Check validity of hints

[kiritsuku]

object PicklingTest extends App {
  import scala.pickling.Defaults._
  import scala.pickling.json._

  case class X(i: Int)
  implicit val shitPickler: Pickler[X] = new Pickler[X] {
    override val tag = FastTypeTag[X]
    override def pickle(x: X, builder: PBuilder): Unit = {
      builder.beginEntry(x)
      AllPicklers.intPickler.pickle(x.i, builder)
      builder.endEntry()
    }
  }
  val s = X(5)
  println(s.pickle)
}

Results in:

Exception in thread "main" java.lang.NullPointerException
    at scala.pickling.json.JSONPickleBuilder$$anonfun$beginEntry$1.apply(JSONPickleFormat.scala:109)
    at scala.pickling.json.JSONPickleBuilder$$anonfun$beginEntry$1.apply(JSONPickleFormat.scala:102)
    at scala.pickling.PickleTools$class.withHints(Tools.scala:521)
    at scala.pickling.json.JSONPickleBuilder.withHints(JSONPickleFormat.scala:39)
    at scala.pickling.json.JSONPickleBuilder.beginEntry(JSONPickleFormat.scala:102)
    at scala.pickling.pickler.PrimitivePickler.pickle(Primitives.scala:23)
    at test.pickling.PicklingTest$$anon$8.pickle(PicklingTest.scala:25)
    at test.pickling.PicklingTest$$anon$8.pickle(PicklingTest.scala:21)
    at scala.pickling.functions$.pickleInto(functions.scala:36)
    at scala.pickling.functions$.pickle(functions.scala:19)
    at scala.pickling.PickleOps.pickle(Ops.scala:9)
    at test.pickling.PicklingTest$.delayedEndpoint$test$pickling$PicklingTest$1(PicklingTest.scala:30)
    at test.pickling.PicklingTest$delayedInit$body.apply(PicklingTest.scala:10)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:76)
    at scala.App$$anonfun$main$1.apply(App.scala:76)
    at scala.collection.immutable.List.foreach(List.scala:381)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
    at scala.App$class.main(App.scala:76)
    at test.pickling.PicklingTest$.main(PicklingTest.scala:10)
    at test.pickling.PicklingTest.main(PicklingTest.scala)

It should give an explanation what went wrong and how to fix the problem.

[kiritsuku]

These two methods are broken:

trait PickleTools {

  def popHints(): this.type = {
    hints = hints.tail
    this
  }

  def withHints[T](body: Hints => T): T = {
    val hints = this.hints.head
    if (!hints.pinned) this.hints = Hints() :: this.hints.tail
    body(hints)
  }
}

popHints can't pop infinitely and withHints need to check if hints.tag is not null.

[jsuereth]

@sschaef Two things:

1. hintTag is no longer option, and is now included in beginEntry for a pickler. That should help solve the first issue.
2. push/pop hints may need some lovin/fixing soon. Would like to suggestions on how to fix.