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.