About memory alignment of container values

Hey @lemire,

I am currently in the process of greatly improving the deserialization algorithm of the roaring-rs library. The binary serialization format is quite good. It allows the Bitmap types to borrow a reference to the memory from which it deserializes.

However, there is a little problem, a memory alignment problem. In Rust (and other languages), it is undefined behavior to read a type that isn't well aligned in memory, e.g., reading a bitmap, aligned on 64bits, at address 33 (33 % 8 is 1, not 0, therefore unaligned).

I have two solutions to this problem in mind:

• The easy one could be to backup on copying the bytes of a container into an aligned buffer (a Vec) if those are not valid. This is a little bit frustrating as it could happen a lot.
• The alternative is more a question: Do you think that I could use the header offsets when serializing to force the containers to be correctly aligned, introducing some garbage bytes between containers? Or do I need to keep no space between contiguous containers?

Thank you!

Start by reviewing the roaring-rs deserialization algorithm and the binary format's header offsets, focusing on how borrowed container memory is aligned. Compare the proposed aligned-copy fallback with adding padding between containers. Done means the format specification documents a clear alignment decision and its serialization implications.