yes, i don't think the desire to move directory contents was at issue,
for that and other examples.
the question was whether to implement an optimisation for the
case where source and target names are on the same file store.

since in both the Plan 9 and Unix cases, it's necessary to copy,
plan 9 settles for renaming files and directories (in their containing directories),
but does not allow either files or directories to be moved without copying.
(depending on what the move operation means in the presence of
arbitrary per-process binds and mounts, it might be impossible to
provide a sensible definition of move from a user's point of view anyway.)

some versions of unix attempt to allow directories to be moved,
that typically doesn't work across file systems, so mv ends
up copying anyway, doing pretty much what plan 9 would do.
on unix, i might like to do {mv /tmp/superbigvideofile $home/videofarm/birthday}
without having the system copy a big file, but that usually doesn't work.
so on unix i probably would do {recordvideo $home/videofarm/tmp.$pid}
and rename it later, which is how i'd get round the problem on plan 9.