 
+portable-atomic = { version = "0.3", optional = true }
+
 [dev-dependencies]
 futures = "0.3.5"

src/lib.rs

@@ -1,10 +1,26 @@
 //! `futures::task::AtomicWaker` extracted into its own crate.
+//!
+//! # Features
+//!
+//! This crate adds a feature, `atomic-polyfill`, which uses a polyfill
+//! from the [`atomic-polyfill`] crate in order to provide functionality
+//! to targets without atomics. See the [`README`] for the [`atomic-polyfill`]
+//! crate for more information on how this is implemented by the end-user.
+//!
+//! [`atomic-polyfill`]: https://crates.io/crates/atomic-polyfill
+//! [`README`]: https://github.com/embassy-rs/atomic-polyfill/blob/main/README.md
 
-use std::cell::UnsafeCell;
-use std::fmt;
-use std::sync::atomic::AtomicUsize;
-use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
-use std::task::Waker;
+#![no_std]
+
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::sync::atomic::Ordering::{AcqRel, Acquire, Release};
+use core::task::Waker;
+
+#[cfg(not(feature = "portable-atomic"))]
+use core::sync::atomic::AtomicUsize;
+#[cfg(feature = "portable-atomic")]
+use portable_atomic::AtomicUsize;
 
 /// A synchronization primitive for task wakeup.
 ///