Add test for rust-lang#59972

Author: tmandry

src/test/run-pass/generator/issue-59972.rs

@@ -0,0 +1,23 @@
+// compile-flags: --edition=2018
+
+#![feature(async_await, await_macro)]
+
+pub enum Uninhabited { }
+
+fn uninhabited_async() -> Uninhabited {
+    unreachable!()
+}
+
+async fn noop() { }
+
+#[allow(unused)]
+async fn contains_never() {
+    let error = uninhabited_async();
+    await!(noop());
+    let error2 = error;
+}
+
+#[allow(unused_must_use)]
+fn main() {
+    contains_never();
+}