From its timid introduction to the Mercurial Version Control System back in 2017 to its more than 50k lines of code today, Rust has enabled a wide range of improvements, some of which we wager would have been impossible if not for Rust.

This talk shows how we reach far beyond the obvious point of "Rust runs faster than Python". It discusses aspects like maintainability, dependency management, API re-designs, opportunities for more advanced algorithms, on disk data-structures, safe parallelism, etc.

We present our rare perspective of working on a 20 year-old codebase with half-a-million lines of Python code, in a software niche with quite extreme goals. Mercurial aims to provide instant-feeling commands with short lived processes for a local database of tens of millions of revisions for millions of files with fully distributed replication.