module Minitest::Assertions

def diff exp, act
  require "tempfile"
  expect = mu_pp_for_diff exp
  butwas = mu_pp_for_diff act
  result = nil
  need_to_diff =
    Minitest::Assertions.diff &&
    (expect.include?("\n")    ||
     butwas.include?("\n")    ||
     expect.size > 30         ||
     butwas.size > 30         ||
     expect == butwas)
  return "Expected: #{mu_pp exp}\n  Actual: #{mu_pp act}" unless
    need_to_diff
  Tempfile.open("expect") do |a|
    a.puts expect
    a.flush
    Tempfile.open("butwas") do |b|
      b.puts butwas
      b.flush
      result = `#{Minitest::Assertions.diff} #{a.path} #{b.path}`
      result.sub!(/^\-\-\- .+/, "--- expected")
      result.sub!(/^\+\+\+ .+/, "+++ actual")
      if result.empty? then
        klass = exp.class
        result = [
                  "No visible difference in the #{klass}#inspect output.\n",
                  "You should look at the implementation of #== on ",
                  "#{klass} or its members.\n",
                  expect,
                 ].join
      end
    end
  end
  result
end