Add bounds for WorkerLocal's Send and Sync

Author: ammaraskar

rayon-core/src/worker_local.rs

@@ -15,8 +15,10 @@ pub struct WorkerLocal<T> {
     registry: Arc<Registry>,
 }
 
-unsafe impl<T> Send for WorkerLocal<T> {}
-unsafe impl<T> Sync for WorkerLocal<T> {}
+/// We prevent concurrent access to the underlying value in the
+/// Deref impl, thus any values safe to send across threads can
+/// be used with WorkerLocal.
+unsafe impl<T: Send> Sync for WorkerLocal<T> {}
 
 impl<T> WorkerLocal<T> {
     /// Creates a new worker local where the `initial` closure computes the