Add -Yprint-debug to hide pretty-printing debug info by default

Currently, this flag only hides the "^" we display on inferred types,
but in the future we can hide more stuff that shouldn't be seen by the
user in error messages and in the IDE.

Author: smarter

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

@@ -92,6 +92,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val YnoDeepSubtypes = BooleanSetting("-Yno-deep-subtypes", "throw an exception on deep subtyping call stacks.")
   val YplainPrinter = BooleanSetting("-Yplain-printer", "Pretty-print using a plain printer.")
   val YprintSyms = BooleanSetting("-Yprint-syms", "when printing trees print info in symbols instead of corresponding info in trees.")
+  val YprintDebug = BooleanSetting("-Yprint-debug", "when printing trees, print some extra information useful for debugging.")
   val YtestPickler = BooleanSetting("-Ytest-pickler", "self-test for pickling functionality; should be used with -Ystop-after:pickler")
   val YcheckReentrant = BooleanSetting("-Ycheck-reentrant", "check that compiled program does not contain vars that can be accessed from a global root.")
   val YkeepComments = BooleanSetting("-Ykeep-comments", "Keep comments when scanning source files.")

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

@@ -190,13 +190,13 @@ class PlainPrinter(_ctx: Context) extends Printer {
         toTextLocal(tycon) ~ "[" ~ Text(args.map(argText), ", ") ~ "]"
       case tp: TypeVar =>
         if (tp.isInstantiated)
-          toTextLocal(tp.instanceOpt) ~ "^" // debug for now, so that we can see where the TypeVars are.
+          toTextLocal(tp.instanceOpt) ~ ("^" provided ctx.settings.YprintDebug.value)
         else {
           val constr = ctx.typerState.constraint
           val bounds =
             if (constr.contains(tp)) constr.fullBounds(tp.origin)(ctx.addMode(Mode.Printing))
             else TypeBounds.empty
-          if (bounds.isAlias) toText(bounds.lo) ~ "^"
+          if (bounds.isAlias) toText(bounds.lo) ~ ("^" provided ctx.settings.YprintDebug.value)
           else if (ctx.settings.YshowVarBounds.value) "(" ~ toText(tp.origin) ~ "?" ~ toText(bounds) ~ ")"
           else toText(tp.origin)
         }