rustup override set nightly. The directory is stored in
overridessection) separately from the project itself.
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).