Description:
The sysuser.conf for the prosody package creates a "prosody" user. By default the server will run under this user. Since it forks, it creates a PID file in /run/prosody. The server doesn't run as root, so systemd-tmpfiles is used to setup /run/prosody, though the prosody.tmpfile.d file is:

d /run/prosody 0755 412 412

Note the usage of "412" for UID and GID, wich I assume should be the name of the prosody user, but that is not guaranteed, as it is created by name in sysuser.conf.

In this configuration, the server won't start. When I try to run systemd-tmpfiles manually, I get:

# systemd-tmpfiles --create /usr/lib/tmpfiles.d/prosody.conf
[/usr/lib/tmpfiles.d/prosody.conf:1] Unknown user '412'.

And so, prosody fails to start (log in reversed order):

Feb 12 19:13:43 wilhelm.re prosody[392]: general: Shutdown complete
Feb 12 19:13:43 wilhelm.re prosody[392]: general: Shutdown status: Cleaning up
Feb 12 19:13:43 wilhelm.re prosody[392]: general: Shutting down...
Feb 12 19:13:43 wilhelm.re prosody[392]: general: Shutting down: Couldn't write pidfile
Feb 12 19:13:43 wilhelm.re prosody[392]: mod_posix: Couldn't write pidfile at /run/prosody/prosody.pid; /run/prosody/prosody.pid: No such file or directory
Feb 12 19:13:43 wilhelm.re prosody[392]: mod_posix: Successfully daemonized to PID 392
Feb 12 19:13:43 wilhelm.re prosody[390]: mod_posix: Prosody is about to detach from the console, disabling further console output

Additional info:
* prosody-0.9.7-2