diff --git a/src/librustc_error_codes/error_codes.rs b/src/librustc_error_codes/error_codes.rs index 428cecf13a3b0..325b79ba25f68 100644 --- a/src/librustc_error_codes/error_codes.rs +++ b/src/librustc_error_codes/error_codes.rs @@ -381,6 +381,7 @@ E0700: include_str!("./error_codes/E0700.md"), E0701: include_str!("./error_codes/E0701.md"), E0704: include_str!("./error_codes/E0704.md"), E0705: include_str!("./error_codes/E0705.md"), +E0706: include_str!("./error_codes/E0706.md"), E0712: include_str!("./error_codes/E0712.md"), E0713: include_str!("./error_codes/E0713.md"), E0714: include_str!("./error_codes/E0714.md"), @@ -595,7 +596,6 @@ E0744: include_str!("./error_codes/E0744.md"), E0696, // `continue` pointing to a labeled block // E0702, // replaced with a generic attribute input check E0703, // invalid ABI - E0706, // `async fn` in trait // E0707, // multiple elided lifetimes used in arguments of `async fn` E0708, // `async` non-`move` closures with parameters are not currently // supported diff --git a/src/librustc_error_codes/error_codes/E0706.md b/src/librustc_error_codes/error_codes/E0706.md new file mode 100644 index 0000000000000..bee9219af7cf6 --- /dev/null +++ b/src/librustc_error_codes/error_codes/E0706.md @@ -0,0 +1,57 @@ + `async fn`s are not yet supported in traits in Rust. + +Erroneous code example: + +```compile_fail,edition2018 +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: + +```edition2018,ignore (example-of-desugaring-equivalence) +async fn foo() -> User { + unimplemented!() +} +// The async fn above gets desugared as follows: +fn foo(&self) -> impl Future + '_ { + unimplemented!() +} +``` + +But when it comes to supporting this in traits, there are [a few implementation +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: + +```edition2018,ignore (example-of-desugaring-equivalence) +impl MyDatabase { + async fn get_user(&self) -> User { + unimplemented!() + } +} + +impl MyDatabase { + fn get_user(&self) -> impl Future + '_ { + 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>`). + +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. + +[`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/ +[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 diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs index edb91d5bf18ed..c4032ec5dd0db 100644 --- a/src/librustc_passes/ast_validation.rs +++ b/src/librustc_passes/ast_validation.rs @@ -173,8 +173,11 @@ impl<'a> AstValidator<'a> { fn check_trait_fn_not_async(&self, span: Span, asyncness: IsAsync) { if asyncness.is_async() { - struct_span_err!(self.session, span, E0706, - "trait fns cannot be declared `async`").emit() + struct_span_err!(self.session, span, E0706, "trait fns cannot be declared `async`") + .note("`async` trait functions are not currently supported") + .note("consider using the `async-trait` crate: \ + https://crates.io/crates/async-trait") + .emit(); } } diff --git a/src/test/ui/async-await/async-trait-fn.rs b/src/test/ui/async-await/async-trait-fn.rs new file mode 100644 index 0000000000000..786100e916da0 --- /dev/null +++ b/src/test/ui/async-await/async-trait-fn.rs @@ -0,0 +1,7 @@ +// edition:2018 +trait T { + async fn foo() {} //~ ERROR trait fns cannot be declared `async` + async fn bar(&self) {} //~ ERROR trait fns cannot be declared `async` +} + +fn main() {} diff --git a/src/test/ui/async-await/async-trait-fn.stderr b/src/test/ui/async-await/async-trait-fn.stderr new file mode 100644 index 0000000000000..9acfa2cc06912 --- /dev/null +++ b/src/test/ui/async-await/async-trait-fn.stderr @@ -0,0 +1,21 @@ +error[E0706]: trait fns cannot be declared `async` + --> $DIR/async-trait-fn.rs:3:5 + | +LL | async fn foo() {} + | ^^^^^^^^^^^^^^^^^ + | + = note: `async` trait functions are not currently supported + = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait + +error[E0706]: trait fns cannot be declared `async` + --> $DIR/async-trait-fn.rs:4:5 + | +LL | async fn bar(&self) {} + | ^^^^^^^^^^^^^^^^^^^^^^ + | + = note: `async` trait functions are not currently supported + = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait + +error: aborting due to 2 previous errors + +For more information about this error, try `rustc --explain E0706`. diff --git a/src/test/ui/async-await/edition-deny-async-fns-2015.stderr b/src/test/ui/async-await/edition-deny-async-fns-2015.stderr index 7633825eb32ab..bb09ee9a93296 100644 --- a/src/test/ui/async-await/edition-deny-async-fns-2015.stderr +++ b/src/test/ui/async-await/edition-deny-async-fns-2015.stderr @@ -57,7 +57,11 @@ error[E0706]: trait fns cannot be declared `async` | LL | async fn foo() {} | ^^^^^^^^^^^^^^^^^ + | + = note: `async` trait functions are not currently supported + = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait error: aborting due to 10 previous errors -For more information about this error, try `rustc --explain E0670`. +Some errors have detailed explanations: E0670, E0706. +For more information about an error, try `rustc --explain E0670`.