It's called "polonius": https://github.com/rust-lang/polonius

There are some posts on Niko Matsakis' blog, starting with this one: https://smallcultfollowing.com/babysteps/blog/2018/04/27/an-...

More recently a really good talk, with slides here: https://nikomatsakis.github.io/rust-belt-rust-2019/