This is arguably the most important lint of all, and it's a pity that it's allow by default. Whenever we create an API, we must document what the API does. This will make it much, much easier for new developers to use it. The #![warn(missing_docs)] lint will make sure that at least all of your public API has some documentation. I personally usually have this as a warning during the development and change it to deny or even forbid once the project enters into production.