Fix doc-tests on exported macro_rules!.

src/librustdoc/test.rs

@@ -537,4 +537,8 @@ impl<'a, 'hir> intravisit::Visitor<'hir> for HirCollector<'a, 'hir> {
             intravisit::walk_struct_field(this, f);
         });
     }
+
+    fn visit_macro_def(&mut self, macro_def: &'hir hir::MacroDef) {
+        self.visit_testable(macro_def.name.to_string(), &macro_def.attrs, |_| ());
+    }
 }

src/test/rustdoc/issue-38219.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+// compile-flags:--test
+// should-fail
+
+/// ```
+/// fail
+/// ```
+#[macro_export]
+macro_rules! foo { () => {} }