Make Label a final class

Author: odersky

compiler/src/dotty/tools/dotc/transform/DropBreaks.scala

@@ -123,6 +123,7 @@ class DropBreaks extends MiniPhase, RecordStackChange:
             Some((id.symbol, arg))
           case _ => None
       case _ => None
+  end Break
 
   /** The LabelUsage data associated with `lbl` in the current context */
   private def labelUsage(lbl: Symbol)(using Context): Option[LabelUsage] =

library/src/scala/util/boundary.scala

@@ -23,7 +23,7 @@ object boundary:
 
   /** Labels are targets indicating which boundary will be exited by a `break`.
    */
-  class Label[-T]:
+  final class Label[-T]:
     def break(value: T): Nothing = throw Break(this, value)
 
   /** Run `body` with freshly generated label as implicit argument. Catch any