Gem::Commands::SetupCommand#install_default_bundler_gem

# File tmp/rubies/ruby-3.0.5/lib/rubygems/commands/setup_command.rb, line 362
def install_default_bundler_gem(bin_dir)
  specs_dir = File.join(default_dir, "specifications", "default")
  mkdir_p specs_dir, :mode => 0755

  bundler_spec = Dir.chdir("bundler") { Gem::Specification.load("bundler.gemspec") }

  # 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)

  # The base_dir value for a specification is inferred by walking up from the
  # folder where the spec was `loaded_from`. In the case of default gems, we
  # walk up two levels, because they live at `specifications/default/`, whereas
  # in the case of regular gems we walk up just one level because they live at
  # `specifications/`. However, in this case, the gem we are installing is
  # misdetected as a regular gem, when it's a default gem in reality. This is
  # because when there's a `:destdir`, the `loaded_from` path has changed and
  # doesn't match `Gem.default_specifications_dir` which is the criteria to
  # tag a gem as a default gem. So, in that case, write the correct
  # `@base_dir` directly.
  bundler_spec.instance_variable_set(:@base_dir, File.dirname(File.dirname(specs_dir)))

  # Remove gemspec that was same version of vendored bundler.
  normal_gemspec = File.join(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 = bundler_spec.bin_dir
  mkdir_p bundler_bin_dir, :mode => 0755
  bundler_spec.executables.each do |e|
    cp File.join("bundler", bundler_spec.bindir, e), File.join(bundler_bin_dir, e)
  end

  require_relative '../installer'

  Dir.chdir("bundler") do
    built_gem = Gem::Package.build(bundler_spec)
    begin
      Gem::Installer.at(
        built_gem,
        env_shebang: options[:env_shebang],
        format_executable: options[:format_executable],
        force: options[:force],
        install_as_default: true,
        bin_dir: bin_dir,
        install_dir: default_dir,
        wrappers: true
      ).install
    ensure
      FileUtils.rm_f built_gem
    end
  end

  bundler_spec.executables.each {|executable| bin_file_names << target_bin_path(bin_dir, executable) }

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