LibMarkdown: Implement introspection of the document tree

Author: https://github.com/BenWiederhake
Commit: 24e7196158
Pull-request: https://github.com/SerenityOS/serenity/pull/10420
Reviewed-by: https://github.com/petelliott ✅

Userland/Libraries/LibMarkdown/HorizontalRule.cpp

@@ -7,6 +7,7 @@
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <LibMarkdown/HorizontalRule.h>
+#include <LibMarkdown/Visitor.h>
 
 namespace Markdown {
 
@@ -24,6 +25,15 @@ String HorizontalRule::render_for_terminal(size_t view_width) const
     return builder.to_string();
 }
 
+RecursionDecision HorizontalRule::walk(Visitor& visitor) const
+{
+    RecursionDecision rd = visitor.visit(*this);
+    if (rd != RecursionDecision::Recurse)
+        return rd;
+    // Normalize return value.
+    return RecursionDecision::Continue;
+}
+
 OwnPtr<HorizontalRule> HorizontalRule::parse(LineIterator& lines)
 {
     if (lines.is_end())