Apply cargo hint

src/cache.rs

@@ -214,7 +214,7 @@ impl SyncCacheCommandChannel {
 }
 
 impl SyncCacheCommandSender {
-    pub fn is_channel_open(&self) -> bool {
+    pub fn _is_channel_open(&self) -> bool {
         Rc::strong_count(&self.channel) > 1
     }