Could you borrow a bit?

bascule:

A particularly fun question for bitstring reference type libraries like what I'm writing is: is how do you implement split_at_mut?

I think it should be possible by not restricting ourselves to relying on references.

As I wrote in the other discussion, a quasi-reference type like this could work:

struct BitSliceMut<'a> {
    origin_ptr: *mut u8,
    bit_offset: usize,
    bit_len: usize,
    _pd: PhantomData<&'a mut u8>,
}

By keeping it non-Send it should be possible to construct a safe API for creating (e.g. with split_at_mut) and manipulating such type.

Yes, there is a number of unfortunate ergonomic issues with such custom reference types, but IMO it's the most practical option right now.