New debug option: -Yshow-tree-ids

When enabled, we print trees with their uniqueIds. This was useful
in diagnosing the issue, because that way we could find out when a
tree that was missing in one version was created in the other.

Author: odersky

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -99,6 +99,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val YdumpSbtInc = BooleanSetting("-Ydump-sbt-inc", "For every compiled foo.scala, output the API representation and dependencies used for sbt incremental compilation in foo.inc, implies -Yforce-sbt-phases.")
   val YcheckAllPatmat = BooleanSetting("-Ycheck-all-patmat", "Check exhaustivity and redundancy of all pattern matching (used for testing the algorithm)")
   val YretainTrees = BooleanSetting("-Yretain-trees", "Retain trees for top-level classes, accessible from ClassSymbol#tree")
+  val YshowTreeIds = BooleanSetting("-Yshow-tree-ids", "Uniquely tag all tree nodes in debugging output.")
 
   /** Area-specific debug output */
   val Yexplainlowlevel = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -617,6 +617,8 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       val clsStr = ""//if (tree.isType) tree.getClass.toString else ""
       txt = (txt ~ "@" ~ pos.toString ~ clsStr).close
     }
+    if (ctx.settings.YshowTreeIds.value)
+      txt = (txt ~ "#" ~ tree.uniqueId.toString).close
     tree match {
       case Block(_, _) | Template(_, _, _, _) => txt
       case _ => txt.close