From 8de461f44aed5df8a5ff04d446a135ad38c29bda Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Thu, 19 Nov 2020 13:46:14 -0800
Subject: [PATCH] Improve async fn error message with link to workaround

---
 syntax/parse.rs          |  6 ++++++
 tests/ui/async_fn.stderr | 23 +++--------------------
 2 files changed, 9 insertions(+), 20 deletions(-)

diff --git a/syntax/parse.rs b/syntax/parse.rs
index ec89cbb7..78e6a3f2 100644
--- a/syntax/parse.rs
+++ b/syntax/parse.rs
@@ -373,6 +373,12 @@ fn parse_extern_fn(
             "variadic function is not supported yet",
         ));
     }
+    if foreign_fn.sig.asyncness.is_some() {
+        return Err(Error::new_spanned(
+            foreign_fn,
+            "async function is not directly supported yet, but see https://cxx.rs/async.html for a working approach",
+        ));
+    }
 
     let mut doc = Doc::new();
     let mut cxx_name = None;
diff --git a/tests/ui/async_fn.stderr b/tests/ui/async_fn.stderr
index 63e3666a..dedec7bb 100644
--- a/tests/ui/async_fn.stderr
+++ b/tests/ui/async_fn.stderr
@@ -1,22 +1,5 @@
-error[E0308]: mismatched types
- --> $DIR/async_fn.rs:4:18
+error: async function is not directly supported yet, but see https://cxx.rs/async.html for a working approach
+ --> $DIR/async_fn.rs:4:9
   |
 4 |         async fn f();
-  |                  ^
-  |                  |
-  |                  expected `()`, found opaque type
-  |                  possibly return type missing here?
-...
-8 | async fn f() {}
-  |              - the `Output` of this `async fn`'s found opaque type
-  |
-  = note: expected unit type `()`
-           found opaque type `impl Future`
-help: consider `await`ing on the `Future`
-  |
-4 |         async fn f.await();
-  |                   ^^^^^^
-help: try adding a semicolon
-  |
-4 |         async fn f;();
-  |                   ^
+  |         ^^^^^^^^^^^^^