Improve pickling of holes

 - Avoid using a mismatched sourcefile/span compbination for a hole filling
 - This can be achieved by always pickling the position of holes.

Author: odersky

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

@@ -62,7 +62,10 @@ class PositionPickler(pickler: TastyPickler, addrOfTree: untpd.Tree => Option[Ad
           | _: Trees.DefTree[_]
 
           // package defs might be split into several Tasty files
-          | _: Trees.PackageDef[_] => true
+          | _: Trees.PackageDef[_]
+          // holes can change source files when filled, which means
+          // they might lose their position
+          | _: TreePickler.Hole => true
       case _ => false
     }
 

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

@@ -1290,10 +1290,14 @@ class TreeUnpickler(reader: TastyReader,
       }
       // We need to make sure a hole is created with the source file of the surrounding context, even if
       // it filled with contents a different source file. Otherwise nodes containing holes might end
-      // up without a position. TODO: It would probably more systematic to always create an Inlined argument node
-      // at the position of the hole.
+      // up without a position. PositionPickler makes sure that holes always get spans assigned,
+      // so we can just return the filler tree with the new source and no span here.
       if (filled.source == ctx.source) filled
-      else filled.cloneIn(ctx.source)
+      else {
+        val filled1 = filled.cloneIn(ctx.source)
+        filled1.span = NoSpan
+        filled1
+      }
     }
 
 // ------ Setting positions ------------------------------------------------