debug

{ plan: "pro", tags: ["ruby", "rails"] }

"<b>Fish</b> & Chips"

Article.new(title: "Hello")

nil

Struct.new(:plan).new("pro")

Converted to YAML with to_yaml, escaped, and used as the content. Before that, it tries Marshal.dump; an object that cannot be dumped (an instance of an anonymous class, a proc, an object with singleton methods, etc.) is not converted to YAML, and a <code class="debug_dump"> with its inspect string as the content is returned instead.