#![feature(marker_trait_attr)] #[marker] trait Marker { const N: usize = 0; fn do_something() {} } struct OverrideConst; impl Marker for OverrideConst { //~^ ERROR impls for marker traits cannot contain items const N: usize = 1; } struct OverrideFn; impl Marker for OverrideFn { //~^ ERROR impls for marker traits cannot contain items fn do_something() { println!("Hello world!"); } } fn main() {}