Do we construct a SyscallReturn from C at all? I don't see any, but just want to make sure we're not doing a SyscallReturn ret = { 0 }; or something that would cause issues for rust.

Yeah, I don't think so. I created the helper functions for safely creating the union variants and switched all the constructions over to that a while ago.

I think technically SyscallReturn ret = {0} would currently be ok, since the enum tags start at 0, but would certainly be better to avoid.