1 `async fn`s are not yet supported in traits in Rust.
3 Erroneous code example:
5 ```compile_fail,edition2018
7 // Neither case is currently supported.
13 `async fn`s return an `impl Future`, making the following two examples
16 ```edition2018,ignore (example-of-desugaring-equivalence)
17 async fn foo() -> User {
20 // The async fn above gets desugared as follows:
21 fn foo(&self) -> impl Future<Output = User> + '_ {
26 But when it comes to supporting this in traits, there are [a few implementation
27 issues][async-is-hard]. One of them is returning `impl Trait` in traits is not
28 supported, as it would require [Generic Associated Types] to be supported:
30 ```edition2018,ignore (example-of-desugaring-equivalence)
32 async fn get_user(&self) -> User {
38 fn get_user(&self) -> impl Future<Output = User> + '_ {
44 Until these issues are resolved, you can use the [`async-trait` crate], allowing
45 you to use `async fn` in traits by desugaring to "boxed futures"
46 (`Pin<Box<dyn Future + Send + 'async>>`).
48 Note that using these trait methods will result in a heap allocation
49 per-function-call. This is not a significant cost for the vast majority of
50 applications, but should be considered when deciding whether to use this
51 functionality in the public API of a low-level function that is expected to be
52 called millions of times a second.
54 You might be interested in visiting the [async book] for further information.
56 [`async-trait` crate]: https://crates.io/crates/async-trait
57 [async-is-hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
58 [Generic Associated Types]: https://github.com/rust-lang/rust/issues/44265
59 [async book]: https://rust-lang.github.io/async-book/07_workarounds/06_async_in_traits.html