I think the way you do this is set things up so that no bits that are written in Rust are built by default, and make sure that the build system is set up such that Rust bindings for C code are only built when there's Rust code that's enabled that requires them.

Then sure, some people who download a kernel release might enable a Rust driver, and that makes the build fail. But until Rust is considered a first-class, fully-supported language in the kernel, that's fine.

In practice, though, I would expect that the Rust maintainers would fix those sorts of things up before an actual release is cut, after the 2-week merge window, during the period when only fixes are accepted. Maybe not every single time, but most of the time; if no one is available to fix a particular bit of breakage, then it's broken for that release. And that's fine too, even if it might be annoying to some users.