Unsafe code in rust

As a side project, I had the idea of writing an alternative implementation of core and std (I just want to learn how they work inside and get a feel of it).

However I have seen that unsafe rust is frowned upon (maybe my impression so please correct me if I am wrong) and rewriting these libraries is quite literally impossible without unsafe.

So am I allowed to use them here under the reason that these libraries can't be written without unsafe?