What's the fastest way to convert from a subclass of String to a String?

which_string_coercion_is_faster.rb

#!/usr/bin/env ruby
require 'benchmark'

TIMES_TO_REPEAT = 10_000_000

class SpecialString < String; end

Benchmark.bmbm do|b|
  b.report("''.to_s") do
    str = ''
    TIMES_TO_REPEAT.times { str.to_s }
  end
  
  b.report('"".to_s') do
    str = ""
    TIMES_TO_REPEAT.times { str.to_s }
  end
  
  b.report("String(str)") do
    str = SpecialString.new("")
    TIMES_TO_REPEAT.times { String(str) }
  end

  b.report("str.to_s") do
    str = SpecialString.new("")
    TIMES_TO_REPEAT.times { str.to_s }
  end
end