some fix

[mibamur]

# config/initializers/sassc_rails.rb
require "sprockets/engines"

module Extensions
  module Sprockets
    module Engines
      def register_engine(ext, klass)
        return if [
          Sass::Rails::SassTemplate,
          Sass::Rails::ScssTemplate
        ].include?(klass)
        super
      end
    end
  end
end

Sprockets::Base.send(:prepend, Extensions::Sprockets::Engines)

Rails.application.config.assets.configure do |env|
  env.register_engine '.sass', SassC::Rails::SassTemplate
  env.register_engine '.scss', SassC::Rails::ScssTemplate
end

# Gemfile
#...
gem 'sass-rails', require: false
gem 'sassc-rails', github: 'sass/sassc-rails'
#...

[ridem]

With Sprockets 4 and gems using register_engine, this was the correct call for me:

# config/initializers/sassc_rails.rb
require "sprockets/engines"

module Extensions
  module Sprockets
    module Engines
      def register_engine(ext, klass, opts={})
        return if [
          Sass::Rails::SassTemplate,
          Sass::Rails::ScssTemplate
        ].include?(klass)
        super
      end
    end
  end
end
Sprockets::Base.send(:prepend, Extensions::Sprockets::Engines)

Rails.application.config.assets.configure do |env|
  if env.respond_to?(:register_transformer)
    env.register_transformer 'text/sass', 'text/css', SassC::Rails::SassTemplate.new
    env.register_transformer 'text/scss', 'text/css', SassC::Rails::ScssTemplate.new
  else
    env.register_engine '.sass', SassC::Rails::SassTemplate
    env.register_engine '.scss', SassC::Rails::ScssTemplate
  end
end