error[E0658]: async closures are unstable --> $DIR/trait-bounds-in-macro.rs:8:28 | LL | async fn foo() -> impl async Fn() { } | ^^^^^ | = note: see issue #62290 for more information = help: add `#![feature(async_closure)]` to the crate attributes to enable = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date = help: to use an async block, remove the `||`: `async {` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.