The Rust Async Working Group is pleased to announce a major milestone in the use of async fn in traits. Rust 1.75, which will be released next week, will support both the use of -> impl Trait notation and async fn in traits. While there are still some missing features, this progress is significant. It is important to note that the use of -> impl Trait in public traits and APIs is discouraged, as additional bounds cannot be added to the return type. However, a solution using the trait-variant crate allows for the use of async fn in public traits. The group plans to continue improving async Rust and will publish a more detailed roadmap in the future.
https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html