class Sass::Tree::Visitors::Perform

def visit_warn(node)
  res = node.expr.perform(@environment)
  res = res.value if res.is_a?(Sass::Script::Value::String)
  @environment.stack.with_directive(node.filename, node.line, "@warn") do
    msg = "WARNING: #{res}\n         "
    msg << @environment.stack.to_s.gsub("\n", "\n         ") << "\n"
    Sass::Util.sass_warn msg
  end
  []
end