silence_assets.rb

class NullLogger < Logger
  def initialize(*args); end
  def write(*input); true; end
end

Rails.application.assets.logger = NullLogger.new

class TemporaryLoggerLevel
  LEVEL = Logger::ERROR

  def initialize(env)
    @env = env
    @initial_level = ::Rails.logger.level
  end

  def swap
    Rails.logger.level = determine_level
    yield
  ensure
    Rails.logger.level = @initial_level
  end

  private

  def determine_level
    assets_path? ? LEVEL : @initial_level
  end

  def assets_path?
    @env['PATH_INFO'] =~ /^\/assets\//
  end
end

class Rails::Rack::Logger
  alias_method :standard_call, :call

  def call(env)
    TemporaryLoggerLevel.new(env).swap do
      standard_call(env)
    end
  end
end