A way to ask about uniqueness of class references, without a struct wrapper

CowBuffer.swift

extension ObjectIdentifier {
  /// Returns true if the object identified by `self` is uniquely referenced.
  ///
  /// - Requires: the object identified by `self` exists.
  /// - Note: will only work when called from a mutating method
  internal func _liveObjectIsUniquelyReferenced() -> Bool {
    var me = self
    return withUnsafeMutablePointer(to: &me) {
      $0.withMemoryRebound(to: AnyObject.self, capacity: 1) {
        _isUnique(&$0.pointee)
      }
    }    
  }
}

protocol COWBuffer { }

extension COWBuffer {
  var id: ObjectIdentifier {
    if let _=Self.self as? AnyObject.Type, let me = self as Any as? AnyObject {
      return ObjectIdentifier(me)
    }
    fatalError()
  }
  
  mutating func isUniquelyReferenced() -> Bool {
    defer { _fixLifetime(self) }
    return id._liveObjectIsUniquelyReferenced()
  }

  mutating func mutate<R>(body: (inout Self)->R) -> R {
    return body(&self)
  }
}

class X : COWBuffer {
  init() {  }
}

func test() {
  var x1 = X()
  x1.mutate {  print($0.isUniquelyReferenced() ? "in-place" : "copy") }
  do
  {
    var x2 = x1
    x1.mutate  {  print($0.isUniquelyReferenced() ? "in-place": "copy")}
    x2.mutate  {  print($0.isUniquelyReferenced() ? "in-place": "copy")}
  }
  x1.mutate {  print($0.isUniquelyReferenced() ? "in-place": "copy")}
}
test()