Drop unnecessary compatibility hack

... and bump major Tasty version

Also: drop unused method

Author: odersky

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -978,8 +978,6 @@ object Trees {
       protected def finalize(tree: Tree, copied: untpd.MemberDef): copied.ThisTree[T] =
         postProcess(tree, copied.withSpan(tree.span).withAttachmentsFrom(tree))
 
-      protected def srcCtx(tree: Tree)(implicit ctx: Context) = ctx.withSource(tree.source)
-
       def Ident(tree: Tree)(name: Name)(implicit ctx: Context): Ident = tree match {
         case tree: BackquotedIdent =>
           if (name == tree.name) tree

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

@@ -244,7 +244,7 @@ Standard Section: "Comments" Comment*
 object TastyFormat {
 
   final val header: Array[Int] = Array(0x5C, 0xA1, 0xAB, 0x1F)
-  val MajorVersion: Int = 11
+  val MajorVersion: Int = 12
   val MinorVersion: Int = 0
 
   /** Tags used to serialize names */

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

@@ -111,16 +111,9 @@ class TreeUnpickler(reader: TastyReader,
     val owner = ctx.owner
     val source = ctx.source
     def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
-      val sourceToUse = if (source.exists) source else ctx.source
-        // TODO: remove. This is a hack to get pickling tests (notable tuple-cons-2.scala
-        // to pass. The problem is that without a bootstrapped library some unpicked
-        // files are lacking a source, and if these are completed from Definitions
-        // there's also no external source given in `context`. In that case we switch
-        // late and pick the calling context's source. We can drop this hack once
-        // all Tasty trees have a SOURCE entry, which will determine the context source
       treeAtAddr(currentAddr) =
         new TreeReader(reader).readIndexedDef()(
-          ctx.withPhaseNoLater(ctx.picklerPhase).withOwner(owner).withSource(sourceToUse))
+          ctx.withPhaseNoLater(ctx.picklerPhase).withOwner(owner).withSource(source))
     }
   }