# File lib/sass/tree/visitors/perform.rb, line 163
  def visit_mixin(node)
    handle_include_loop!(node) if @environment.mixins_in_use.include?(node.name)

    original_env = @environment
    original_env.push_frame(:filename => node.filename, :line => node.line)
    original_env.prepare_frame(:mixin => node.name)
    raise Sass::SyntaxError.new("Undefined mixin '#{node.name}'.") unless mixin = @environment.mixin(node.name)

    passed_args = node.args.dup
    passed_keywords = node.keywords.dup

    raise Sass::SyntaxError.new("Mixin \#{node.name} takes \#{mixin.args.size} argument\#{'s' if mixin.args.size != 1}\n but \#{node.args.size} \#{node.args.size == 1 ? 'was' : 'were'} passed.\n".gsub("\n", "")) if mixin.args.size < passed_args.size

    passed_keywords.each do |name, value|
      # TODO: Make this fast
      unless mixin.args.find {|(var, default)| var.underscored_name == name}
        raise Sass::SyntaxError.new("Mixin #{node.name} doesn't have an argument named $#{name}")
      end
    end

    environment = mixin.args.zip(passed_args).
      inject(Sass::Environment.new(mixin.environment)) do |env, ((var, default), value)|
      env.set_local_var(var.name,
        if value
          value.perform(@environment)
        elsif kv = passed_keywords[var.underscored_name]
          kv.perform(@environment)
        elsif default
          default.perform(env)
        end)
      raise Sass::SyntaxError.new("Mixin #{node.name} is missing parameter #{var.inspect}.") unless env.var(var.name)
      env
    end

    with_environment(environment) {node.children = mixin.tree.map {|c| visit(c)}.flatten}
    node
  rescue Sass::SyntaxError => e
    if original_env # Don't add backtrace info if this is an @include loop
      e.modify_backtrace(:mixin => node.name, :line => node.line)
      e.add_backtrace(:line => node.line)
    end
    raise e
  ensure
    original_env.pop_frame if original_env
  end