sync: add CancellationToken (#2263)

As a first step towards structured concurrency, this change adds a
CancellationToken for graceful cancellation of tasks.

The task can be awaited by an arbitrary amount of tasks due to the usage
of an intrusive list.

The token can be cloned. In addition to this child tokens can be derived.
When the parent token gets cancelled, all child tokens will also get
cancelled.

1 files changed, 2 insertions, 0 deletions

diff --git a/tokio/src/sync/tests/mod.rs b/tokio/src/sync/tests/mod.rs
index d571754c..6ba8c1f9 100644
--- a/tokio/src/sync/tests/mod.rs
+++ b/tokio/src/sync/tests/mod.rs
@@ -7,6 +7,8 @@ cfg_not_loom! {
 cfg_loom! {
     mod loom_atomic_waker;
     mod loom_broadcast;
+    #[cfg(tokio_unstable)]
+    mod loom_cancellation_token;
     mod loom_list;
     mod loom_mpsc;
     mod loom_notify;