Error code E0714

A #[marker] trait contained an associated item.

Erroneous code example:

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

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

fn main() {}

The items of marker traits cannot be overridden, so there's no need to have them when they cannot be changed per-type anyway. If you wanted them for ergonomic reasons, consider making an extension trait instead.