Enable an option for portable-atomic (#2670)

futures-core/Cargo.toml

@@ -16,6 +16,7 @@ std = ["alloc"]
 alloc = []
 
 [dependencies]
+portable-atomic = { version = "0.3.15", default-features = false, optional = true }
 
 [dev-dependencies]
 futures = { path = "../futures" }

futures-core/src/task/__internal/atomic_waker.rs

@@ -1,9 +1,16 @@
 use core::cell::UnsafeCell;
 use core::fmt;
-use core::sync::atomic::AtomicUsize;
-use core::sync::atomic::Ordering::{AcqRel, Acquire, Release};
 use core::task::Waker;
 
+use atomic::AtomicUsize;
+use atomic::Ordering::{AcqRel, Acquire, Release};
+
+#[cfg(feature = "portable-atomic")]
+use portable_atomic as atomic;
+
+#[cfg(not(feature = "portable-atomic"))]
+use core::sync::atomic;
+
 /// A synchronization primitive for task wakeup.
 ///
 /// Sometimes the task interested in a given event will change over time.

futures-util/Cargo.toml

@@ -21,6 +21,7 @@ io-compat = ["io", "compat", "tokio-io"]
 sink = ["futures-sink"]
 io = ["std", "futures-io", "memchr"]
 channel = ["std", "futures-channel"]
+portable-atomic = ["futures-core/portable-atomic"]
 
 # Unstable features
 # These features are outside of the normal semver guarantees and require the