macros: add #[allow(unused_mut)] to select! (#2858)

tokio/src/macros/select.rs

@@ -404,6 +404,7 @@ macro_rules! select {
                                 // The future returned a value, check if matches
                                 // the specified pattern.
                                 #[allow(unused_variables)]
+                                #[allow(unused_mut)]
                                 match &out {
                                     $bind => {}
                                     _ => continue,

tokio/tests/macros_select.rs

@@ -462,3 +462,20 @@ async fn async_never() -> ! {
         tokio::time::delay_for(Duration::from_millis(10)).await;
     }
 }
+
+// From https://github.com/tokio-rs/tokio/issues/2857
+#[tokio::test]
+async fn mut_on_left_hand_side() {
+    let v = async move {
+        let ok = async { 1 };
+        tokio::pin!(ok);
+        tokio::select! {
+            mut a = &mut ok => {
+                a += 1;
+                a
+            }
+        }
+    }
+    .await;
+    assert_eq!(v, 2);
+}