Understanding Rust Error E0701: Incorrect #[non_exhaustive] Usage

Introduction to Rust Error E0701

Rust Error E0701 occurs when the #[non_exhaustive] attribute is used incorrectly, i.e., it is placed on something other than a struct or enum. This error prevents potential issues stemming from improper usage of the #[non_exhaustive] attribute.

An Erroneous Code Example

#![allow(unused)]
fn main() {
    #[non_exhaustive]
    trait Foo { }
}

In this example, the #[non_exhaustive] attribute is applied to a trait named `Foo`. As traits are not valid targets for this attribute, it causes Rust error E0701.

What is #[non_exhaustive]?

The #[non_exhaustive] attribute in Rust allows library authors to extend structs and enums without breaking code that depends on them. It indicates that the struct or enum may be extended in the future, requiring users to explicitly handle additional cases, thus avoiding errors.

How to Fix Rust Error E0701

To fix Rust error E0701, ensure that the #[non_exhaustive] attribute is only applied to structs and enums. The following sections provide examples of proper #[non_exhaustive] usage.

Correct #[non_exhaustive] Usage on Structs

#![allow(unused)]
#[non_exhaustive]
struct Foo {
    a: u32,
    b: String,
}

In this example, the #[non_exhaustive] attribute is correctly applied to a struct named `Foo`. This informs users that the struct may be extended in the future without breaking their code.

Correct #[non_exhaustive] Usage on Enums

#![allow(unused)]
#[non_exhaustive]
enum Bar {
    A,
    B,
}

In this example, the #[non_exhaustive] attribute is applied to an enum named `Bar`, indicating that more variants may be added in the future without causing breaking changes.

Conclusion

To avoid Rust error E0701, ensure that the #[non_exhaustive] attribute is only applied to structs and enums. By properly using this attribute, you can effectively extend your library while maintaining backward compatibility with dependent code.