You could read "Rust will become mandatory" as "all contributors will need to be able to code Rust" or even "all new code has to be written in Rust" or similar variations
I see. No, I understood it the way it is, as introducing it as a new hard dependency in git 3. I suppose it is a pilot for making it mandatory for contributions / incrementally replacing the existing code in the future, though.
Git is pretty modular, and it already includes multiple languages. I guess that significant parts of it will remain in C for a long time, including incremental improvements to those parts. Though it wouldn't surprise me if some parts of git did become all-Rust over time.
My last company used Jenkins, so our build infrastructure depended on Java. We used zero code outside of supporting Jenkins. So Java was required to build our stuff, but not to write or run it.
Edit: nope, I’m wrong. On reading the link, they’re setting up the build infrastructure to support Rust in the Git code itself.