- simple bug where when clearing the IC I forgot to clear
  the old list of gc references.
- keep gc references alive during the duration of the rewrite.
  There are some cases where the thing we are guarding on would
  normally get destroyed by the end of the operation.
  So make the ref an owned ref.  But at the end if we see
  that we hold the only reference to the object, abort the rewrite
  (since the guard could never pass in the future)