What do you mean? I have the Rust working in Kbuild; I haven't published the patches. At Linux Plumber's conf for later this year, I've submitted a proposal to discuss the barriers to adoption within the kernel proper. Some very senior kernel devs are supportive.

You have it working, but if you're writing drivers and stuff the kernel might change out from under it and you'll have to continuously keep it up-to-date if it's not in-tree, right?

Yes; but what I'm talking about (Kbuild) is in-tree support for Rust. Bindings are regenerated when the kernel is built.

As far as "how the Linux kernel likes to work" either it's in tree, or it doesn't matter. (Not my opinion per se, IDC, but look at upstream's collective treatment of ZFS and other out of tree drivers in general; they remove APIs they know external modules are using). I do generally find out of tree drivers to be a PITA to build outside of the kernel, and sometimes find curious things in their build scripts. Updating NVIDIA drivers seems to be...not great.