Gem::Commands::SetupCommand#make_destination_dirs

# File tmp/rubies/ruby-4.0.0/lib/rubygems/commands/setup_command.rb, line 420
def make_destination_dirs
  lib_dir, bin_dir = Gem.default_rubygems_dirs

  unless lib_dir
    lib_dir, bin_dir = generate_default_dirs
  end

  mkdir_p lib_dir, mode: 0o755
  mkdir_p bin_dir, mode: 0o755

  [lib_dir, bin_dir]
end