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diff --git a/third_party/rust/crossbeam-utils-0.6.6/src/sync/wait_group.rs b/third_party/rust/crossbeam-utils-0.6.6/src/sync/wait_group.rs
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+++ b/third_party/rust/crossbeam-utils-0.6.6/src/sync/wait_group.rs
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+use std::fmt;
+use std::sync::{Arc, Condvar, Mutex};
+
+/// Enables threads to synchronize the beginning or end of some computation.
+///
+/// # Wait groups vs barriers
+///
+/// `WaitGroup` is very similar to [`Barrier`], but there are a few differences:
+///
+/// * `Barrier` needs to know the number of threads at construction, while `WaitGroup` is cloned to
+///   register more threads.
+///
+/// * A `Barrier` can be reused even after all threads have synchronized, while a `WaitGroup`
+///   synchronizes threads only once.
+///
+/// * All threads wait for others to reach the `Barrier`. With `WaitGroup`, each thread can choose
+///   to either wait for other threads or to continue without blocking.
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_utils::sync::WaitGroup;
+/// use std::thread;
+///
+/// // Create a new wait group.
+/// let wg = WaitGroup::new();
+///
+/// for _ in 0..4 {
+///     // Create another reference to the wait group.
+///     let wg = wg.clone();
+///
+///     thread::spawn(move || {
+///         // Do some work.
+///
+///         // Drop the reference to the wait group.
+///         drop(wg);
+///     });
+/// }
+///
+/// // Block until all threads have finished their work.
+/// wg.wait();
+/// ```
+///
+/// [`Barrier`]: https://doc.rust-lang.org/std/sync/struct.Barrier.html
+pub struct WaitGroup {
+    inner: Arc<Inner>,
+}
+
+/// Inner state of a `WaitGroup`.
+struct Inner {
+    cvar: Condvar,
+    count: Mutex<usize>,
+}
+
+impl WaitGroup {
+    /// Creates a new wait group and returns the single reference to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_utils::sync::WaitGroup;
+    ///
+    /// let wg = WaitGroup::new();
+    /// ```
+    pub fn new() -> WaitGroup {
+        WaitGroup {
+            inner: Arc::new(Inner {
+                cvar: Condvar::new(),
+                count: Mutex::new(1),
+            }),
+        }
+    }
+
+    /// Drops this reference and waits until all other references are dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_utils::sync::WaitGroup;
+    /// use std::thread;
+    ///
+    /// let wg = WaitGroup::new();
+    ///
+    /// thread::spawn({
+    ///     let wg = wg.clone();
+    ///     move || {
+    ///         // Block until both threads have reached `wait()`.
+    ///         wg.wait();
+    ///     }
+    /// });
+    ///
+    /// // Block until both threads have reached `wait()`.
+    /// wg.wait();
+    /// ```
+    pub fn wait(self) {
+        if *self.inner.count.lock().unwrap() == 1 {
+            return;
+        }
+
+        let inner = self.inner.clone();
+        drop(self);
+
+        let mut count = inner.count.lock().unwrap();
+        while *count > 0 {
+            count = inner.cvar.wait(count).unwrap();
+        }
+    }
+}
+
+impl Drop for WaitGroup {
+    fn drop(&mut self) {
+        let mut count = self.inner.count.lock().unwrap();
+        *count -= 1;
+
+        if *count == 0 {
+            self.inner.cvar.notify_all();
+        }
+    }
+}
+
+impl Clone for WaitGroup {
+    fn clone(&self) -> WaitGroup {
+        let mut count = self.inner.count.lock().unwrap();
+        *count += 1;
+
+        WaitGroup {
+            inner: self.inner.clone(),
+        }
+    }
+}
+
+impl fmt::Debug for WaitGroup {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let count: &usize = &*self.inner.count.lock().unwrap();
+        f.debug_struct("WaitGroup")
+            .field("count", count)
+            .finish()
+    }
+}