async can begin expressions

src/libsyntax/parse/token.rs

@@ -104,6 +104,7 @@ pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
     !ident_token.is_reserved_ident() ||
     ident_token.is_path_segment_keyword() ||
     [
+        keywords::Async.name(),
         keywords::Do.name(),
         keywords::Box.name(),
         keywords::Break.name(),

src/test/ui/async-matches-expr.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+// edition:2018
+
+#![feature(async_await, await_macro)]
+
+macro_rules! match_expr {
+    ($x:expr) => {}
+}
+
+fn main() {
+    match_expr!(async {});
+    match_expr!(async || {});
+}