install_default_bundler_gem

# File lib/rubygems/commands/setup_command.rb, line 350
  def install_default_bundler_gem
    return unless Gem::USE_BUNDLER_FOR_GEMDEPS

    specs_dir = Gem::Specification.default_specifications_dir
    specs_dir = File.join(options[:destdir], specs_dir) unless Gem.win_platform?
    mkdir_p specs_dir

    # Workaround for non-git environment.
    gemspec = File.open('bundler/bundler.gemspec', 'rb'){|f| f.read.gsub(/`git ls-files -z`/, "''") }
    File.open('bundler/bundler.gemspec', 'w'){|f| f.write gemspec }

    bundler_spec = Gem::Specification.load("bundler/bundler.gemspec")
    bundler_spec.files = Dir.chdir("bundler") { Dir["{*.md,{lib,exe,man}/**/*}"] }
    bundler_spec.executables -= ]bundler bundle_ruby]

    # Remove bundler-*.gemspec in default specification directory.
    Dir.entries(specs_dir).
      select {|gs| gs.start_with?("bundler-") }.
      each {|gs| File.delete(File.join(specs_dir, gs)) }

    default_spec_path = File.join(specs_dir, "#{bundler_spec.full_name}.gemspec")
    Gem.write_binary(default_spec_path, bundler_spec.to_ruby)

    bundler_spec = Gem::Specification.load(default_spec_path)

    # Remove gemspec that was same version of vendored bundler.
    normal_gemspec = File.join(Gem.default_dir, "specifications", "bundler-#{bundler_spec.version}.gemspec")
    if File.file? normal_gemspec
      File.delete normal_gemspec
    end

    # Remove gem files that were same version of vendored bundler.
    if File.directory? bundler_spec.gems_dir
      Dir.entries(bundler_spec.gems_dir).
        select {|default_gem| File.basename(default_gem) == "bundler-#{bundler_spec.version}" }.
        each {|default_gem| rm_r File.join(bundler_spec.gems_dir, default_gem) }
    end

    bundler_bin_dir = File.join(Gem.default_dir, 'gems', bundler_spec.full_name, bundler_spec.bindir)
    bundler_bin_dir = File.join(options[:destdir], bundler_bin_dir) unless Gem.win_platform?
    mkdir_p bundler_bin_dir
    bundler_spec.executables.each do |e|
      cp File.join("bundler", bundler_spec.bindir, e), File.join(bundler_bin_dir, e)
    end

    if Gem.win_platform?
      require 'rubygems/installer'

      installer = Gem::Installer.for_spec bundler_spec
      bundler_spec.executables.each do |e|
        installer.generate_windows_script e, bundler_spec.bin_dir
      end
    end

    say "Bundler #{bundler_spec.version} installed"
  end