Simplify readTopLevel

Author: nicolasstucki

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

@@ -82,9 +82,7 @@ class TreeUnpickler(reader: TastyReader,
   /** The unpickled tree */
   def unpickle()(implicit ctx: Context): Tree = {
     assert(roots != null, "unpickle without previous enterTopLevel")
-    val rdr = new TreeReader(reader)
-    if (rdr.isTopLevel) rdr.readTopLevel().head
-    else rdr.readTerm()
+    new TreeReader(reader).readTopLevel()
   }
 
   class Completer(owner: Symbol, reader: TastyReader) extends LazyType {
@@ -851,19 +849,14 @@ class TreeUnpickler(reader: TastyReader,
       }
     }
 
-    def isTopLevel(implicit ctx: Context): Boolean =
-      nextByte == IMPORT || nextByte == PACKAGE
+    def isTopLevel(implicit ctx: Context): Boolean = nextByte == PACKAGE
 
-    def readTopLevel()(implicit ctx: Context): List[Tree] = {
-      @tailrec def read(acc: ListBuffer[Tree]): List[Tree] = {
-        if (isTopLevel) {
-          acc += readIndexedStat(NoSymbol)
-          if (!isAtEnd) read(acc) else acc.toList
-        }
-        else // top-level trees which are not imports or packages are not part of tree
-          acc.toList
-      }
-      read(new ListBuffer[tpd.Tree])
+    /** Read the top level of a tree.
+     *  It may be a package in case of class file tasty or a term if in a term.
+     */
+    def readTopLevel()(implicit ctx: Context): Tree = {
+      if (isTopLevel) readIndexedStat(NoSymbol)
+      else readTerm()
     }
 
     def readIndexedStat(exprOwner: Symbol)(implicit ctx: Context): Tree = nextByte match {