6. non-exhaustive enums

Use non-exhaustive attribute to indicate that enum may have more variants in future.

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#[non_exhaustive]
enum Event{
  Error,
  Completed(String)
}

If such enum is used in match pattern, it is required to handle ’_’ pattern.

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let event = Event::Error;
match event {
    Event::Error => {
        println!("error")
    }
    Event::Completed(s) => {
        println!("completed {}", s)
    }
    _ => {
        println!("unknown")
    }
}

How is this useful?

If you are a maintainer of a library and you’d expect to add more variants in the future.

This approach helps you to build a library with non-breaking code changes for library users.