Fix handling of empty package and ASTs name

Author: nicolasstucki

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

@@ -16,7 +16,7 @@ object DottyUnpickler {
   class BadSignature(msg: String) extends RuntimeException(msg)
 
   class TreeSectionUnpickler(posUnpickler: Option[PositionUnpickler], commentUnpickler: Option[CommentUnpickler])
-  extends SectionUnpickler[TreeUnpickler]("ASTs") {
+  extends SectionUnpickler[TreeUnpickler](TreePickler.sectionName) {
     def unpickle(reader: TastyReader, nameAtRef: NameTable): TreeUnpickler =
       new TreeUnpickler(reader, nameAtRef, posUnpickler, commentUnpickler, Seq.empty)
   }

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

@@ -34,7 +34,7 @@ class TastyPrinter(bytes: Array[Byte])(implicit ctx: Context) {
     unpickle(new CommentSectionUnpickler)
   }
 
-  class TreeSectionUnpickler extends SectionUnpickler[Unit]("ASTs") {
+  class TreeSectionUnpickler extends SectionUnpickler[Unit](TreePickler.sectionName) {
     import TastyFormat._
     def unpickle(reader: TastyReader, tastyName: NameTable): Unit = {
       import reader._

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

@@ -17,6 +17,8 @@ import printing.Texts._
 
 object TreePickler {
 
+  val sectionName = "ASTs"
+
   case class Hole(idx: Int, args: List[tpd.Tree]) extends tpd.Tree {
     override def fallbackToText(printer: Printer): Text =
       s"[[$idx|" ~~ printer.toTextGlobal(args, ", ") ~~ "]]"
@@ -25,7 +27,7 @@ object TreePickler {
 
 class TreePickler(pickler: TastyPickler) {
   val buf: TreeBuffer = new TreeBuffer
-  pickler.newSection("ASTs", buf)
+  pickler.newSection(TreePickler.sectionName, buf)
   import TreePickler._
   import buf._
   import pickler.nameBuffer.nameIndex

compiler/src/dotty/tools/dotc/fromtasty/TastyFileUtil.scala

@@ -6,7 +6,7 @@ import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.core.NameKinds
 import dotty.tools.dotc.core.Names.SimpleName
 import dotty.tools.dotc.core.StdNames.nme
-import dotty.tools.dotc.core.tasty.TastyUnpickler
+import dotty.tools.dotc.core.tasty.{TastyUnpickler, TreePickler}
 
 object TastyFileUtil {
 
@@ -25,14 +25,16 @@ object TastyFileUtil {
     val bytes = Files.readAllBytes(path)
     val unpickler: TastyUnpickler = new TastyUnpickler(bytes)
     val className =
-      unpickler.nameAtRef.contents.iterator.takeWhile {
+      unpickler.nameAtRef.contents.iterator.dropWhile {
+        name => name.toString == TreePickler.sectionName || name == nme.EMPTY_PACKAGE
+      }.takeWhile {
         case name: SimpleName => name != nme.CONSTRUCTOR
         case name => !name.is(NameKinds.ModuleClassName)
       }.collect {
         case name: SimpleName => name.toString
-      }.drop(1).toList
-
-    val classpath = path.toString.replace(className.mkString("", "/", ".tasty"), "")
+      }.toList
+    val classInPath = className.mkString("", "/", ".tasty")
+    val classpath = path.toString.replace(classInPath, "")
     (classpath, className.mkString("."))
   }