Another example, let's say you're parsing a subscriber name for an email list, and you have a few checks such as whether it's too long, has whitespace, orcontains forbidden characters:

    pub fn parse(s: String) -> SubscriberName {
        let is_empty_or_whitespace = s.trim().is_empty();

        let is_too_long = s.graphemes(true).count() > 256;

        let forbidden_characters = ['/', '(', ')', '"', '<', '>', '\\', '{', '}'];

        let contains_forbidden_characters = s
            .chars()
            .any(|g| forbidden_characters.contains(&g));

        if is_empty_or_whitespace || is_too_long || contains_forbidden_characters {
            panic!("{} is not a valid subscriber name.", s)
        } else {
            Self(s)
        }
    }

And let's say your code to add a new subscriber to the email list only accepts a SubscriberName, not a String:

    pub struct NewSubscriber {
        pub email: SubscriberEmail,
        pub name: SubscriberName,
    }

Because `parse` gives back a SubscriberName if and only if all of the parse checks pass, you simply cannot add a new subscriber without all checks passing; the function will simply fail otherwise.

This example comes from the excellent book Zero To Production In Rust, which has a chapter on type-driven development [0].

[0] https://zero2prod.com