Converting the entire codebase to `private[this]` might not be an unreasonable thing to do.  Only in very rare instances does `private[this]` not work where plain `private` does (it wouldn't surprise me if every instance of `private` in Spark could safely be made `private[this]`), and it can provide some performance benefits (not everything that you would expect to get inlined away actually does), so arguably `private[this]` should be the default instead of `private`.

If someone has a few spare cycles, doing the search and replace and running the spark-perf numbers could at least give us some idea of how much we are giving away by using the slightly simpler syntax.  Digging out just when `private[this]` makes a difference would be more work, and maybe not worth doing.