const_added.rb

constants    = []
current_expr = nil

set_trace_func lambda { |event, file, line, id, binding, klass|
  if current_expr
    e, b = current_expr

    if b.eval("defined? #{e}") == 'constant'
      const = b.eval(e)
      constants << const unless constants.include?(const)
    end

    current_expr = nil
  end

  case event
  when 'class'
    const = binding.eval('self')
    constants << const unless constants.include?(const)
  when 'c-return'
    if (klass == Module || klass == Class) && id == :new
      expr = File.open(file, 'r') { |f| f.each_line.take(line) }.last.strip

      begin
        if binding.eval("defined? #{expr}") == 'assignment'
          left_expr = expr.split('=', 2)[0].strip
          current_expr = [left_expr, binding]
        end
      rescue SyntaxError
      end
    elsif klass == Module && id == :const_set
      expr = File.open(file, 'r') { |f| f.each_line.take(line) }.last.strip
      left_expr, right_expr = expr.split('const_set', 2).map(&:strip)
      left_expr.chomp!('.')
      left_expr = 'self' if left_expr.empty?
      _, right_expr = /:([^,]+),/.match(right_expr).to_a
      current_expr = ["#{left_expr}::#{right_expr}", binding]
    end
  end
}

class Neko
end

class Neko
  module Mike
  end

  const_set :Tama, Module.new
end

Neko.const_set(:Poyo, Class.new)

Inu   = Class.new
Usagi = Module.new # これ以降に何かしらの呼び出しがないとset_trace_funcが呼ばれないので、Usagiだけ取れない、とかもアリ

a = Class.new

p constants