diff --git a/compiler/src/dotty/tools/dotc/parsing/Scanners.scala b/compiler/src/dotty/tools/dotc/parsing/Scanners.scala
index 4e8e3cdce803..e026b7fad9a2 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Scanners.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Scanners.scala
@@ -177,8 +177,11 @@ object Scanners {
     /** All doc comments kept by their end position in a `Map` */
     private[this] var docstringMap: SortedMap[Int, Comment] = SortedMap.empty
 
-    /* All comments positions will be stored in this */
-    var commentPositions:mutable.MutableList[Position] = mutable.MutableList()
+    /* A Buffer for comment positions */
+    private[this] val commentPosBuf = new mutable.ListBuffer[Position]
+
+    /** Return a list of all the comment positions */
+    def commentPositions: List[Position] = commentPosBuf.toList
 
     private[this] def addComment(comment: Comment): Unit = {
       val lookahead = lookaheadReader()
@@ -619,7 +622,7 @@ object Scanners {
         if (keepComments) {
           val pos = Position(start, charOffset - 1, start)
           val comment = Comment(pos, flushBuf(commentBuf))
-          commentPositions += pos
+          commentPosBuf += pos
 
           if (comment.isDocComment) {
             addComment(comment)
diff --git a/compiler/src/dotty/tools/dotc/printing/SyntaxHighlighting.scala b/compiler/src/dotty/tools/dotc/printing/SyntaxHighlighting.scala
index 6ab82e22c397..a7a8ea420df5 100644
--- a/compiler/src/dotty/tools/dotc/printing/SyntaxHighlighting.scala
+++ b/compiler/src/dotty/tools/dotc/printing/SyntaxHighlighting.scala
@@ -49,11 +49,6 @@ object SyntaxHighlighting {
           highlightRange(pos.start, pos.end, color)
       }
 
-      def highlightComments(commentsPos: mutable.MutableList[Position]): Unit = {
-       for (pos <- commentsPos)
-         highlightPosition(pos,CommentColor)
-      }
-      
       val scanner = new Scanner(source)
       while (scanner.token != EOF) {
         val start = scanner.offset
@@ -87,7 +82,9 @@ object SyntaxHighlighting {
         }
       }
 
-      highlightComments(scanner.commentPositions)
+      scanner.commentPositions.foreach {
+        highlightPosition(_,CommentColor)
+      }
 
       object TreeHighlighter extends untpd.UntypedTreeTraverser {
         import untpd._