It looks like in their last sprint meeting (end of Sept[0][1]) there was a lot of planning and talk about moving parts of mercurial to rust. From the history of sprints[2] it sounds like facebook first started experimenting with some mercurial implementations in rust and may be one of the big contributors spearheading this (though I also see indygreg and durin42 around the phabricator projet giving mercurial advice). I'm a fan of both rust and mercurial so this is exciting news to hear.

[0] https://www.mercurial-scm.org/wiki/4.4sprint#Rust

[1] https://public.etherpad-mozilla.org/p/sprint-hg4.4-NOSPAMREM... (remove everything after-and-including the last hyphen, I left it in since it seems like they don't want a direct link that's easily scraped?)

[2] https://www.mercurial-scm.org/wiki/4.0sprint