Prevent Breaking Code Changes in Future Releases using `non exhaustive` enums in Rust

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Hello everyone, recently I have come across a feature in Rust, known as non_exhaustive. It was introduced in Rust 1.40.0 . This attribute prevents source code-breaking changes in projects downstream.

enums can be non-exhaustive or exhaustive, depending upon their needs. A non-exhaustive enum indicates that this enum may get new value in the future. By adding #[non_exhaustive] attribute, we can create a non-exhaustive enum which forces downstream crates to use wildcard pattern (_) in the match expression and make it non-exhaustive. Otherwise, we will get the compilation error.

Let’s see with a simple example.

• Create a project using cargo new and define a rust module movie_genresunder this project.
• Create a file lib.rs and add an enum for movie genres with #[non_exhaustive] attribute.

#[non_exhaustive]
pub enum Genres {
    Horror,
    Comedy,
    Romance,
}

• In main Cargo.toml add the dependency.

movie_genres = {version = "0.1.0", path="./movie_genres"}

• Now import movie_genres in main.rs and start testing.

extern crate movie_genres;
use movie_genres::Genres;

fn main() {
    let genres = Genres::Horror;

    match genres {
        Genres::Horror => println!("Horror Movie!!"),
        Genres::Comedy => println!("Comedy Movie!!"),
        Genres::Romance => println!("Romance Movie!!"),
    }
}

• Now if we hit cargo run, we will get a compilation error.
  

• Now add wildcard arm in match expression.

    match genres {
        Genres::Horror => println!("Horror Movie!!"),
        Genres::Comedy => println!("Comedy Movie!!"),
        Genres::Romance => println!("Romance Movie!!"),
        _ => println!("Other movie category"),
    }

• Add hit cargo run.

You can find full example here https://github.com/knoldus/non_exhaustive_rust_template.

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