You can write a patch for the Linux network stack in any language you want. Whether Linus will merge it is a different matter. :P

EDIT: Downvotes eh? Then I suppose I'll state outright what I left implicit: Linux is not inextricably tied to any single language, and it's a strawman to suppose that any attempt to integrate Rust with Linux would first have to reinvent the universe from scratch. Rust has excellent interoperability with C (though, notably, unions are iffy), and it's common to write Rust code that gets called from C with C being none the wiser.