Their ring logger was really enlightening to me about the value of Rust enums. Heterogeneous log events are dropped into the ring with some holding only the fact that something happened, and others holding additional data about what happened. Then Humility is able to print out the contents of the ring either online or in crash dumps. This is how you get logging in nostdlib Rust without ending up without half of a badly implemented printf. Instead, Humility, which has the full stdlib available, formats the enums for the firmware.