add regression test for issue-64391

Author: nikomatsakis

src/test/ui/async-await/issue-64391.rs

@@ -0,0 +1,14 @@
+// Regression test for Issue #64391. The goal here is that this
+// function compiles. In the past, due to incorrect drop order for
+// temporaries in the tail expression, we failed to compile this
+// example. The drop order itself is directly tested in
+// `drop-order/drop-order-for-temporary-in-tail-return-expr.rs`.
+//
+// check-pass
+// edition:2018
+
+async fn add(x: u32, y: u32) -> u32 {
+    async { x + y }.await
+}
+
+fn main() { }