Gem::Commands::SetupCommand#install_default_bundler_gem

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/commands/setup_command.rb, line 340
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)

    previous_specs_dir = File.dirname(loaded_from)

    # Remove previous default gem executables if they were not shadowed by a
    # regular gem. They live under the same root as the previous default
    # gemspec, which is not necessarily default_dir.
    if all_specs_current_version.size == 1
      previous_root = File.dirname(File.dirname(previous_specs_dir))
      FileUtils.rm_rf File.join(previous_root, "gems", current_default_spec.full_name)
    end

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

  # Root directory that specs_dir belongs to. It may differ from default_dir
  # when Gem.default_specifications_dir is customized to live under a
  # different root, like Homebrew does. Executables must be extracted under
  # the same root as the default gemspec, since that's where activation of
  # the default gem will look for them.
  bundler_install_dir = File.dirname(File.dirname(specs_dir))

  new_bundler_spec = 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)

  # 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. A regular
  # gem lives under default_dir, which is not necessarily the same root as
  # the default gemspec.
  normal_gems_dir = File.join(default_dir, "gems")
  if File.directory? normal_gems_dir
    Dir.entries(normal_gems_dir).
      select {|normal_gem| File.basename(normal_gem) == full_name }.
      each {|normal_gem| rm_r File.join(normal_gems_dir, normal_gem) }
  end

  require_relative "../installer"

  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: bundler_install_dir,
      wrappers: true
    )
    # We only need to install the executables here. The default spec was
    # already written above, and lib/bundler.rb and lib/bundler/* are
    # available under the site_ruby directory.
    installer.extract_bin
    installer.generate_bin
  ensure
    FileUtils.rm_f built_gem
  end

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

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