Gem::Commands::SetupCommand#install_default_bundler_gem

# File tmp/rubies/ruby-4.0.0/lib/rubygems/commands/setup_command.rb, line 350
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
    all_specs_current_version = Gem::Specification.stubs.select {|s| s.full_name == current_default_spec.full_name }

    Gem::Specification.remove_spec current_default_spec
    loaded_from = current_default_spec.loaded_from
    File.delete(loaded_from)

    # Remove previous default gem executables if they were not shadowed by a regular gem
    FileUtils.rm_rf current_default_spec.full_gem_path if all_specs_current_version.size == 1

    File.dirname(loaded_from)
  else
    target_specs_dir = File.join(default_dir, "specifications", "default")
    mkdir_p target_specs_dir, mode: 0o755
    target_specs_dir
  end

  new_bundler_spec = Dir.chdir("bundler") { Gem::Specification.load("bundler.gemspec") }
  full_name = new_bundler_spec.full_name
  gemspec_path = "#{full_name}.gemspec"

  default_spec_path = File.join(specs_dir, gemspec_path)
  Gem.write_binary(default_spec_path, new_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", gemspec_path)
  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) == full_name }.
      each {|default_gem| rm_r File.join(bundler_spec.gems_dir, default_gem) }
  end

  require_relative "../installer"

  Dir.chdir("bundler") do
    built_gem = Gem::Package.build(new_bundler_spec)
    begin
      installer = Gem::Installer.at(
        built_gem,
        env_shebang: options[:env_shebang],
        format_executable: options[:format_executable],
        force: options[:force],
        bin_dir: bin_dir,
        install_dir: default_dir,
        wrappers: true
      )
      # We need to install only executable and default spec files.
      # lib/bundler.rb and lib/bundler/* are available under the site_ruby directory.
      installer.extract_bin
      installer.generate_bin
      installer.write_default_spec
    ensure
      FileUtils.rm_f built_gem
    end
  end

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

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