From e565329144fec8ab2aa87afa170c41dc17ad018e Mon Sep 17 00:00:00 2001 From: Esteban Küber Date: Mon, 18 Nov 2019 12:34:22 -0800 Subject: Add error code documentation --- src/librustc_error_codes/error_codes/E0706.md | 78 +++++++++++++++++++++++++++ 1 file changed, 78 insertions(+) create mode 100644 src/librustc_error_codes/error_codes/E0706.md (limited to 'src/librustc_error_codes/error_codes') 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 00000000000..1eb1271a9e8 --- /dev/null +++ b/src/librustc_error_codes/error_codes/E0706.md @@ -0,0 +1,78 @@ + `async fn`s are not yet supported 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 normally 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 which is that 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], which allows you to use +this feature by sidesteping the language feature issue by desugaring to "boxed futures" +(`Pin>`): + +```edition2018,ignore (example-of-desugaring-equivalence) +#[async_trait] +impl MyDatabase { + async fn get_user(&self) -> User { + unimplemented!() + } +} + +// The annotated impl above gets desugared as follows: +impl MyDatabase { + fn get_user<'async>( + &'async self, + ) -> Pin + Send + 'async>> + where + Self: Sync + 'async, + { + unimplemented!() + } +} +``` + +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 \ No newline at end of file -- cgit 1.4.1-3-g733a5 From 846f5e6bb97c269da4feb171b1837c43de66a38b Mon Sep 17 00:00:00 2001 From: Dylan DPC Date: Tue, 19 Nov 2019 17:09:39 +0100 Subject: Update E0706.md --- src/librustc_error_codes/error_codes/E0706.md | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) (limited to 'src/librustc_error_codes/error_codes') diff --git a/src/librustc_error_codes/error_codes/E0706.md b/src/librustc_error_codes/error_codes/E0706.md index 1eb1271a9e8..909b08fb174 100644 --- a/src/librustc_error_codes/error_codes/E0706.md +++ b/src/librustc_error_codes/error_codes/E0706.md @@ -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: @@ -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) async fn foo() -> User { @@ -23,7 +23,7 @@ fn foo(&self) -> impl Future + '_ { ``` 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: ```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 -this feature by sidesteping the language feature issue by desugaring to "boxed futures" +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>`): ```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-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 \ No newline at end of file +[async book]: https://rust-lang.github.io/async-book/07_workarounds/06_async_in_traits.html -- cgit 1.4.1-3-g733a5 From a079159c64bfd7b379a9c574d21dbcedb78c5b0c Mon Sep 17 00:00:00 2001 From: Esteban Kuber Date: Tue, 19 Nov 2019 11:29:20 -0800 Subject: Remove desugared `async-trait` example --- src/librustc_error_codes/error_codes/E0706.md | 23 +---------------------- 1 file changed, 1 insertion(+), 22 deletions(-) (limited to 'src/librustc_error_codes/error_codes') diff --git a/src/librustc_error_codes/error_codes/E0706.md b/src/librustc_error_codes/error_codes/E0706.md index 909b08fb174..bee9219af7c 100644 --- a/src/librustc_error_codes/error_codes/E0706.md +++ b/src/librustc_error_codes/error_codes/E0706.md @@ -42,28 +42,7 @@ impl MyDatabase { 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>`): - -```edition2018,ignore (example-of-desugaring-equivalence) -#[async_trait] -impl MyDatabase { - async fn get_user(&self) -> User { - unimplemented!() - } -} - -// The annotated impl above gets desugared as follows: -impl MyDatabase { - fn get_user<'async>( - &'async self, - ) -> Pin + Send + 'async>> - where - Self: Sync + 'async, - { - unimplemented!() - } -} -``` +(`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 -- cgit 1.4.1-3-g733a5