Is it not possible to build a wrapper that does this? It seems like it should be.
For comparison, while Rust offers a very rich string library, it's also very strict about what you can/cannot do with strings, so if your use case falls outside of that you're out of luck. With Zig, you can pretty easily roll your own and make it do what you want. (and when Zig is post 1.0, I imagine there will be some very nice pre-made string libraries by the community etc.)
In C you have pain. Does the caller allocate a buffer? How does it know how big to make it? Do you have to have separate calls to calculate the required length? Etc. Can Zig work like Rust/C++ and not like C? My impression is that it can't.
Apparently Zig doesn't have a global allocator so it seems like you can't. (And it seems like the allocation details aren't encoded in the type anyway so it will be a disaster if you start mixing up global and non-global allocator types anyway.)