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Understanding Rust Error E0739

What is Rust Error E0739?

Rust Error E0739 occurs when the #[track_caller] attribute is applied to a struct. The #[track_caller] attribute is used to derive better error messages and panics by providing the location (file, line number, column number) of the caller of a function. This attribute is helpful for functions that deal with unwrapping values, assertions, or panics. However, applying this attribute to a struct is not allowed and will result in Rust Error E0739.

Example of Erroneous Code

#![allow(unused)]
fn main() {
#[track_caller]
struct Bar {
    a: u8,
}
}

How to Fix Rust Error E0739

To fix Rust Error E0739, you should remove the #[track_caller] attribute from the struct definition. Remember, the #[track_caller] attribute should only be applied to functions. If you want to track the caller for a specific method within the struct, you can apply the attribute to that method instead.

Example of Fixed Code

struct Bar {
    a: u8,
}

impl Bar {
    #[track_caller]
    fn unwrap_a(&self) -> u8 {
        if self.a == 0 {
            panic!("a must not be 0");
        }
        self.a
    }
}

fn main() {
    let bar = Bar { a: 0 };
    bar.unwrap_a();
}

Conclusion

Rust Error E0739 occurs when applying the #[track_caller] attribute to a struct, which is not allowed. To fix the error, remove the attribute from the struct definition and apply it to methods within the struct if necessary. By doing so, you'll ensure your code compiles and runs without issues, while also providing better error messages when panicking or using assertions.

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