Copyable protocol (thanks to Mike Ash for help)

copyable.swift

protocol Copyable: class {
  init(copy: Self)
}

//
// this is a 'diffing' constructor, in your callback modify properties of $0 as needed
//

func _copy<T:Copyable, U>(obj: T, @noescape fun: inout T -> ()) -> U {
  var cp = obj.dynamicType.init(copy: obj)
  fun(&cp)
  return cp as! U
}

extension Copyable {
  func copy() -> Self {
    return self.dynamicType.init(copy: self)
  }
}

//
// copy struct
//
func _scopy<T>(obj: T, @noescape fun: inout T -> ()) -> T {
  assert(!(obj is AnyObject))
  var cp = obj
  fun(&cp)
  return cp
}