Understanding Rust Error E0774

What is Rust Error E0774?

Rust Error E0774 occurs when the 'derive' attribute is applied to an entity that is not a struct, union, or enum. Derive is a built-in Rust feature that automatically provides specific trait implementations for a given data structure.

Erroneous code example

#![allow(unused)]
fn main() {
trait Foo {
    #[derive(Clone)] // error!
    type Bar;
}
}

In this example, the derive attribute is wrongly applied to 'type Bar' within the trait 'Foo'. The compiler will raise the E0774 error because 'derive' is only allowed for structs, unions, or enums.

Correct usage of derive

To demonstrate the correct usage of the derive attribute, let's consider the following example:

#![allow(unused)]
fn main() {
#[derive(Clone)] // ok!
struct Bar {
    field: u32,
}
}

In this case, the derive attribute is applied correctly to the struct 'Bar', and the compiler will not raise any errors.

Deriving multiple traits

You can also derive multiple traits at once. Below is an example demonstrating how to derive multiple traits for the struct 'Bar':

#![allow(unused)]
fn main() {
#[derive(Clone, Debug, PartialEq)] // ok!
struct Bar {
    field: u32,
}
}

Here, the struct 'Bar' derives the Clone, Debug, and PartialEq traits without any issues.

Conclusion

Rust Error E0774 results from applying the 'derive' attribute to an unsupported entity. To fix the error, ensure that the 'derive' attribute is only applied to structs, unions, or enums. Use appropriate syntax and multiple traits if necessary for the desired functionality.