Add more realistic example of async error

src/test/rustdoc-ui/error-in-impl-trait/realistic-async.rs

@@ -0,0 +1,28 @@
+// edition:2018
+// check-pass
+
+mod windows {
+    pub trait WinFoo {
+        fn foo(&self) {}
+    }
+
+    impl WinFoo for () {}
+}
+
+#[cfg(any(windows, doc))]
+use windows::*;
+
+mod unix {
+    pub trait UnixFoo {
+        fn foo(&self) {}
+    }
+
+    impl UnixFoo for () {}
+}
+
+#[cfg(any(unix, doc))]
+use unix::*;
+
+async fn bar() {
+    ().foo()
+}