Better printing of TyperStates

Identify them by number. Helps in the same way other fixed numbering schemes
help understand debug output.

Author: odersky

compiler/src/dotty/tools/dotc/core/TyperState.scala

@@ -12,9 +12,19 @@ import printing.Texts._
 import config.Config
 import collection.mutable
 import java.lang.ref.WeakReference
+import Decorators._
+
+object TyperState {
+  @sharable private var nextId: Int = 0
+}
 
 class TyperState(previous: TyperState /* | Null */) extends DotClass with Showable {
 
+  val id = TyperState.nextId
+  TyperState.nextId += 1
+
+  //assert(id != 146)
+
   private[this] var myReporter =
     if (previous == null) new ConsoleReporter() else previous.reporter
 
@@ -171,7 +181,7 @@ class TyperState(previous: TyperState /* | Null */) extends DotClass with Showab
       constraint = constraint.remove(poly)
   }
 
-  override def toText(printer: Printer): Text = constraint.toText(printer)
+  override def toText(printer: Printer): Text = s"TS[$id]"
 
   def hashesStr: String =
     if (previous == null) "" else hashCode.toString + " -> " + previous.hashesStr