Nothing. You can always vibe-code in Rust even when the rust-lang/rust repository itself largely forbids vibe coding.
At this point they will probably just fork yet again and maintain some vibe compiler.
[0]: https://x.com/jarredsumner/status/2051600118886138262
This policy does not seem to forbid vibe coding?
> Solicited, non-critical, high-quality, well-tested, and well-reviewed code changes that are originally authored by an LLM are allowed, with disclosure.
Vibe coding (in its original meaning) would have hard time arguing it's of high quality.