Labeled nested breaks in Scala

labeled-breaks.scala

object lib:
  import scala.annotation.implicitNotFound
  import scala.util.boundary, boundary.*

  opaque type Lab[T <: Singleton] >: Unit = Unit
  object Lab:
    inline def apply[T <: Singleton]: Lab[T] = ()

  inline def label[T <: Singleton & String](
      inline f: Label[Lab[T]] ?=> Unit
  ) = boundary[Lab[T]](f)

  inline def breakL[T <: Singleton & String](using
      @implicitNotFound("No label [${T}] to break out of")
      l: Label[Lab[T]]
  ) = break(Lab[T])

@main def hello =
  import lib.*
  label["outer"]:
    while true do
      println("Entered the outer loop")

      label["inner"]:
        while true do
          println("Entered the inner loop")

          breakL["outer"]

      println("This point will never be reached")

  println("Exited the outer loop")