It doesn't make sense to convert a C array to a Vec, the Vec type is a growable array but the C array isn't growable. It makes sense to convert to Rust's array type, which has a fixed size, and we realise there's a problem at API boundaries because C's arrays decay to pointers, so the moment we touch an API boundary all safety is destroyed.
Firstly, that's not an array. C has actual arrays, even though they decay to pointers at API edges and what you've made with malloc is not an array. I'll disregard C++ new and new[]
But also, it's definitely not a growable array. Box::new_uninit_slice makes the thing you've got here, a heap allocation of some specific size, which doesn't magically grow (or shrink) and isn't initialized yet.
