Harden display logic

Displaying stuff should never report an exception or cause more messages to be displayed that
relate to the displaying.

Author: odersky

src/dotty/tools/dotc/printing/Showable.scala

@@ -5,6 +5,7 @@ import core._
 
 import Contexts._, Texts._, Decorators._
 import config.Config.summarizeDepth
+import scala.util.control.NonFatal
 
 trait Showable extends Any {
 
@@ -20,7 +21,11 @@ trait Showable extends Any {
   def fallbackToText(printer: Printer): Text = toString
 
   /** The string representation of this showable element. */
-  def show(implicit ctx: Context): String = toText(ctx.printer).show
+  def show(implicit ctx: Context): String =
+    try toText(ctx.printer).show
+    catch {
+      case NonFatal(ex) => s"[cannot display due to $ex, raw string = $toString]"
+    }
 
   /** The summarized string representation of this showable element.
    *  Recursion depth is limited to some smallish value. Default is

src/dotty/tools/dotc/reporting/Reporter.scala

@@ -11,6 +11,7 @@ import config.Settings.Setting
 import config.Printers
 import java.lang.System.currentTimeMillis
 import typer.ErrorReporting.DiagnosticString
+import typer.Mode
 
 object Reporter {
 
@@ -215,7 +216,7 @@ abstract class Reporter {
   }
 
   def report(d: Diagnostic)(implicit ctx: Context): Unit = if (!isHidden(d)) {
-    doReport(d)
+    doReport(d)(ctx.addMode(Mode.Printing))
     d match {
       case d: ConditionalWarning if !d.enablingOption.value => unreportedWarnings(d.enablingOption.name) += 1
       case d: Warning => warningCount += 1
@@ -248,7 +249,7 @@ abstract class Reporter {
   }
 
   /** Should this diagnostic not be reported at all? */
-  def isHidden(d: Diagnostic)(implicit ctx: Context): Boolean = false
+  def isHidden(d: Diagnostic)(implicit ctx: Context): Boolean = ctx.mode.is(Mode.Printing)
 
   /** Does this reporter contain not yet reported errors or warnings? */
   def hasPending: Boolean = false

src/dotty/tools/dotc/typer/Mode.scala

@@ -63,6 +63,9 @@ object Mode {
    */
   val AllowDependentFunctions = newMode(9, "AllowDependentFunctions")
 
+  /** We are currently printing something: avoid to produce more logs about
+   *  the printing
+   */
   val Printing = newMode(10, "Printing")
 
   val PatternOrType = Pattern | Type