Understanding Rust Error E0436

Introduction to Rust Error E0436

Rust Error E0436 occurs when the functional record update syntax is used on something other than a struct. In Rust, the functional record update syntax allows you to update a struct by creating a new instance with a specified set of changed fields, while keeping the remaining fields the same.

The functional record update syntax cannot be used with struct-like enum variants, which may cause confusion and lead to this error.

Erroneous Code Example

#![allow(unused)]
fn main() {
enum PublicationFrequency {
    Weekly,
    SemiMonthly { days: (u8, u8), annual_special: bool },
}

fn one_up_competitor(competitor_frequency: PublicationFrequency)
                     -> PublicationFrequency {
    match competitor_frequency {
        PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
            days: (1, 15), annual_special: false
        },
        c @ PublicationFrequency::SemiMonthly{ .. } =>
            PublicationFrequency::SemiMonthly {
                annual_special: true, ..c // error: functional record update
                                          //        syntax requires a struct
        }
    }
}
}

How to Fix Rust Error E0436

To fix Rust Error E0436, you need to rewrite the code without using the functional record update syntax on non-structs. In the case of an enum with variants containing struct-like values, you can update the fields manually.

Corrected Code Example

#![allow(unused)]
fn main() {
enum PublicationFrequency {
    Weekly,
    SemiMonthly { days: (u8, u8), annual_special: bool },
}

fn one_up_competitor(competitor_frequency: PublicationFrequency)
                     -> PublicationFrequency {
    match competitor_frequency {
        PublicationFrequency::Weekly => PublicationFrequency::SemiMonthly {
            days: (1, 15), annual_special: false
        },
        PublicationFrequency::SemiMonthly{ days, .. } =>
            PublicationFrequency::SemiMonthly {
                days, annual_special: true // ok!
        }
    }
}
}

Conclusion

Rust Error E0436 is caused by the incorrect use of the functional record update syntax on non-structs. Fixing this error requires updating the code to ensure that the functional record update syntax is used only with structs, and manually updating non-struct values when needed. By following these guidelines, you can write more robust and error-free Rust code.