Since unsoundness is a property of an interface, that private safe function would be unsound but could be used as part of the implementation details of a sound public API.
It's not something I would do personally, and would be something I'd put down as a black mark against using a library as a dependency, but it's nowhere near as bad as doing the same thing on a public API.
It's not something I would do personally, and would be something I'd put down as a black mark against using a library as a dependency, but it's nowhere near as bad as doing the same thing on a public API.