You're right that this is generally a golden rule. But rules can have exceptions, and this seems to be one of them; the Linux kernel is now so large and complex, and C so obviously outdated now, that it's worth the pain to start writing drivers in Rust. And because of the modularity of the kernel, and the care taken to make Rust binary-compatible with C, this looks to be actually practical, as individual subsystems will be either entirely Rust or entirely C, particularly when new drivers are involved.