Using #[non_exhaustive] for Non-exhaustive Structs

Last Updated on January 11, 2020

Non-exhaustive structs will future-proof our codes by preventing client codes from using Struct expressions. We use Struct expressions to instantiate structs and they’re JSON-like expression in appearance with all fields present. When we update a struct with a new field, all expressions must be updated to include that field.

The non_exhaustive attribute indicates that a type or variant may have more fields or variants added in the future. It can be applied to structs, enums, and enum variants.

For example, we have a struct Pet with 2 initial fields name and age.

struct Pet {
    name: String,
    age: i32
}

struct Pet {

name: String,

age: i32

}

We can instantiate it using a struct expression as follows.

let my_pet = Pet { 
                 name: "Jake".to_string(), 
                 age: 2
             };

let my_pet = Pet {

name: "Jake".to_string(),

age: 2

};

The codes work until we add a new field owner to the struct.

Wouldn’t it be better if we could just disallow struct expressions? We could use #[non_exhaustive]!

Crate-Level-Only Structs

There is one important thing to note about #[non_exhaustive], it only works at the crate level.

Using #[non_exhaustive] for Non-exhaustive Structs

How then do we instantiate structs? Through factory functions provided by the crate with non-exhaustive structs.

Crate mycrate lib.rs

This crate has the non-exhaustive structs. These are just structs marked with #[non_exhaustive]. The following codes have a struct with 2 public factory functions to instantiate it in the client codes. They are using #[non_exhaustive] on structs which in this case is just Person .

#[non_exhaustive]
#[derive(Debug)]
pub struct Person {
    pub first_name: String,
    pub last_name: String
}

impl Person {
    pub fn new() -> Person {
        Person {
            first_name: "John".to_string(),
            last_name: "Doe".to_string()
        }
    }

    pub fn new_with_fname_lname(fname: String, lname: String) -> Person {
        Person {
            first_name: fname,
            last_name: lname
        }
    }
}

#[non_exhaustive]

#[derive(Debug)]

pub struct Person {

pub first_name: String,

pub last_name: String

}

impl Person {

pub fn new() -> Person {

Person {

first_name: "John".to_string(),

last_name: "Doe".to_string()

}

pub fn new_with_fname_lname(fname: String, lname: String) -> Person {

Person {

first_name: fname,

last_name: lname

}

Crate myapp

This crate uses mycrate that has those non-exhaustive structs. Any attempts to use struct expressions to instantiate those structs will fail the building process.

Cargo.toml

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

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

[package]

name = "rust1_40features"

version = "0.1.0"

authors = ["Karl San Gabriel"]

edition = "2018"

[dependencies]

# Change path accordingly

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

main.rs

extern crate mycrate;
use mycrate::Person;

fn main() {
    let p = Person::new();
    println!("{:?}", p);

    let another_p = Person::new_with_fname_lname("Karl".to_string(),
                                                 "San Gabriel".to_string());
    println!("{:?}", another_p);
}

extern crate mycrate;

use mycrate::Person;

fn main() {

let p = Person::new();

println!("{:?}", p);

let another_p = Person::new_with_fname_lname("Karl".to_string(),

"San Gabriel".to_string());

println!("{:?}", another_p);

}

Output:

Person { first_name: "John", last_name: "Doe" }
Person { first_name: "Karl", last_name: "San Gabriel" }

1 2	Person { first_name: "John", last_name: "Doe" } Person { first_name: "Karl", last_name: "San Gabriel" }

Tested with Rust 1.40.0.

Crate-Level-Only Structs

Crate mycrate lib.rs

Crate myapp

Cargo.toml

main.rs

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Karl San Gabriel