When attempting to use rsync with a URL such as that shown below which uses "rsync:" as a protocol prefix and "::" as a module separator the error shown beneath is produced:
rsync -a rsync://rsync.mad-hacking.net::hacking-gentoo-overlay ~/test/
ssh: Could not resolve hostname rsync: Name or service not known
Using the "/" as a module separator, or omitting the rsync: protocol prefix, as shown below, works normally.
rsync -a rsync://rsync.mad-hacking.net/hacking-gentoo-overlay/ ~/test/
rsync -a rsync.mad-hacking.net::hacking-gentoo-overlay ~/test/
The last two syntaxes are described in the man-page and of course they work.
The first syntax and anything else you invent will of course not work.
I see no bug.
Fair enough - the command in question was being generated by some other software. If you do a quick Google it would seem it is not alone in generating such malformed code.
At first glance the man page doesn't seem to say it is invalid although closer inspection does seem to imply that I agree. Perhaps better wording of the examples section would make it clearer?