EnumIterator.swift

func enumIterator<T: Hashable>(_: T.Type) -> AnyGenerator<T> {
  var cast: (Int -> T)!
  switch sizeof(T) {
  case 0: return anyGenerator(GeneratorOfOne(unsafeBitCast((), T.self)))
  case 1: cast = { unsafeBitCast(UInt8(truncatingBitPattern: $0), T.self) }
  case 2: cast = { unsafeBitCast(UInt16(truncatingBitPattern: $0), T.self) }
  case 4: cast = { unsafeBitCast(UInt32(truncatingBitPattern: $0), T.self) }
  case 8: cast = { unsafeBitCast(UInt64($0), T.self) }
  default: fatalError("cannot be here")
  }
  
  var i = 0
  return anyGenerator {
    let next = cast(i)
    return next.hashValue == i++ ? next : nil
  }
}