rustdoc: Add a test for fixed issue #16019

Closes #16019

src/test/rustdoc/issue-16019.rs

@@ -0,0 +1,19 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+macro_rules! define_struct {
+    ($rounds:expr) => (
+        struct Struct {
+            sk: [u32; $rounds + 1]
+        }
+        )
+}
+
+define_struct!(2);