Rust Error E0715: Marker Trait Impls Overriding Associated Items

Understanding Rust Error E0715

Rust Error E0715 occurs when the implementation of a #[marker] trait tries to override an associated item. Marker traits allow for multiple implementations for the same type; however, they are not permitted to override anything in those implementations because this would result in ambiguity concerning which override should be used.

Here's an example of code that causes Rust Error E0715:

#![feature(marker_trait_attr)]

#[marker]
trait Marker {
    const N: usize = 0;
    fn do_something() {}
}

struct OverrideConst;
impl Marker for OverrideConst { // error!
    const N: usize = 1;
}
fn main() {}

Fixing Rust Error E0715

To resolve Rust Error E0715, you must ensure that any trait marked with #[marker] doesn't try to override an associated item in its implementation.

One possible way to fix the issue is by removing the const N or the do_something() function from the marker trait. Removing the function would make the implementation for OverrideConst valid and the error would be resolved. Here's an example:

#![feature(marker_trait_attr)]

#[marker]
trait Marker {
    const N: usize = 0;
}

struct OverrideConst;
impl Marker for OverrideConst {
    // This is now a valid implementation
}
fn main() {}

Alternatively, you can remove the marker trait attribute and use a standard trait, which can have overridden associated items. Here's another way to fix the issue:

trait NonMarker {
    const N: usize = 0;
    fn do_something() {}
}

struct OverrideConst;
impl NonMarker for OverrideConst {
    const N: usize = 1;
}
fn main() {}

Conclusion

Rust Error E0715 is caused when the implementation of a #[marker] trait attempts to override an associated item, which is not allowed due to potential ambiguity. To resolve Rust Error E0715, either remove the associated items from the marker trait or change it to a standard trait.