CircuitousShapeshift.swift

struct Navigation: ~Copyable {
    var antenna: Bool
}

struct Engine: ~Copyable {
    var warp_coils: [Int]
}

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

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

func maintenance(_ ship: Spaceship) {
  ship.v = Variant.navigation(Navigation(antenna: true))
}

func foo(_ ship: Spaceship) {
  switch ship.v { // Borrows ship.v since it's a noncopyable type
    case .navigation(let navigation):
      break
    case .engine(let engine): // engine is a borrow reference
      maintenance(ship)
      // Theoretically we'd get the same error if we write to fuel here,
      // though Swift doesn't have mutable references.
      let warp_coils = engine.warp_coils
      print("Warp coils:", warp_coils)
  }
}

func main() {
  let ship = Spaceship(Variant.engine(Engine(warp_coils: [42, 73])))
  foo(ship)
}

main()



// verdagon@Evans-MBP swiftub % swift run -c release
// Building for production...
// [1/1] Write swift-version-24CDE65C6C26C763.txt
// Build of product 'SwiftUB' complete! (0.08s)
// Simultaneous accesses to 0x16f563110, but modification requires exclusive access.
// Previous access (a read) started at SwiftUB`foo(_:) + 44 (0x10089f458).
// Current access (a modification) started at:
// 0    libswiftCore.dylib                 0x00000001aab0cdf0 swift::runtime::AccessSet::insert(swift::runtime::Access*, void*, void*, swift::ExclusivityFlags) + 432
// 1    libswiftCore.dylib                 0x00000001aab0d008 swift_beginAccess + 84
// 2    SwiftUB                            0x000000010089f42c foo(_:) + 84
// 3    SwiftUB                            0x000000010089f30c SwiftUB_main + 104
// 4    dyld                               0x000000019a7217a8 start + 2360
// Fatal access conflict detected.


// Previous version, which also crashes:
//
// 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)
// }