Ultimately all software has to touch hardware somewhere. There is no way to verify that the hardware always does what it is supposed to be because reality is not a computer. At the bottom of every dependency tree in any Rust code there always has to be unsafe code. But because Rust is the way it is those interfaces are the only places you need to check for incorrectly written code. Everywhere else that is just using safe code is automatically correct as long as the unsafe code was correct.