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Understanding Rust Error E0714: Marker Traits with Associated Items

Rust Error E0714 Example

Given the Rust code that demonstrates Rust Error E0714:

#![feature(marker_trait_attr)]
#![feature(associated_type_defaults)]

#[marker]
trait MarkerConst {
    const N: usize; // error!
}

fn main() {}

Explanation of Rust Error E0714

This error occurs when a #[marker] trait contains an associated item. The #[marker] attribute in Rust indicates that a trait should not have any items (methods or associated constants). When you add an associated item to a marker trait, the compiler will raise Error E0714.

Marker traits are meant to be simple, zero-sized traits, often used as a means of signaling certain behaviors or properties relative to the types implementing the traits. As marker traits cannot contain any items, this makes the mistake of trying to include an associated item in the trait.

Solving Rust Error E0714

To solve this error, you can either remove the #[marker] attribute from the trait definition or remove the associated item. If you want to maintain an associated item within your trait for ergonomic reasons, you should consider using an extension trait instead. Extension traits can be used to extend the functionality of an existing trait without modifying its original definition.

Example: Using an Extension Trait

In the following example, an extension trait is used to add the associated constant 'N' to the original 'Marker' trait:

trait Marker {}

trait MarkerConstExt: Marker {
    const N: usize;
}

struct MyStruct;

impl Marker for MyStruct {}

impl MarkerConstExt for MyStruct {
    const N: usize = 5;
}

fn main() {
    println!("MyStruct N: {}", MyStruct::N);
}

In this example, we have the original 'Marker' trait and an extension trait 'MarkerConstExt'. The struct 'MyStruct' now implements both the 'Marker' and 'MarkerConstExt' traits, satisfying the requirements without raising Rust Error E0714.

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