asynchronousrusttraitssendrust-tokio

How do I make a async function in trait return a future that is Send?

I have a trait defined like this:

trait MyTrait {
    async fn my_async_fn(arg: SendType) -> Result<Self, Box<dyn Error>>;
}

How do make the future returned by my_async_fn be Send for everything that Implements the trait?

Solution

  • If you will make the trait public the compiler will help you:

    warning: use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
 --> src\main.rs:6:5
  |
6 |     async fn my_async_fn(arg: SendType) -> Result<Self, Box<dyn Error>>;
  |     ^^^^^
  |
  = note: you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
  = note: `#[warn(async_fn_in_trait)]` on by default
help: you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`, but these cannot be relaxed without a breaking API change
  |
6 -     async fn my_async_fn(arg: SendType) -> Result<Self, Box<dyn Error>>;
6 +     fn my_async_fn(arg: SendType) -> impl std::future::Future<Output = Result<Self, Box<dyn Error>>> + Send;

    So:

    fn my_async_fn(
    arg: SendType,
) -> impl std::future::Future<Output = Result<Self, Box<dyn Error>>> + Send;