ruby/test/rubygems/test_config.rb

require 'rubygems/test_case'
require 'rubygems'

class TestConfig < Gem::TestCase

  def test_datadir
    _, err = capture_io do
      datadir = RbConfig::CONFIG['datadir']
      assert_equal "#{datadir}/xyz", RbConfig.datadir('xyz')
    end

    assert_match(/deprecate/, err)
  end

end