Rust Error E0718: Correctly Using #[lang] Attribute

Understanding Rust Error E0718

Rust Error E0718 occurs when an incorrect item type has been assigned the #[lang] attribute. The #[lang] attribute is used to implement compiler-built-in traits and provide certain language features. Misusing this attribute will result in the error.

Here's an example of code causing the E0718 error:

#![allow(unused)]
#![feature(lang_items)]

fn main() {
    #[lang = "owned_box"]
    static X: u32 = 42;
}

Correcting Rust Error E0718

To fix the E0718 error, it is essential to apply the #[lang] attribute to the correct item type. The exact item type depends on the specific language item you are working with. To give you a better understanding of how to use the #[lang] attribute correctly, let's look at some examples.

Using #[lang] to Implement the Drop Trait

When implementing the Drop trait for a particular type, you will need to use the #[lang = "drop"] attribute. In this case, you must apply the attribute to an impl block for the Drop trait. Here's an example of how to do this correctly:

#![feature(lang_items)]

struct Exemplar;

#[lang = "drop"]
impl Drop for Exemplar {
    fn drop(&mut self) {
        println!("Dropping Exemplar!");
    }
}

fn main() {
    let _x = Exemplar;
}

Using #[lang] to Implement the Add Trait

In another example, we will implement the Add trait for a custom type using the #[lang] attribute. In this case, you must apply the attribute to an impl block for the Add trait. Here's an example of how to do this correctly:

#![feature(lang_items)]

use std::ops::Add;

struct MyInt(i32);

#[lang = "add"]
impl Add for MyInt {
    type Output = MyInt;

    fn add(self, other: MyInt) -> MyInt {
        MyInt(self.0 + other.0)
    }
}

fn main() {
    let a = MyInt(42);
    let b = MyInt(24);
    let result = a + b;
}

Conclusion

Always ensure that you apply the #[lang] attribute to the correct item type when implementing Rust's built-in traits or language features. Misusing this attribute will result in Rust Error E0718. By using the examples provided in this article, you should now be able to make the necessary adjustments to avoid the E0718 error in your Rust code.