add test for rust-lang#96084

Author: b-naber

tests/ui/async-await/issue-96084.rs

@@ -0,0 +1,33 @@
+// run-pass
+// edition:2018
+
+use std::mem;
+
+async fn foo() {
+    let x = [0u8; 100];
+    async {}.await;
+    println!("{}", x.len());
+}
+
+async fn a() {
+    let fut = foo();
+    let fut = fut;
+    fut.await;
+}
+
+async fn b() {
+    let fut = foo();
+    println!("{}", mem::size_of_val(&fut));
+    let fut = fut;
+    fut.await;
+}
+
+fn main() {
+    assert_eq!(mem::size_of_val(&foo()), 102);
+
+    // 1 + sizeof(foo)
+    assert_eq!(mem::size_of_val(&a()), 103);
+
+    // 1 + (sizeof(foo) * 2)
+    assert_eq!(mem::size_of_val(&b()), 103);
+}