stringOf eagerly prints infinite iterable

[som-snytt]

Haven't tried dotr yet.

scala> val vs = new Iterable[Int] { def iterator = Iterator.continually(42) }
java.lang.OutOfMemoryError: Java heap space
  at java.base/java.util.Arrays.copyOf(Arrays.java:3745)
  at java.base/java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:172)
  at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:538)
  at java.base/java.lang.StringBuilder.append(StringBuilder.java:174)
  at java.base/java.lang.StringBuilder.append(StringBuilder.java:168)
  at scala.collection.IterableOnceOps.addString(IterableOnce.scala:1170)
  at scala.collection.IterableOnceOps.addString$(IterableOnce.scala:1162)
  at $anon$1.addString(<console>:1)
  at scala.collection.IterableOnceOps.mkString(IterableOnce.scala:1112)
  at scala.collection.IterableOnceOps.mkString$(IterableOnce.scala:1110)
  at $anon$1.mkString(<console>:1)
  at scala.collection.Iterable.toString(Iterable.scala:77)
  at scala.collection.Iterable.toString$(Iterable.scala:77)
  at $anon$1.toString(<console>:1)
  at scala.runtime.ScalaRunTime$.inner$1(ScalaRunTime.scala:232)
  at scala.runtime.ScalaRunTime$.stringOf(ScalaRunTime.scala:243)
  at scala.runtime.ScalaRunTime$.replStringOf(ScalaRunTime.scala:251)
  at .$print$lzycompute(<synthetic>:8)
  ... 14 elided

[Jasper-M]

knownSize should have more possible states.

sealed trait Size
case class Known(n: Int) extends Size
case object Unknown extends Size
case object UnknownFinite extends Size
case object Infinite extends Size

[joshlemer]

Should the default Iterable#toString implementation be changed to not even force its elements? View and LazyList for instance override their toString to avoid forcing elements, maybe the forcing implementation here could be migrated to StrictOptimizedIterableOps or somewhere else.

[eed3si9n]

Here's my PR - scala/scala#8554