Trait inheritance/composition

[catamorphism]

I know that there are multiple proposals floating around for how to do interface inheritance (of which traits may be one), but just noting it here so that FIXMEs can refer to issues.

[catamorphism]

Though this is an old bug, it's still useful since inheritance isn't quite finished yet. I moved it from enhancement => completion.

[dustinlacewell]

struct Point {
  mut x: int,
  mut y: int
}

trait Positioned {
  fn X()-> int;
  fn Y()-> int;
  fn SetX(int);
  fn SetY(int);
}

trait Movable: Positioned {
  fn translate(dx: int, dy: int) {
    self.SetX(self.X() + dx);
    self.SetY(self.Y() + dy);
  }
}

struct Entity {
  mut pos: Point,
}

impl Entity: Positioned {
  fn X()-> int { self.pos.x }
  fn Y()-> int { self.pos.y }
  fn SetX(v: int) { self.pos.x = v; }
  fn SetY(v: int) { self.pos.y = v; }
}

impl Entity: Movable { }

fn main() {
  let mut e: Entity = Entity {
    pos: Point {x: 0, y: 0},
  };

  io::println(fmt!("%?, %?", e.X(), e.Y()));
  e.translate(5, 10);
  io::println(fmt!("%?, %?", e.X(), e.Y()));
}

"error: attempted access of field SetX on type self, but no field or method with that name was found"

If you change the Movable.translate method to:

trait Movable: Positioned {
  fn translate(dx: int, dx: int) {
    let s = self as Positioned;
    s.SetX(s.X() + dx);
    s.SetY(s.Y() + dy);
  }
}

It says: "error: failed to find an implementation of trait @positioned for self"

[catamorphism]

@dustinlacewell I filed your bug as a separate issue: #3979

[catamorphism]

Inheritance basically works, closing the catchall bug.