Fun milestone: I now have fully green CI on a branch in which options parsing is done by both the Python and Rust parsers, with an error thrown if they are not identical.

I've been trying to grok this from the sidelines. Is the plan here to move away from Python's implementation?

Eventually, yes

Gotcha, that's what I was thinking - just wasn't sure why we'd error the parser instead of logging in your note above. Does this change how the python side consumes options though (e.g. thinking about shell completions)

We wouldn't force that error on users, that's for my debugging use

👍

Here's the big one! https://github.com/pantsbuild/pants/pull/20887