[Nemo157]

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.

[mlindner]

Every single function call is using an interface. Your public API is not the only interface.

It's an extremely bad habit to pick up as this ruins one of the main purposes of Rust, namely it's ability to allow collaboration and while not causing undefined behavior. Any sort of internal "don't do this or it'll cause undefined behavior" is just falling back to C/C++ land.

Some developer later will modify that code later, maybe even the same developer and will forget about the invariants that need to be held if they're not documented in unsafety comments around unsafe code. Anything that relies on code review to catch unsoundness outside of unsafe code is automatically wrong.