Add more context to `async fn` trait error. Suggest `async-trait`.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index edb91d5bf18..b7ec2893bb3 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -174,7 +174,12 @@ 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()
+                             "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.")
+                .emit();
         }
     }