@david:  sounds like you've never been bitten by intending a simple
rename or move presumably within a filesystem and seeing mv instead
launch into copying.  if i want a copy i'll use cp.  and i'll have a
look before i rm any original.  those safety checks you refer to are
almost entirely around the copy, which i don't even want.  rhel rename
comes with util-linux-ng, ubuntu rename comes packaged with perl,
neither manpage has the WARNING you mention.  reads like the author
had something ominous in mind but they don't say what.  would rename
consent to lose a directory within it's children?  you mean to suggest
mv protects you in some important way from something rename might do?
my understanding of mv is if rename(2) succeeds, it's done, but if
not, well that's when the trouble begins..

@munir:  and there is no reliable test for "same filesystem".  even
/proc/mounts is not always reliable, eg with persistence ala
overlayfs, or if /proc itself is unavailable, eg in a chroot