Testing if it's possible to have Scala class parameters not becoming instance attributes

Testing.scala

// Regarding these discussions about Scala class constructor parameters:
// http://stackoverflow.com/questions/15639078/scala-class-constructor-parameters
// http://stackoverflow.com/questions/1889454/in-scala-how-do-you-define-a-local-parameter-in-the-primary-constructor-of-a-cl

// Class parameter `fake` is not prefixed by either `val` or `var`
class Testing(fake: Boolean) {
  val buffer = if (fake) null else new StringBuilder()

  def output(str: Any): Unit = {
    // The class parameter `fake` is accessible beyond the construction phase
    if (!fake) {
      buffer.append(str.toString)
    }
  }

  override def toString: String = {
    // We're accessing it here in the `toString` method as well
    if (fake) "()" else s"(${buffer.toString()})"
  }
}

// Entry point just to make this example runnable
object Testing {
  def main (args: Array[String]): Unit = {
    println(test(new Testing(fake = true)))
    println(test(new Testing(fake = false)))
  }

  private def test(instance: Testing): String = {
    instance.output("Hello World")
    instance.toString
  }
}

def output(str: Any): Unit = {
// The class parameter fake is accessible beyond the construction phase
if (!fake) {
buffer.append(str.toString)
}
}

My guess : Scala may be feeding "fake" arg to "output" function using "Partially applied functions and currying"
PS: I cant update stackoverflow as I dont have 50 pts yet :)