Gem::Commands::SetupCommand#default_dir

# File tmp/rubies/ruby-3.4.1/lib/rubygems/commands/setup_command.rb, line 605
def default_dir
  prefix = options[:prefix]

  if prefix.empty?
    dir = Gem.default_dir
  else
    dir = prefix
  end

  prepend_destdir_if_present(dir)
end