[scotty79]

Ok. So that's just triggering auto-insertion of &mut *

It's completely optional fearure. You don't have to rely on it. Like auto-insertion of semicolons in JS maybe it's better if you don't. Although likelihood of this biting you is way lower than in case of relying on semicolons autoinsertion.

[legerdemain]

It's "completely optional" in the sense that in several years of reading other people's Rust code, I have never seen anyone explicitly use it throughout a code base. It is utterly ubiquitous, rarely explained, and poorly (if at all) understood by a large majority of Rust users I've mentioned it to.

My claim is that for most Rust users, understanding the borrow checker is limited to a few heuristics that seem to work for them 80% of the time, and throwing up their hands and declaring an approach unworkable the remaining 20% of the time. To a first approximation, no one can mentally model the borrow checker beyond extremely simple examples of 4-5 lines of code.