It must be possible, when specifying methods, to refer to the type that the interface is implemented on. For example, an add method on iface num, which takes an extra parameter of the same type and returns a value of the same type, can't be expressed in the current system without adding a lot of noisy type parameters.

iface num {
  fn add(x: self) -> self;
  fn sub(x: self) -> self;
  /* etc */
}

impl of num for int {
  fn add(x: int) -> int { self + x }
  fn sub(x: int) -> int { self - x }
}

The self type is an implicit type parameter refers to the specialized type. An interface that uses it can not be cast to, since it'd be impossible to type its methods in the absence of a concrete self type.