Print typerstate nesting info as part of constr printing

When printing info about adding to constraints, show the hashes
of the typerstate stack, so that we know where constraints are added.

Author: odersky

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

@@ -79,12 +79,12 @@ trait ConstraintHandling {
       if (Config.failOnInstantiationToNothing) assert(false, msg)
       else ctx.log(msg)
     }
-    constr.println(i"adding $description")
+    constr.println(i"adding $description in ${ctx.typerState.hashesStr}")
     val lower = constraint.lower(param)
     val res =
       addOneBound(param, bound, isUpper = true) &&
       lower.forall(addOneBound(_, bound, isUpper = true))
-    constr.println(i"added $description = $res")
+    constr.println(i"added $description = $res in ${ctx.typerState.hashesStr}")
     res
   }
 
@@ -95,7 +95,7 @@ trait ConstraintHandling {
     val res =
       addOneBound(param, bound, isUpper = false) &&
       upper.forall(addOneBound(_, bound, isUpper = false))
-    constr.println(i"added $description = $res")
+    constr.println(i"added $description = $res in ${ctx.typerState.hashesStr}")
     res
   }
 
@@ -108,12 +108,12 @@ trait ConstraintHandling {
         val up2 = p2 :: constraint.exclusiveUpper(p2, p1)
         val lo1 = constraint.nonParamBounds(p1).lo
         val hi2 = constraint.nonParamBounds(p2).hi
-        constr.println(i"adding $description down1 = $down1, up2 = $up2")
+        constr.println(i"adding $description down1 = $down1, up2 = $up2 ${ctx.typerState.hashesStr}")
         constraint = constraint.addLess(p1, p2)
         down1.forall(addOneBound(_, hi2, isUpper = true)) &&
         up2.forall(addOneBound(_, lo1, isUpper = false))
       }
-    constr.println(i"added $description = $res")
+    constr.println(i"added $description = $res ${ctx.typerState.hashesStr}")
     res
   }
 

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

@@ -79,6 +79,9 @@ class TyperState(r: Reporter) extends DotClass with Showable {
   def tryWithFallback[T](op: => T)(fallback: => T)(implicit ctx: Context): T = unsupported("tryWithFallBack")
 
   override def toText(printer: Printer): Text = "ImmutableTyperState"
+
+  /** A string showing the hashes of all nested mutable typerstates */
+  def hashesStr: String = ""
 }
 
 class MutableTyperState(previous: TyperState, r: Reporter, override val isCommittable: Boolean)
@@ -207,4 +210,7 @@ extends TyperState(r) {
   }
 
   override def toText(printer: Printer): Text = constraint.toText(printer)
+
+  override def hashesStr: String = hashCode.toString + " -> " + previous.hashesStr
+
 }