feat: introduce stack-allocated PyBuffer

[winstxnhdw]

Summary

This PR implements a pinned stack-allocated PyUntypedBuffer variant, PyUntypedBufferView.

Closes #5836

[davidhewitt]

Thanks very much for this. Various thoughts around the accessor methods.

Also, out of scope for this PR, but I keep wondering if we should provide iterators for these structures. Especially with the strides / suboffsets etc, it's not necessarily trivial to get this right.

[winstxnhdw]

Also, out of scope for this PR, but I keep wondering if we should provide iterators for these structures. Especially with the strides / suboffsets etc, it's not necessarily trivial to get this right.

Yes, I think it would definitely be useful as well. I am not yet sure how this would look like though.

[davidhewitt]

Thanks for the continued work here, looking great!

Implementing Drop directly on PyUntypedBufferView is functionally equivalent to what I had in mind from the "drop guard", so gets a 👍 from me.

Afraid I had quite a few more thoughts around some of the edge cases (plus some ideas we can probably ignore). Hopefully helps us get to the right eventual abstraction!

[davidhewitt]

I wonder if there's some way with const generics that we could make a compile-time guarantee on the nullability of this field (and others).

Probably leads to a horrendously messy type system where in practice these null checks are not the bottleneck.

[winstxnhdw]

No, not at all. I agree with you. It's a great idea. Let me think about this.

[winstxnhdw]

Okay, I think I came up with something good, but it makes a lot of the other suggestions moot now.