    # File lib/rubygems.rb, line 583
      def self.required_location(gemname, libfile, *version_constraints)
        version_constraints = Gem::Requirement.default if version_constraints.empty?
        matches = Gem.source_index.find_name(gemname, version_constraints)
        return nil if matches.empty?
        spec = matches.last
        spec.require_paths.each do |path|
          result = File.join(spec.full_gem_path, path, libfile)
          return result if File.exist?(result)
        end
        nil
      end