Builder patterns are also super helpful for libraries. Allows you to create APIs with lots of optional fields without having call sites that pass in a dozen None values.

There's also the type state pattern where you have a struct take a generic so you can allow certain methods depending on the generic type. Like say you have a struct with a field that you want to be optional, but you know statically that at a certain point in the code it'll be set.

You can do:

    struct User<IdType> {
       id: IdType
    }

    impl User<()> {
       fn create() -> User<u64> {
         ...
       }
    }

    impl User<u64> {
       fn id() -> u64 {
         user.id
       }
    }

That way you can have some methods where you can be certain that the ID is set and some methods where it's not set.