Catch stackoverflow errors in the highlighter (#19836)

Co-authored-by: Matt Bovel <matthieu@bovel.net>
Co-authored-by: Sébastien Doeraene <sjrdoeraene@gmail.com>

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

@@ -124,24 +124,30 @@ object SyntaxHighlighting {
         }
       }
 
-      val parser = new Parser(source)
-      val trees = parser.blockStatSeq()
-      TreeHighlighter.highlight(trees)
-
-      val highlighted = new StringBuilder()
-
-      for (idx <- colorAt.indices) {
-        val prev = if (idx == 0) NoColor else colorAt(idx - 1)
-        val curr = colorAt(idx)
-        if (curr != prev)
-          highlighted.append(curr)
-        highlighted.append(in(idx))
-      }
+      try
+        val parser = new Parser(source)
+        val trees = parser.blockStatSeq()
+        TreeHighlighter.highlight(trees)
+
+
+        val highlighted = new StringBuilder()
 
-      if (colorAt.last != NoColor)
-        highlighted.append(NoColor)
+        for (idx <- colorAt.indices) {
+          val prev = if (idx == 0) NoColor else colorAt(idx - 1)
+          val curr = colorAt(idx)
+          if (curr != prev)
+            highlighted.append(curr)
+          highlighted.append(in(idx))
+        }
+
+        if (colorAt.last != NoColor)
+          highlighted.append(NoColor)
 
-      highlighted.toString
+        highlighted.toString
+      catch
+        case e: StackOverflowError =>
+          in
     }
   }
+
 }

compiler/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -497,3 +497,16 @@ class ReplVerboseTests extends ReplTest(ReplTest.defaultOptions :+ "-verbose"):
   }
 
 end ReplVerboseTests
+
+class ReplHighlightTests extends ReplTest(ReplTest.defaultOptions.filterNot(_.startsWith("-color")) :+ "-color:always"):
+  @Test def i18596: Unit = initially:
+    run("""(1 to 500).foldRight("x") { case (_, n) => s"<x>$n</x>" }""")
+
+  @Test def i16904: Unit = initially:
+    run(""""works not fine"* 10000""")
+
+    run("""
+      case class Tree(left: Tree, right: Tree)
+      def deepTree(depth: Int): Tree
+      deepTree(300)""")
+