fix

src/sync/mod.rs

@@ -192,5 +192,8 @@ cfg_unstable! {
 
 pub(crate) mod waker_set;
 pub(crate) use waker_set::WakerSet;
-pub(crate) mod spin_lock;
-pub(crate) use spin_lock::Spinlock;
+
+cfg_default! {
+    pub(crate) mod spin_lock;
+    pub(crate) use spin_lock::Spinlock;
+}