There's actually a good example of this in the rewrite [1], in `PathString::slice`. They are doing an unsafe operation to return a slice that could be a use-after-free, if the caller had not already guaranteed that an invariant will remain true. Following proper rust idiomatic practices, claude has added a SAFETY comment to the unsafe block to explain why it's safe: "caller guarantees the borrowed memory outlives this".
Now, normally, you'd communicate this contract to your API users by marking the type's constructor (PathString::init) as "unsafe", and including the contract in its documentation. Unfortunately in this case, this invariant does not exist - it appears to have been fabricated out of thin air by the LLM [2]. So, not only does this particular codebase have UB problems caused by unsafe code, the SAFETY blocks for the unsafe code are also, well, lies.
Now, normally, you'd communicate this contract to your API users by marking the type's constructor (PathString::init) as "unsafe", and including the contract in its documentation. Unfortunately in this case, this invariant does not exist - it appears to have been fabricated out of thin air by the LLM [2]. So, not only does this particular codebase have UB problems caused by unsafe code, the SAFETY blocks for the unsafe code are also, well, lies.
[1] https://github.com/oven-sh/bun/blob/63035b3e37/src/bun_core/...
[2] https://github.com/oven-sh/bun/blob/63035b3e37/src/bun_core/...