Improve trace for TASTy without valid line numbers

Ref: scala#18882

compiler/src/dotty/tools/dotc/transform/init/Trace.scala

@@ -22,12 +22,16 @@ object Trace:
 
   val empty: Trace = Vector.empty
 
+  val CONNECTING_INDENT = "\u2502   "            // "| "
+  val CHILD             = "\u251c\u2500\u2500 "  // "|-- "
+  val LAST_CHILD        = "\u2514\u2500\u2500 "  // "\-- "
+
   extension (trace: Trace)
     def add(node: Tree): Trace = trace :+ node
     def toVector: Vector[Tree] = trace
     def ++(trace2: Trace): Trace = trace ++ trace2
 
-  def show(using trace: Trace, ctx: Context): String = buildStacktrace(trace, "\n")
+  def show(using trace: Trace, ctx: Context): String = buildStacktrace(trace, "")
 
   def position(using trace: Trace): Tree = trace.last
 
@@ -42,7 +46,6 @@ object Trace:
     var lines: mutable.ArrayBuffer[String] = new mutable.ArrayBuffer
     trace.foreach { tree =>
       val pos = tree.sourcePos
-      val prefix = "-> "
       val line =
         if pos.source.exists then
           val loc = "[ " + pos.source.file.name + ":" + (pos.line + 1) + " ]"
@@ -52,18 +55,19 @@ object Trace:
           tree match
             case defDef: DefTree =>
               // The definition can be huge, avoid printing the whole definition.
-              defDef.symbol.show
+              defDef.symbol.showFullName
             case _ =>
-              tree.show
+              tree.show.split(System.lineSeparator(), 2).nn.head.nn
       val positionMarkerLine =
         if pos.exists && pos.source.exists then
-          positionMarker(pos)
+          CONNECTING_INDENT + positionMarker(pos)
         else ""
 
       // always use the more precise trace location
-      if lastLineNum == pos.line then
+      if lastLineNum >= 0 && lastLineNum == pos.line then
         lines.dropRightInPlace(1)
 
+      val prefix = if tree `eq` trace.last then LAST_CHILD else CHILD
       lines += (prefix + line + "\n" + positionMarkerLine)
 
       lastLineNum = pos.line