Add ui test testing the recommendation to use async-trait

Author: dtolnay

tests/ui/no-attribute-macro.rs

@@ -0,0 +1,11 @@
+pub trait Trait {
+    async fn method(&self);
+}
+
+pub struct Struct;
+
+impl Trait for Struct {
+    async fn method(&self) {}
+}
+
+fn main() {}

tests/ui/no-attribute-macro.stderr

@@ -0,0 +1,25 @@
+error[E0706]: functions in traits cannot be declared `async`
+ --> tests/ui/no-attribute-macro.rs:2:5
+  |
+2 |     async fn method(&self);
+  |     -----^^^^^^^^^^^^^^^^^^
+  |     |
+  |     `async` because of this
+  |
+  = note: `async` trait functions are not currently supported
+  = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+  = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+  = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
+
+error[E0706]: functions in traits cannot be declared `async`
+ --> tests/ui/no-attribute-macro.rs:8:5
+  |
+8 |     async fn method(&self) {}
+  |     -----^^^^^^^^^^^^^^^^^
+  |     |
+  |     `async` because of this
+  |
+  = note: `async` trait functions are not currently supported
+  = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+  = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+  = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable