Backport "Make sure that the stacktrace is shown with -Ydebug-unpickling" to LTS (#20801)

Backports #19115 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

compiler/src/dotty/tools/dotc/core/TypeErrors.scala

@@ -22,7 +22,11 @@ abstract class TypeError(using creationContext: Context) extends Exception(""):
    *  This is expensive and only useful for debugging purposes.
    */
   def computeStackTrace: Boolean =
-    ctx.debug || (cyclicErrors != noPrinter && this.isInstanceOf[CyclicReference] && !(ctx.mode is Mode.CheckCyclic))
+    ctx.debug
+    || (cyclicErrors != noPrinter && this.isInstanceOf[CyclicReference] && !(ctx.mode is Mode.CheckCyclic))
+    || ctx.settings.YdebugTypeError.value
+    || ctx.settings.YdebugError.value
+    || ctx.settings.YdebugUnpickling.value
 
   override def fillInStackTrace(): Throwable =
     if computeStackTrace then super.fillInStackTrace().nn
@@ -188,3 +192,14 @@ object CyclicReference:
     ex
 end CyclicReference
 
+class UnpicklingError(denot: Denotation, where: String, cause: Throwable)(using Context) extends TypeError:
+  override def toMessage(using Context): Message =
+    val debugUnpickling = cause match
+      case cause: UnpicklingError => ""
+      case _ =>
+        if ctx.settings.YdebugUnpickling.value then
+          cause.getStackTrace().nn.mkString("\n    ", "\n    ", "")
+        else "\n\nRun with -Ydebug-unpickling to see full stack trace."
+    em"""Could not read definition $denot$where. Caused by the following exception:
+        |$cause$debugUnpickling"""
+end UnpicklingError

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -128,12 +128,7 @@ class TreeUnpickler(reader: TastyReader,
         def where =
           val f = denot.symbol.associatedFile
           if f == null then "" else s" in $f"
-        if ctx.settings.YdebugUnpickling.value then throw ex
-        else throw TypeError(
-          em"""Could not read definition of $denot$where
-              |An exception was encountered:
-              |  $ex
-              |Run with -Ydebug-unpickling to see full stack trace.""")
+        throw UnpicklingError(denot, where, ex)
       treeAtAddr(currentAddr) =
         try
           atPhaseBeforeTransforms {