sync: add drop guard for cancellation token (#3839)

tokio-util/src/sync/cancellation_token.rs

@@ -1,5 +1,6 @@
 //! An asynchronously awaitable `CancellationToken`.
 //! The token allows to signal a cancellation request to one or more tasks.
+pub(crate) mod guard;
 
 use crate::loom::sync::atomic::AtomicUsize;
 use crate::loom::sync::Mutex;
@@ -11,6 +12,8 @@ use core::ptr::NonNull;
 use core::sync::atomic::Ordering;
 use core::task::{Context, Poll, Waker};
 
+use guard::DropGuard;
+
 /// A token which can be used to signal a cancellation request to one or more
 /// tasks.
 ///
@@ -275,6 +278,14 @@ impl CancellationToken {
         }
     }
 
+    /// Creates a `DropGuard` for this token.
+    ///
+    /// Returned guard will cancel this token (and all its children) on drop
+    /// unless disarmed.
+    pub fn drop_guard(self) -> DropGuard {
+        DropGuard { inner: Some(self) }
+    }
+
     unsafe fn register(
         &self,
         wait_node: &mut ListNode<WaitQueueEntry>,