Gem::Commands::SetupCommand#generate_default_dirs

# File tmp/rubies/ruby-3.0.5/lib/rubygems/commands/setup_command.rb, line 460
def generate_default_dirs
  prefix = options[:prefix]
  site_or_vendor = options[:site_or_vendor]

  if prefix.empty?
    lib_dir = RbConfig::CONFIG[site_or_vendor]
    bin_dir = RbConfig::CONFIG['bindir']
  else
    # Apple installed RubyGems into libdir, and RubyGems <= 1.1.0 gets
    # confused about installation location, so switch back to
    # sitelibdir/vendorlibdir.
    if defined?(APPLE_GEM_HOME) and
      # just in case Apple and RubyGems don't get this patched up proper.
      (prefix == RbConfig::CONFIG['libdir'] or
       # this one is important
       prefix == File.join(RbConfig::CONFIG['libdir'], 'ruby'))
      lib_dir = RbConfig::CONFIG[site_or_vendor]
      bin_dir = RbConfig::CONFIG['bindir']
    else
      lib_dir = File.join prefix, 'lib'
      bin_dir = File.join prefix, 'bin'
    end
  end

  [prepend_destdir_if_present(lib_dir), prepend_destdir_if_present(bin_dir)]
end