Illustration of Rubinius tagged nil that enables tracing where a nil value originated

result.txt

rbx tagged_nil.rb
                                    main # Rubinius::Loader at core/loader.rb:852 (+90)
                                  script # Rubinius::Loader at core/loader.rb:667 (+343)
                             load_script # Rubinius::CodeLoader at
        core/code_loader.rb:280 (+60)
                                    load # Rubinius::CodeLoader::Source(Rubinius::CodeLoader::Script) at
        core/code_loader.rb:119 (+81)
                              run_script # Rubinius::CodeLoader::Source(Rubinius::CodeLoader::Script) at
        core/code_loader.rb:132 (+21)
                              __script__ # Object at tagged_nil.rb:50 (+153)
                                       m # B at tagged_nil.rb:40 (+31)
                                       m # C at tagged_nil.rb:46 (+3)
   catch_me_if_you_can! (method_missing) # NilClass at tagged_nil.rb:34 (+361)

undefined method catch_me_if_you_can! called on an instance of NilClass originating in 'who_me?' at (dynamic):1 (NoMethodError)

An exception occurred running tagged_nil.rb

tagged_nil.rb

class A
  dynamic_method :who_me? do |g|
    r0 = g.new_register

    g.noop
    g.noop

    g.r_load_nil r0, 0
    g.r_store_stack r0
    g.ret
  end
end

class NilClass
  def method_missing(*_, **)
    sym,  = _

    m = Rubinius::Mirror::Object.reflect self
    code_id = m.nil_code_id.to_s 16
    ip = m.nil_ip

    cc = nil

    ObjectSpace.each_object(Rubinius::CompiledCode) do |c|
      cc = c if c.code_id.start_with? code_id
    end

    if cc
      msg = "undefined method #{sym} called on an instance of NilClass originating in '#{cc.name}' at #{cc.file}:#{cc.line_from_ip(ip)}"
    else
      msg = "undefined method #{sym} called on an instance of NilClass"
    end

    raise NoMethodError, msg
  end
end

class B
  def m(a)
    C.new.m a
  end
end

class C
  def m(a)
    a.catch_me_if_you_can!
  end
end

B.new.m A.new.who_me?