Use crossbeam_utils::BackOff as spin hint

Author: al8n

Diff for: Cargo.toml

@@ -6,15 +6,15 @@ homepage = "https://github.com/al8n/wg"
 repository = "https://github.com/al8n/wg.git"
 documentation = "https://docs.rs/wg/"
 readme = "README.md"
-version = "0.9.0"
+version = "0.9.1"
 license = "MIT OR Apache-2.0"
 keywords = ["waitgroup", "async", "sync", "notify", "wake"]
 categories = ["asynchronous", "concurrency", "data-structures", "no-std"]
 edition = "2021"
 
 [features]
 default = ["std", "parking_lot", "triomphe"]
-alloc = ["event-listener"]
+alloc = ["crossbeam-utils"]
 std = ["triomphe?/default", "event-listener?/default", "futures-core?/default"]
 triomphe = ["dep:triomphe"]
 parking_lot = ["dep:parking_lot"]
@@ -28,6 +28,8 @@ event-listener = { version = "5", optional = true, default-features = false, fea
 pin-project-lite = { version = "0.2", optional = true }
 futures-core = { version = "0.3", default-features = false, optional = true }
 
+crossbeam-utils = { version = "0.8", optional = true, default-features = false }
+
 [dev-dependencies]
 agnostic-lite = { version = "0.3", features = ["smol", "async-std", "tokio", "time"] }
 tokio = { version = "1", features = ["full"] }

Diff for: src/future.rs

@@ -273,7 +273,6 @@ pin_project_lite::pin_project! {
     /// A future returned by [`AsyncWaitGroup::wait()`].
     #[derive(Debug)]
     #[must_use = "futures do nothing unless you `.await` or poll them"]
-    #[cfg_attr(docsrs, doc(cfg(feature = "tokio")))]
     pub struct WaitGroupFuture<'a> {
         inner: &'a AsyncWaitGroup,
         #[pin]

Diff for: src/no_std.rs

@@ -160,7 +160,7 @@ impl WaitGroup {
     /// ```
     pub fn wait(&self) {
         while self.inner.counter.load(Ordering::SeqCst) != 0 {
-            core::hint::spin_loop();
+            crossbeam_utils::Backoff::new().spin();
         }
     }
 }

Diff for: src/sync.rs

@@ -162,7 +162,6 @@ impl WaitGroup {
     /// [`wait`]: struct.AsyncWaitGroup.html#method.wait
     pub fn add(&self, num: usize) -> Self {
         let mut ctr = self.inner.count.lock_me();
-
         *ctr += num;
         Self {
             inner: self.inner.clone(),