macros: fix unused_braces in generated code (#3404)

tokio-macros/src/entry.rs

@@ -254,7 +254,7 @@ fn parse_knobs(
                 .enable_all()
                 .build()
                 .unwrap()
-                .block_on(async { #body })
+                .block_on(async #body)
         }
     };
 

tokio/tests/macros_test.rs

@@ -17,3 +17,11 @@ async fn test_macro_is_resilient_to_shadowing() {
     .await
     .unwrap();
 }
+
+// https://github.com/tokio-rs/tokio/issues/3403
+#[rustfmt::skip] // this `rustfmt::skip` is necessary because unused_braces does not warn if the block contains newline.
+#[tokio::main]
+async fn unused_braces_main() { println!("hello") }
+#[rustfmt::skip] // this `rustfmt::skip` is necessary because unused_braces does not warn if the block contains newline.
+#[tokio::test]
+async fn unused_braces_test() { assert_eq!(1 + 1, 2) }