libsass/sassc working with Rails sprockets - rough impl, Rails 3.2

config_application.rb

    config.before_initialize do
        require_relative '../lib/sassc_template'
        Sprockets::Engines #force autoloading
        Sprockets.register_engine '.scss', SasscTemplate
        Sprockets.register_engine '.sass', SasscTemplate
    end

sassc_template.rb

require 'tilt/template'
require 'open3'

class SasscTemplate < Tilt::Template
  self.default_mime_type = 'text/css'

  class RenderError < StandardError
  end

  # Override Tilt::Template to insist on a file
  def initialize(file=nil, line=1, options={}, &block)
    @file, @line, @options = nil, 1, {}

    [options, line, file].compact.each do |arg|
      case
      when arg.respond_to?(:to_str)  ; @file = arg.to_str
      when arg.respond_to?(:to_int)  ; @line = arg.to_int
      when arg.respond_to?(:to_hash) ; @options = arg.to_hash.dup
      when arg.respond_to?(:path)    ; @file = arg.path
      else raise TypeError
      end
    end

    raise ArgumentError, "file required, block not supported" if @file.nil?

    prepare
  end

  def prepare
    @load_paths = Rails.application.config.assets.paths.map(&:to_s)
  end

  def evaluate scope, locals, &block
    stdout, stderr, s = Open3.capture3("sassc -l -t nested -I #{@load_paths.join(" -I ")} #{@file}")

    if s.success?
      depend_on_imports(scope, stdout)
      stdout
    else
      raise SasscTemplate::RenderError, "\n\n#{stderr}"
    end
  end

  def depend_on_imports scope, css
    # Prune comments from the compiled SCSSand find our dependencies, so we can tell sprockets
    # about them. If we don't, sprockets will not recompile if an import is changed.
    regexp = %r{/\* line \d+, (.+css) \*/}

    css.scan(regexp).sort.uniq.each do |file|
      next unless file[0]
      scope.depend_on(file[0])
    end
  end

  def allows_script?
    false
  end
end