[jcranmer]

C takes backwards compatibility quite seriously, so anything it adds has to be opt-in (not that it stops some people from trying to propose seriously breaking changes, but c'est la vie).

Something like a borrow checker can be added (and there are people on the C committee willing to see it). However, Rust's "shared xor mutable" rules are probably too strong for C, so you'd need to find some weaker model that solves a good deal of memory safety issues (maybe expressing pointer ownership and some form of invalidation is sufficient, but this is just some spitballing by me). The focus by the interested people right now is mostly around bounds-checking rather than a borrow checker.