Add debug help to find out what's compiling

We sometimes see that tests take long time or loop infinitely. Then the first
problem is to find out which test caused this behavior. This can now be shown
by turning on a new flag `debugPrintProgress` in `Run.scala`. If that flag is
set to `true`, the compiler will print out each 10 seconds the list of
currently compiled source files.

compiler/src/dotty/tools/dotc/Run.scala

@@ -38,6 +38,7 @@ import Run.Progress
 import scala.compiletime.uninitialized
 import dotty.tools.dotc.transform.MegaPhase
 import dotty.tools.dotc.transform.Pickler.AsyncTastyHolder
+import java.util.{Timer, TimerTask}
 
 /** A compiler run. Exports various methods to compile source files */
 class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with ConstraintRunInfo {
@@ -382,7 +383,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
         initializeAsyncTasty()
       else () => {}
 
-    runPhases(allPhases = fusedPhases)(using runCtx)
+    showProgress(runPhases(allPhases = fusedPhases)(using runCtx))
     cancelAsyncTasty()
 
     ctx.reporter.finalizeReporting()
@@ -433,6 +434,26 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
       process()(using unitCtx)
     }
 
+  /** If set to true, prints every 10 seconds the files currently being compiled.
+   *  Turn this flag on if you want to find out which test among many takes more time than
+   *  the others or loops infinitely.
+   */
+  private inline val debugPrintProgress = false
+
+  /** Period between progress reports, in ms */
+  private inline val printProgressPeriod = 10000
+
+  /** Shows progress if debugPrintProgress is true */
+  private def showProgress(proc: => Unit)(using Context): Unit =
+    if !debugPrintProgress then proc
+    else
+      val watchdog = new TimerTask:
+        def run() = println(i"[compiling $units]")
+      try
+        new Timer().schedule(watchdog, printProgressPeriod, printProgressPeriod)
+        proc
+      finally watchdog.cancel()
+
   private sealed trait PrintedTree
   private /*final*/ case class SomePrintedTree(phase: String, tree: String) extends PrintedTree
   private object NoPrintedTree extends PrintedTree