The rust guys would have gained a lot more traction by just asking the C guys to keep a bunch of comments up to date detailing the semantics and error checking procedures, and promising to edit their rust API if the C code changes, but I suspect they didn't ask for that because they know that no guarantees come from a comment and they want to be sure that the rust code works across all the possible scenarios and in rust culture, that is always documented in the type system where it can be enforced.

I could be being daft but I thought this is more or less what the Rust guys were asking for. Tell us the current symantics of the system, and if it changes in future let us know what the new semantics are and we will fix the Rust code accordingly.

I do understand what you mean though about enforcing restrictions on what the C guys can do without breaking the Rust code. I think you run into situations wherever two languages meet. The way most projects handle this is the upstream releases a new version, or a release candidate of a new version with their breaking changes documented and then downstream updates their stuff accordingly when they get time. Obviously this is one project, but I imagine it's possible for the C guys to update stuff in a pull request and then drop an email in LKML to the Rust guys so they know stuff needs fixing. None of this seems that hard to me.

Ultimately though everything here is Linus decision. Either your in or your out. If Linus says yes to Rust doing whatever then that's what's going to happen. Likewise if he says no, then it's not going to happen that way. Until he weighs in no one can really say how this will end.

Personally though I disagree with the C guys. Safety features are important and should be used where it is practical to do so. Until now C has had the justification that it's still the fastest language and by a significant margin. Now a somewhat safe language like Rust exists with the same speed and capabilities I don't think we can afford to continue ignoring safety for the sake of a few bruised egos. If this was a proper industry like aviation safety would always come first, and if that means adopting new technologies and forcing people to adapt. I can understand if C devs have a hard time adapting, I don't expect it to happen overnight. The expectation though should be they should learn some Rust eventually, even if it's just enough to know the type signatures and what not that they might break with their changes to C code. Kernel devs are supposed to be some of the smartest computer people out there. If they can't learn even that small amount of another language then should they really still be kernel developers?