Add setting to show error stacktraces during printing

Author: odersky

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

@@ -101,6 +101,7 @@ class ScalaSettings extends Settings.SettingGroup {
   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 YshowPrintErrors = BooleanSetting("-Yshow-print-errors", "don't suppress exceptions thrown during tree printing.")
   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/Formatting.scala

@@ -31,7 +31,9 @@ object Formatting {
       case arg: Showable =>
         try arg.show
         catch {
-          case NonFatal(ex) if !ctx.mode.is(Mode.PrintShowExceptions) =>
+          case NonFatal(ex)
+          if !ctx.mode.is(Mode.PrintShowExceptions) &&
+             !ctx.settings.YshowPrintErrors.value =>
             s"[cannot display due to $ex, raw string = $toString]"
         }
       case _ => arg.toString