What options do I have if my rear derailleur breaks when I'm out on a trail?
If you carry a multi-tool which happens to include a chain tool, you could also convert your bike to a single speed. If you do so, you would have to replace the chain afterwards as well (since you shorten it) but in some situations it could get you out of a rural area.

If you do so, carefully select the gear you would put it in, as you won't be able to shift anymore afterwards.

There is a nice video online illustrating the idea: how to do a roadside fix of a broken rear derailleur.
GCN has some more videos online explaining roadside repairs of various issues, which may also interest you. 

Additionally, I always take a missing link with me, as it allows you to get home after a broken chain. This speed link can also be used while converting your bike to a single speed.