gsutil rsync supports sync files from source

Currently, gsutil rsync does not support using file name as src, will get an error like:
CommandException: arg (/medpop/esp/ESP_cohorts/du.txt) does not name a directory, bucket, or bucket subdir.

Is it possible to support file name as src, then it is a unique interface for files and folders, otherwise, we have to invoke the cp command separately.

Thanks much!
Wallace