fix default build

Signed-off-by: Xinye <[email protected]>

Author: Xinye

src/lib.rs

@@ -612,6 +612,7 @@ impl FailPoint {
 
     fn set_actions(&self, actions_str: &str, actions: Vec<Action>) {
         loop {
+            #[cfg(feature = "async")]
             self.async_pause_notify.notify_waiters();
             // TODO: maybe busy waiting here.
             match self.actions.try_write() {
@@ -674,7 +675,10 @@ impl FailPoint {
             Task::Callback(f) => {
                 f.run();
             }
-            Task::CallbackAsync(_) => unreachable!(),
+            #[cfg(feature = "async")]
+            Task::CallbackAsync(_) => panic!(
+                "to use async callback, please enable `async` feature and use `async_fail_point`"
+            ),
         }
         None
     }