Gem::Commands::SetupCommand#install_default_bundler_gem

# File tmp/rubies/ruby-3.1.3/lib/rubygems/commands/setup_command.rb, line 362
def install_default_bundler_gem(bin_dir)
  current_default_spec = Gem::Specification.default_stubs.find {|s| s.name == "bundler" }
  specs_dir = if current_default_spec && default_dir == Gem.default_dir
    Gem::Specification.remove_spec current_default_spec
    loaded_from = current_default_spec.loaded_from
    File.delete(loaded_from)
    File.dirname(loaded_from)
  else
    target_specs_dir = File.join(default_dir, "specifications", "default")
    mkdir_p target_specs_dir, :mode => 0755
    target_specs_dir
  end

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