E0706 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0706

#### Note: this error code is no longer emitted by the compiler.

`async fn`s are not yet supported in traits in Rust.

Erroneous code example:

    trait T {
        // Neither case is currently supported.
        async fn foo() {}
        async fn bar(&self) {}
    }

`async fn`s return an `impl Future`, making the following two examples equivalent:

    async fn foo() -> User {
        unimplemented!()
    }
    // The async fn above gets desugared as follows:
    fn foo(&self) -> impl Future<Output = User> + '_ {
        unimplemented!()
    }

But when it comes to supporting this in traits, there are a few implementation issues. One of them is returning `impl Trait` in traits is not supported, as it would require Generic Associated Types to be supported:

    impl MyDatabase {
        async fn get_user(&self) -> User {
            unimplemented!()
        }
    }

    impl MyDatabase {
        fn get_user(&self) -> impl Future<Output = User> + '_ {
            unimplemented!()
        }
    }

Until these issues are resolved, you can use the `async-trait` crate, allowing you to use `async fn` in traits by desugaring to "boxed futures" (`Pin<Box<dyn Future + Send + 'async>>`).

Note that using these trait methods will result in a heap allocation per-function-call. This is not a significant cost for the vast majority of applications, but should be considered when deciding whether to use this functionality in the public API of a low-level function that is expected to be called millions of times a second.

You might be interested in visiting the async book for further information.