"Trying to represent this API in Rust just doesn't work. The language never lets mutable references alias each other,"

Thank you for the detailed example; it's always great to discuss concrete code. See my other comment for a response about the API, but for other readers following along at home:

Unless I'm misunderstanding something, these examples use *mut [u8] at the same time as a *const [u8], which I agree won't cause any problems. The part I thought would be broken is using a &mut [u8] at the same time as a &[u8] pointing to the same memory.

Here's one of your example tests lightly modified to use references instead of pointers:
https://play.rust-lang.org/?version=stable&mode=debug...

Running it via MIRI does show that it triggers undefined behavior.