Do not store context creation trace in -Ydebug

This makes the compiler unusably slow.

Author: smarter

src/dotty/tools/dotc/core/Contexts.scala

@@ -249,25 +249,6 @@ object Contexts {
     final def withPhaseNoLater(phase: Phase) =
       if (ctx.phase.id > phase.id) withPhase(phase) else ctx
 
-    /** If -Ydebug is on, the top of the stack trace where this context
-     *  was created, otherwise `null`.
-     */
-    private var creationTrace: Array[StackTraceElement] = _
-
-    private def setCreationTrace() =
-      if (this.settings.debug.value)
-        creationTrace = (new Throwable).getStackTrace().take(20)
-
-    /** Print all enclosing context's creation stacktraces */
-    def printCreationTraces() = {
-      println("=== context creation trace =======")
-      for (ctx <- outersIterator) {
-        println(s">>>>>>>>> $ctx")
-        if (ctx.creationTrace != null) println(ctx.creationTrace.mkString("\n"))
-      }
-      println("=== end context creation trace ===")
-    }
-
     /** The current reporter */
     def reporter: Reporter = typerState.reporter
 
@@ -391,7 +372,6 @@ object Contexts {
       this.implicitsCache = null
       this.phasedCtx = this
       this.phasedCtxs = null
-      setCreationTrace()
       this
     }