Add test of select's grammar

Author: dtolnay

futures/tests/async_await_macros.rs

@@ -57,6 +57,18 @@ fn select() {
     assert!(ran);
 }
 
+#[test]
+fn select_grammar() {
+    // Parsing after `=>` using Expr::parse would parse `{}() = future::ready(())`
+    // as one expression.
+    block_on(async {
+        select! {
+            () = future::pending::<()>() => {}
+            () = future::ready(()) => {}
+        }
+    });
+}
+
 #[test]
 fn select_biased() {
     let (tx1, rx1) = oneshot::channel::<i32>();