WithUninitialized.swift

/// Passes a pointer to an uninitialized instance of `type` to `body`.
/// If `body` returns `true`, indicating that the memory has now been
/// initialized, returns the instance; otherwise returns `nil`.
@usableFromInline func withUninitialized<T>(
  _ type: T.Type, do body: (UnsafeMutablePointer<T>) -> Bool
) -> T? {
  let ptr = UnsafeMutablePointer<T>.allocate(capacity: 1)
  defer { ptr.deallocate() }

  guard body(ptr) else {
    return nil
  }

  return ptr.move()
}