From a cursory look, this is one of the most unsafe pieces of code I have seen, with complete disregard to memory alignment requirements and the lifetime of temporary objects passed as arguments to functions.
> It might be better to try Cello out on a hobby project first. Cello does aim to be production ready, but because it is a hack it has its fair share of oddities and pitfalls, and if you are working in a team, or to a deadline, there is much better tooling, support and community for languages such as C++.
The author has already made it clear they consider "memory allocation by unaligned offset into temporary char[] cast into a struct pointer" to be a valid strategy, so frankly I'm not very interested in their opinions on whether it's production ready.
I've seen it on HN before, whenever this project gets mentioned. People who don't know much about C confuse it for a really cool thing you can do with C, as if it's just another legit library that you can pick up and use. It's a lot of undefined behavior. People have enough problems writing safe C as it is, and on top of this complaint about alignment and misuse of temporaries, this thing makes the problem worse in other ways too, removing the few safeguards that exist by treating everything as void* for instance.
Well it shouldn't be used for toy projects either with all of that hackery that it pulls (even if you don't use atomics, violating alignment requirements alone means that your code can never make use of vector instructions).
The new code on GitHub makes it clear that this library is not about fat pointers, but about writing unsafe C in another language, powered by undefined behaviour and the glory of the C preprocessor.
If you want to have safe arrays in C, just use a proper library for that instead of going the route of abusing undefined behaviour. There are plenty of libraries to choose from: https://github.com/search?l=C&q=vector+array&type=Repositori... (not all of them are serious projects, so you should be careful when you pick one. I personally have been using this one: https://github.com/iscgar/cvec2 because I like the easy interface it provides and especially the fact that type safety is a top priority, but other may have different tastes and priorities).
Definitely don't use this in production code.