Gem::Installer#install

# File tmp/rubies/ruby-3.4.1/lib/rubygems/installer.rb, line 289
def install
  pre_install_checks

  run_pre_install_hooks

  # Set loaded_from to ensure extension_dir is correct
  if @options[:install_as_default]
    spec.loaded_from = default_spec_file
  else
    spec.loaded_from = spec_file
  end

  # Completely remove any previous gem files
  FileUtils.rm_rf gem_dir
  FileUtils.rm_rf spec.extension_dir

  dir_mode = options[:dir_mode]
  FileUtils.mkdir_p gem_dir, mode: dir_mode && 0o755

  if @options[:install_as_default]
    extract_bin
    write_default_spec
  else
    extract_files

    build_extensions
    write_build_info_file
    run_post_build_hooks
  end

  generate_bin
  generate_plugins

  unless @options[:install_as_default]
    write_spec
    write_cache_file
  end

  File.chmod(dir_mode, gem_dir) if dir_mode

  say spec.post_install_message if options[:post_install_message] && !spec.post_install_message.nil?

  Gem::Specification.add_spec(spec) unless @install_dir

  load_plugin

  run_post_install_hooks

  spec
rescue Errno::EACCES => e
  # Permission denied - /path/to/foo
  raise Gem::FilePermissionError, e.message.split(" - ").last
end