Force TASTY trees read by unpickler in FromTasty

We want to ensure that the whole tree is read at phase
frontend. To achieve this, we run an empty traverser over the tree.

Author: odersky

src/dotty/tools/dotc/FromTasty.scala

@@ -15,6 +15,7 @@ import util._
 import Decorators._
 import dotty.tools.dotc.transform.Pickler
 import pickling.DottyUnpickler
+import ast.tpd._
 
 /** Compiler for TASTY files.
  *  Usage:
@@ -57,6 +58,10 @@ object FromTasty extends Driver {
     override def toString = s"class file $className"
   }
 
+  object force extends TreeTraverser {
+    def traverse(tree: Tree)(implicit ctx: Context): Unit = traverseChildren(tree)
+  }
+
   class ReadTastyTreesFromClasses extends FrontEnd {
     override def runOn(units: List[CompilationUnit])(implicit ctx: Context): List[CompilationUnit] =
       units.map(readTASTY)
@@ -80,6 +85,7 @@ object FromTasty extends Driver {
                     val (List(unpickled), source) = unpickler.body(readPositions = true)
                     val unit1 = new CompilationUnit(source)
                     unit1.tpdTree = unpickled
+                    force.traverse(unit1.tpdTree)
                     unit1
                   case _ =>
                     cannotUnpickle(s"its class file ${info.classfile} does not have a TASTY attribute")