sync: implement cancellation token getter for drop guard (#8226)

tokio-util/src/sync/cancellation_token/guard.rs

@@ -10,6 +10,13 @@ pub struct DropGuard {
 }
 
 impl DropGuard {
+    /// Returns a reference to the cancellation token wrapped by this guard.
+    pub fn token(&self) -> &CancellationToken {
+        self.inner
+            .as_ref()
+            .expect("`inner` can only be None in a destructor")
+    }
+
     /// Returns stored cancellation token and removes this drop guard instance
     /// (i.e. it will no longer cancel token). Other guards for this token
     /// are not affected.

tokio-util/src/sync/cancellation_token/guard_ref.rs

@@ -13,6 +13,13 @@ pub struct DropGuardRef<'a> {
 }
 
 impl<'a> DropGuardRef<'a> {
+    /// Returns a reference to the cancellation token wrapped by this guard.
+    pub fn token(&self) -> &CancellationToken {
+        self.inner
+            .as_ref()
+            .expect("`inner` can only be None in a destructor")
+    }
+
     /// Returns stored cancellation token and removes this drop guard instance
     /// (i.e. it will no longer cancel token). Other guards for this token
     /// are not affected.