Experiment with changing Buffer::parts_mut to Buffer::as_mut_ptr.

[sunfishcode]

Based on an idea suggested here, experiment with changing from a parts_mut that returns (*mut T, usize) to a as_mut_ptr which returns a *mut [MaybeUninit<T>].

[sunfishcode]

It appears pointer::len requires Rust 1.79.0. So we may not make this change in rustix for now, but we should still think about it for Buffer in general.

[sunfishcode]

I like this idea in theory, but in practice, it ends up making the code more verbose and repetitive. I'm now prototyping Buffer type changes in a standalone repo, where it has simplified the code to have separate getters for the pointer and length, because it's sometimes useful to be able to get the length without having a &mut self. So I'll close this for now.

Another idea here is that the subtle problem above with &mut [MaybeUninit<T>] could be avoided if we could instead use &[MaybeUninitCell<T>], where MaybeUninitCell would be a wrapper around a MaybeUninit which has an interor-mutability way to store initialized values. That way, users with a shared reference could initialize but not deinitialize the elements. That would fix the subtlety above. However, in practice, that wouldn't perform the bookkeeping to track how many elements have been initialized, so the resulting API still wouldn't be very ergonomic.