• make sure you have rsync installed
• start Qt Creator with a fresh settings path
• create a Remote Linux device for localhost in the settings, make sure that rsync is detected as working
• go to the kit settings, and create a kit for localhost as the run device (e.g. by cloning the desktop kit and setting the run device to "localhost")
• create a new console application and use the new "localhost" kit
• click "Run"

The message

Transfer method was downgraded from "rsync" to "generic file copy". If this is unexpected, please re-test device "localhost".

is printed and the generic deploy method is used, even though rsync is available on build & run device (which is the same).

After restarting Qt Creator and trying to run the project again, it uses rsync.

If you just start Qt Creator once with the clean settings path, and then close and restart with the same settings path and do the steps above, it also manages to use rsync, so for some reason it doesn't realize that the Desktop device supports rsync without a restart.