Reword help and add test

Author: estebank

src/librustc_passes/ast_validation.rs

@@ -173,12 +173,10 @@ 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`")
-                .note("Due to technical restrictions rust does not currently support `async` \
-                       trait fns.")
-                .note("Consider using the `async-trait` crate in the meantime until further \
-                       notice.")
+            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();
         }
     }

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() {}

src/test/ui/async-await/async-trait-fn.stderr

@@ -0,0 +1,20 @@
+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
+

src/test/ui/async-await/edition-deny-async-fns-2015.stderr

@@ -57,6 +57,9 @@ 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