Rollup merge of #66526 - estebank:async-fn-trait-information, r=Centril

Add more context to `async fn` trait error

Follow up to #65937. Fix #65899.

2 files changed, 58 insertions, 1 deletions

diff --git a/src/librustc_error_codes/error_codes.rs b/src/librustc_error_codes/error_codes.rs
index e575528a463..b11fe33880c 100644
--- a/src/librustc_error_codes/error_codes.rs
+++ b/src/librustc_error_codes/error_codes.rs
@@ -383,6 +383,7 @@ 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 00000000000..bee9219af7c
--- /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<Output = User> + '_ {
+    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<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.
+
+[`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