rustdoc: Add a pass to prune undocumented items

src/rustdoc/rustdoc.rs

@@ -77,6 +77,8 @@ fn run(source_file: str) {
     let doc = extract::from_srv(srv, default_name);
     let doc = run_passes(srv, doc, [
         attr_pass::run,
+        // FIXME: This pass should be optional
+        prune_undoc_pass::run,
         tystr_pass::run
     ]);
     gen::write_markdown(doc, std::io::stdout());