Fix #11885: Avoid non-determinism of idempotency tests on Windows

The cause of the problem is that we use parallelism in Pickler:

      if !Pickler.ParallelPickling || ctx.settings.YtestPickler.value then force()

Sometimes on Windows, the futures run a little slower, and the later
phase `Inlining` can change the positions of the trees to be pickled,
thus non-determinism.

Author: liufengyun

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

@@ -85,6 +85,21 @@ class CompilationUnit protected (val source: SourceFile) {
   def assignmentSpans(using Context): Map[Int, List[Span]] =
     if myAssignmentSpans == null then myAssignmentSpans = Nullables.assignmentSpans
     myAssignmentSpans
+
+  /** Force pickling at end of Pickler phase for determinism
+   *
+   *  On Windows we observe non-deterministic failure for the
+   *  idempotency test tests/pos/i6507b.scala. The cause of the
+   *  non-determinism is that the positions of the to be pickled
+   *  trees might change in a later phase `Inlining` after pickling.
+   *  Depending on how fast the futures are run, sometimes it may
+   *  observe the mutations in the later phase.
+   */
+  def forcePickling()(using Context): Unit =
+    pickled = pickled.map { (sym, fn) =>
+      val bytes = fn()
+      (sym, () => bytes)
+    }
 }
 
 object CompilationUnit {

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -117,6 +117,7 @@ class Pickler extends Phase {
             .addMode(Mode.ReadPositions)
             .addMode(Mode.ReadComments)
             .addMode(Mode.PrintShowExceptions))
+    units.foreach(_.forcePickling())
     result
   }