inheritance or mix-in?

gistfile1.scala

// There's a class called MappedPassword in Lift. I wanted to create a class that
// holds the plain text when a password is set.

//-------------------------------
// This works, but I didn't like the idea for two reasons.
// 1. The class name is long.
// 2. Seems kinda ugly to me.
abstract class MappedPasswordWithPlainWhenSet[A<:Mapper[A]](override val fieldOwner: A)
extends MappedPassword[A](fieldOwner) {
  var plain: String = ""
  override def set(value: String): String = {
    plain = value
    super.set(value)
  }
}
// In a model class (User)
  override lazy val password = new MyPassword2(this)
  protected class MyPassword2(obj: User)
            extends MappedPasswordWithPlainWhenSet(obj) {
    override def displayName = fieldOwner.passwordDisplayName
  }

//-------------------------------
// I wanted to make it a trait, but the following code doesn't work.
trait HoldsPlainWhenSet[A<:Mapper[A]] {
  self: MappedPassword[A] =>
  var plain: String = ""
  override def set(value: String): String = {
    plain = value
    // self.super.set(value) // This doesn't compile.
    // self.set(value) // This causes an infinite loop.
  }
}

// In a model class (User)
  override lazy val password =
    new MyPassword(this) with HoldsPlainWhenSet[User]