# File lib/sass/tree/warn_node.rb, line 24
      def _perform(environment)
        environment.push_frame(:filename => filename, :line => line)
        res = @expr.perform(environment)
        res = res.value if res.is_a?(Sass::Script::String)
        msg = "WARNING: #{res}\n"
        environment.stack.reverse.each_with_index do |entry, i|
          msg << "        #{i == 0 ? "on" : "from"} line #{entry[:line]}" <<
            " of #{entry[:filename] || "an unknown file"}"
          msg << ", in `#{entry[:mixin]}'" if entry[:mixin]
          msg << "\n"
        end
        Haml::Util.haml_warn msg
        []
      ensure
        environment.pop_frame
      end