According to this article in Mercurial wiki, its core is being re-written in Rust. Mercurial is one of the largest open-source Python projects in existence.
This does not however mean everything will be rewritten in Rust. (Yes, this post’s heading’s not entirely accurate.)
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hgis a Rust binary that embeds and uses a Python interpreter when appropriate (hgis a Python script today)* Python code seemlessly calls out to functionality implemented in Rust
The obvious reason for the decision is startup performance concerns, but the article also (interestingly) states:
In addition to performance concerns, Python is also hindering us because it is a dynamic programming language. Mercurial is a large project by Python standards. Large projects are harder to maintain. Using a statically typed programming language that finds bugs at compile time will enable us to make wide-sweeping changes more fearlessly. This will improve Mercurial’s development velocity.
Git was also designed with a core written in C and a host of shell scripts that call the core for additional functionality. For example, rebase is actually a shell script.
However, usage of shell scripts has made porting Git to Windows a pain. See the Lessons Learned section in this AOSA article.