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Comment by 7e | Hacker News Reader

Allocating memory with C and freeing it with Rust is silly. If you want to free a C-allocated pointer in Rust, just have Rust call back in to C. Expecting that allocators work identically in both runtimes is unreasonable and borderline insane. Heck, I wouldn't expect allocators to work the same even across releases of libc from the same vendor (or across releases of Rust's std).

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I don't agree with your contemptuous framing. It's incorrect, and per the post's author, "dangerous" — but depending on your background it's not "silly" or "borderline insane". It's just naive, and writing a slab allocator as an exercise or making honest explorations like in this blog post will help cure the naivete.

It’s undefined behavior. It will never be stable. Investigating every permutation of zero-utility undefined behavior in the universe is borderline insane. Will the author next investigate exactly how a 2002 Fiat becomes inoperable after a head on collision with a 2025 Volkswagen? These are all deep dives into infinite chaos.

I agree that we shouldn't mix allocators, and so does the post's author. Can you put your technical arguments into terms which aren't so dismissive of people's honest efforts to learn? We ought to be all on the same page. You could affirm and refine the post's conclusions and bring us together, rather than ridicule someone for ever entertaining a notion you consider "insane".

Here's what I got when I asked ChatGPT to rewrite your first comment to be as constructive as possible:

"Totally agree — relying on C and Rust to interoperate at the allocator level is risky at best. Allocating in C and freeing in Rust (or vice versa) assumes a level of compatibility that just isn’t guaranteed. Even within a single ecosystem, allocator behavior can change across versions — whether it’s different versions of libc or updates to Rust’s std. So expecting consistent behavior across language boundaries is, at the very least, unreliable. If you need to free a C-allocated pointer, the safest and cleanest approach is to call back into C to do it."

That's not a drop-in substitute for your original comment and "totally agree" is over-the-top cloying in the usual ChatGPT obsequious way, but I still think it's helpful in suggesting alternative framings.

usually when interfacing with a library written in c the library will export functions for object destruction. it makes sense for that to be part of the interface instead of using the system allocator because it also gives the library freedom to do extra work during object destruction. if you have simple objects then its possible to just use the system allocator, but if you have graphs or trees of objects then its necessary to have a custom destroy function and there is always some risk in the future you might be forced to need to allocate more complex data structures that require multiple allocations.

The article is about how and why mixing allocators fails, not if it fails or how to fix the problem.

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