Add test for issue-54239

Author: JohnTitor

src/test/ui/async-await/issue-54239-private-type-triggers-lint.rs

@@ -0,0 +1,17 @@
+// Regression test for #54239, shouldn't trigger lint.
+// check-pass
+// edition:2018
+
+#![deny(missing_debug_implementations)]
+
+struct DontLookAtMe(i32);
+
+async fn secret() -> DontLookAtMe {
+    DontLookAtMe(41)
+}
+
+pub async fn looking() -> i32 { // Shouldn't trigger lint here.
+    secret().await.0
+}
+
+fn main() {}