The end state described is one where Mercurial is written in Rust so that large scale changes can be made without touching any Python. 3rd party extensions may

The end state described is one where Mercurial is written in Rust so that large scale changes can be made without touching any Python. 3rd party extensions may still be permitted in Python. If that's not what the authors intend I'd suggest they rewrite the document to make clear what the quote above means to describe.