I think sticking close to the python version makes sense for the initial version. Then improvements can be made in follow-ups.

Overall looks good to me. Just a couple of points.

The latest changes are looking really good. I have a couple more comments but I didn't have time for a full review. I'll try to get more reviewed tomorrow. It seems that you still have a lot of stuff still in-flight so I'll try to slowly review the changes as I have time. If you want input/feedback on any particular part just ask and I will prioritize it.

Ah, I forgot to consider the python interop. Now the need for that crate makes sense. Thanks for explaining.

I'm not a windows expert but it seems like the rust OsStr, Path and filesystem APIs should handle these conversions for you. I think the only place where you would need to do os-specific code is when doing serialization and serialization which I think should be handled by https://doc.rust-lang.org/std/os/unix/ffi/trait.OsStringExt.html and https://doc.rust-lang.org/std/os/windows/ffi/trait.OsStringExt.html.

Rust has platform independent types PathBuf and &Path for paths and OsString and &OsStr for strings (owned and references respectively. They do have os-specific extensions but as long as you don't use them it should be cross platform. That being said, if you are serializing and deserializing them you may need to write some platform dependant code.

In D2057#43987, @durin42 wrote:

Mercurial tries to be principled about always treating filenames as bytes. AIUI https://www.mercurial-scm.org/wiki/WindowsUTF8Plan is still the plan of record there?

I will try to finish the review later, but it might be quicker if you incorporate some of the changes first since a lot of them are repeated many times. Overall it looks good, there are a couple of things that i would highlight to make the code easier to read.

I agree with the splitting comments :) In fact there might already be a base85 crate which can be used: https://docs.rs/zero85. Either way I'll hold off on the review, feel free to ping me when you are ready for me to take a look.

Overall it looks great. I added a bunch of nitpicks. The most common suggestion was to add some contextual information to error messages. Other then that mostly minor style improvements. Feel free to push back on anything you don't agree with :)