It is not possible to prentend the emerge of the rsync package because it is an emerge keyword as well. This bug can may be reproduced with other CLI arguments as well.
Steps to Reproduce:
# emerge -p rsync
emerge: the "sync" action does not support "--pretend."
I expected rsync to be installed ...
Try "emerge -p net-misc/rsync" (without the quotes)
*** This bug has been marked as a duplicate of 2365 ***