Gem::Installer#user_install_dir

# File tmp/rubies/ruby-3.4.1/lib/rubygems/installer.rb, line 967
def user_install_dir
  # never install to user home in --build-root mode
  return unless @build_root.nil?

  # Please note that @user_install might have three states:
  # * `true`: `--user-install`
  # * `false`: `--no-user-install` and
  # * `nil`: option was not specified
  if @user_install || (@user_install.nil? && Gem.default_user_install)
    Gem.user_dir
  end
end