> There are some APIs like this but last time I checked they didn’t cover all usecases.

This refers to[1]

    <&mut [T]>::split_at_mut(usize) -> (&mut [T], &mut [T])`

used as

    let (first_42, rest) = mutable_slice.split_at_mut(42);

but you can see that the inner logic is just a bit of pointer twiddling[2]

        let len = self.len();
        let ptr = self.as_ptr();

        // SAFETY: Caller has to check that `0 <= mid <= self.len()`
        unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid)) }

[1]: https://doc.rust-lang.org/std/primitive.slice.html#method.sp...

[2]: https://doc.rust-lang.org/src/core/slice/mod.rs.html#1641-16...