Rust error E0706: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

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

async fns 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 fns 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.

From rustc --explain E0706 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.