Using #[non_exhaustive] for Non-exhaustive Enums

Using non-exhaustive enums in one crate prevents codes in other crates from listing all the possible matches in a match expression. It forces developers to use wildcards and thereby making the match expression non-exhaustive. By having a non-exhaustive match expression, it future-proofs our codes. Note that it has no effect when we use it in the same crate. For our example, we have two projects wherein one uses the other.

Future-Proof Project

This crate does not have dependencies and only has a single public enum.

Cargo.toml

[package]
name = "rust1_40_lib"
version = "0.1.0"
authors = ["Karl San Gabriel"]
edition = "2018"

[dependencies]

[package]

name = "rust1_40_lib"

version = "0.1.0"

authors = ["Karl San Gabriel"]

edition = "2018"

[dependencies]

lib.rs with Non-Exhaustive Enum

The Countries enum contains six items. It uses two metadata. The first makes it printable; while the second makes it non-exhaustive enum using #[non_exhaustive]

#[derive(Debug)]
#[non_exhaustive]
pub enum Countries { PHILIPPINES, MALAYSIA, SINGAPORE, INDONESIA, THAILAND, VIETNAM }

#[derive(Debug)]

#[non_exhaustive]

pub enum Countries { PHILIPPINES, MALAYSIA, SINGAPORE, INDONESIA, THAILAND, VIETNAM }

Build it to make sure there are no errors.

Project With Non-Exhaustive Match Expression

Cargo.toml

This crate uses the abovementioned crate.

[package]
name = "rust1_40features"
version = "0.1.0"
authors = ["Karl San Gabriel"]
edition = "2018"

[dependencies]
# Change path accordingly
countrylib={version="0.1.0", path = "..\\rust\\countrylib" }

[package]

name = "rust1_40features"

version = "0.1.0"

authors = ["Karl San Gabriel"]

edition = "2018"

[dependencies]

# Change path accordingly

countrylib={version="0.1.0", path = "..\\rust\\countrylib" }

main.rs

extern crate countrylib;
use countrylib::Countries;

fn main() {
    let home = Countries::THAILAND;

    match home {
        Countries::PHILIPPINES | Countries::MALAYSIA | Countries::INDONESIA | Countries::SINGAPORE | Countries::THAILAND | Countries::VIETNAM => {
            println!("GROUP 1: {:?}", home)
        },
    }
}

extern crate countrylib;

use countrylib::Countries;

fn main() {

let home = Countries::THAILAND;

match home {

Countries::PHILIPPINES | Countries::MALAYSIA | Countries::INDONESIA | Countries::SINGAPORE | Countries::THAILAND | Countries::VIETNAM => {

println!("GROUP 1: {:?}", home)

}

When we build this project, we will get the following:

To fix this error, modify the codes as follow. In the new codes, the matching expression is no longer exhaustive for the non-exhaustive enum. The codes will remain unchanged even if there are new enum items.

extern crate countrylib;
use countrylib::Countries;

fn main() {
    let home = Countries::THAILAND;

    match home {
        Countries::PHILIPPINES | Countries::MALAYSIA | Countries::INDONESIA | Countries::SINGAPORE | Countries::THAILAND | Countries::VIETNAM => {
            println!("GROUP 1: {:?}", home)
        },
        _ => {
            println!("OTHER GROUP: {:?}", home)
        }
    }
}