For what it's worth, the Oxide Computing people have written illumos driver(s) in rust: https://github.com/oxidecomputer/opte/tree/master/xde

Illumos is based on Solaris, not BSD.

I believe it's a microkernel, so in the sense that every driver is a separate userspace process, yes.

That’s not true. BSD (including all the existing flavors/descendants AFAIK) has a monolithic kernel.

Perhaps you’re confusing it with XNU? (Which is Mach merged with some BSD stuff).