Just added this comment to the PR.

For the confused reader (including myself):
My patch to add libxo supprt to devinfo was added in c759aca606cee8352
and then later reverted in 43b07bdbc5d24febc7a
and then @kgalazka revived this thread with a concrete fix, but there is more to it

Hello! I haven't forgotten about this, I've just been fleshing out the code using it. At some point I realized that it's more appropriate to generate the artifacts this change adds (libcore.rlib and libcompiler-builtins.rlib for the kernel arch) as part of each kernel build using the sources in the rust compiler repo. My bindings are pretty much at a state where developers can try it out (and I have a virtio-snd driver demo anyone can already run in QEMU), but it would be nice to have a better solution for getting a rust compiler when building on FreeBSD hosts before announcing it on the mailing lists.

A clean merge of my patch toghether with this fix on top of e8d6b58ef5a4afe0 would
address the problems I'm aware of, and hopefully provide the feature as intended.
There's deeper issues being raised though, seemingly related to each other.

In D54301#1241028, @brooks wrote:

LGTM. Out of paranoia, I verified using compiler explorer that it's reliably const enough to use in a macro even at -O0 with an old clang.

A similar change has been included in the latest vendor import of mtree.
Thank you!
https://cgit.freebsd.org/src/commit/?id=f8cee1f2c2dfba6223385fd711cce9faeca76451

Approving as mentor, wait for @manu to re-review before commiting

I'd also suggest to change it to SYSCTL_BOOL.

Just reverted instead, "may be" removed in 15.0 is good enough.

I think those issues are not made any worse by your patch? And the fix is something that RISC-V and amd64 would need as well (well, RISC-V also needs this patch). I think you should just merge your current patch and let @markj fix both amd64 and aarch64 when he adds the new arena.

Enable building the driver as a module.

BTW, I've thought about adding an acpi_unregister_ioctl_all or the like that just takes the fn argument and unregisters all commands that use the same function. That would be a nicer way to handle unloading of a module that registers multiple handlers.