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@ -1,4 +1,4 @@
`async fn`s are not yet supported in Rust. `async fn`s are not yet supported in traits in Rust.
Erroneous code example: Erroneous code example:
@ -10,7 +10,7 @@ trait T {
} }
``` ```
`async fn`s normally return an `impl Future`, making the following two examples equivalent: `async fn`s return an `impl Future`, making the following two examples equivalent:
```edition2018,ignore (example-of-desugaring-equivalence) ```edition2018,ignore (example-of-desugaring-equivalence)
async fn foo() -> User { async fn foo() -> User {
@ -23,7 +23,7 @@ fn foo(&self) -> impl Future<Output = User> + '_ {
``` ```
But when it comes to supporting this in traits, there are [a few implementation But when it comes to supporting this in traits, there are [a few implementation
issues][async-is-hard], one of which is that returning `impl Trait` in traits is not supported, issues][async-is-hard]. One of them is returning `impl Trait` in traits is not supported,
as it would require [Generic Associated Types] to be supported: as it would require [Generic Associated Types] to be supported:
```edition2018,ignore (example-of-desugaring-equivalence) ```edition2018,ignore (example-of-desugaring-equivalence)
@ -40,8 +40,8 @@ impl MyDatabase {
} }
``` ```
Until these issues are resolved, you can use the [`async-trait` crate], which allows you to use Until these issues are resolved, you can use the [`async-trait` crate], allowing you to use
this feature by sidesteping the language feature issue by desugaring to "boxed futures" `async fn` in traits by desugaring to "boxed futures"
(`Pin<Box<dyn Future + Send + 'async>>`): (`Pin<Box<dyn Future + Send + 'async>>`):
```edition2018,ignore (example-of-desugaring-equivalence) ```edition2018,ignore (example-of-desugaring-equivalence)
@ -75,4 +75,4 @@ You might be interested in visiting the [async book] for further information.
[`async-trait` crate]: https://crates.io/crates/async-trait [`async-trait` crate]: https://crates.io/crates/async-trait
[async-is-hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/ [async-is-hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
[Generic Associated Types]: https://github.com/rust-lang/rust/issues/44265 [Generic Associated Types]: https://github.com/rust-lang/rust/issues/44265
[async book]: https://rust-lang.github.io/async-book/07_workarounds/06_async_in_traits.html [async book]: https://rust-lang.github.io/async-book/07_workarounds/06_async_in_traits.html