Wrap async fn error message

Author: dtolnay

syntax/parse.rs

@@ -565,7 +565,10 @@ fn parse_extern_fn(
     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, and https://github.com/pcwalton/cxx-async for some helpers; eventually what you wrote will work but it isn't integrated into the cxx::bridge macro yet",
+            "async function is not directly supported yet, but see https://cxx.rs/async.html \
+            for a working approach, and https://github.com/pcwalton/cxx-async for some helpers; \
+            eventually what you wrote will work but it isn't integrated into the cxx::bridge \
+            macro yet",
         ));
     }