rustup override set nightly. The directory is stored in ~/.rustup/settings.toml (in overrides section) separately from the project itself.cargo tree - more details here (works starting from Rust 1.44)move |...| is sometimes needed and why we have to add lifetimes annotations like where F: Fn(i32) -> i32 + 'a>#[lang] annotation. I really enjoyed this article, but for some reason it was quite hard to google to find it again (perhaps because I googled "traits" and "lang" is not a very googleable term).