> If you’re having trouble designing a safe interface for your collection then that should be a signal that maybe what you are doing will result in UB when looked at the wrong way.

Rust is great, but there are some things that are safe (and you could prove them safe in the abstract), but that you can't easily express in Rust's type system.

More specifically, there are some some things and usage pattern of these things that are safe when taken together. But the library can't force the safe usage pattern on the client, with the tools that Rust provides.

If you can't create a safe interface and must have the function then create an unsafe function and clearly document the invariants and then rely on the user to uphold them?

Just take a look at the unsafe functions for the standard library Vec type to see examples of this:

https://doc.rust-lang.org/std/vec/struct.Vec.html#method.fro...