LKHalt.swift

struct Navigation: ~Copyable {
    var antenna: Bool
}

struct Engine: ~Copyable {
    var fuel: Int
}

enum Variant: ~Copyable {
    case navigation(Navigation)
    case engine(Engine)
}

class Spaceship {
  var v: Variant
  init(_ v: consuming Variant) {
    self.v = v
  }
}

func changeAndPrintShip(_ shipA: Spaceship, _ shipB: Spaceship) {
  print("In changeAndPrintShip...")
  switch shipA.v {
    case .navigation(let data):
      break
    case .engine(let data):
      print("In engine case, borrowed the inner Engine...")
      shipB.v = Variant.navigation(Navigation(antenna: true))
      print("Dereferencing the borrow...")
      let fuel = data.fuel
      print("ship.engine.fuel = \(fuel)")
  }
}

func main() {
  print("Starting up!")
  let ship = Spaceship(Variant.engine(Engine(fuel: 42)))
  changeAndPrintShip(ship, ship)
}

main()


// Output:
//
// Starting up!
// In changeAndPrintShip...
// In engine case, borrowed the inner Engine...
// Simultaneous accesses to 0x16b2d3120, but modification requires exclusive access.
// Previous access (a read) started at SwiftUB`changeAndPrintShip(_:_:) + 172 (0x104b2f3f0).
// Current access (a modification) started at:
// 0    libswiftCore.dylib                 0x00000001a562cdf0 swift::runtime::AccessSet::insert(swift::runtime::Access*, void*, void*, swift::ExclusivityFlags) + 432
// 1    libswiftCore.dylib                 0x00000001a562d008 swift_beginAccess + 84
// 2    SwiftUB                            0x0000000104b2f344 changeAndPrintShip(_:_:) + 296
// 3    SwiftUB                            0x0000000104b2f1f4 SwiftUB_main + 192
// 4    dyld                               0x00000001952417a8 start + 2360
// Fatal access conflict detected.