As it stands, this crate appears to make separate fields each with their own Vec. This would duplicate the length and capacity values for each field. This may not be a huge problem, but it would triple the size of the struct as it grows and can lead to the different Vecs falling out of sync.

The main alternative would be to use unsafe and raw pointers (or NonNull pointers). That said, managing the unsafe would almost certainly be more effort than keeping the Vecs in sync. Regardless, I think this could be a useful discussion to be had. Is cutting the struct to a third worth having unsafe code to vet? (probably not)