"Trying to represent this API in Rust just doesn't work. The language never lets mutable references alias each other,"

That was my first thought too. I mean, the caller of this thing knows whether it's the same buffer or not. In-place modification in general allows for different optimizations than knowing that two regions don't alias.

IMHO the more apt comparison isn't greenfield vs. brownfield but, quite plainly, a culture where the C[++] users see the new kid on the block as an opportunity to clean up their own act i.e. their own interfaces, thus improving the C side of the codebase as well … or not.

Since the "or not" side is already served perfectly well by simply continuing to use C, for some value of "perfectly well" anyway. I kindof doubt that Carbon will get much traction. Which is for the best really (again IMHO): I'd rather see one effort to replace C++ with something sane+safe actually succeed, long-term, than three such projects, all with somewhat-conflicting goals and all fighting for developer mindset, not to mention time.