Error code E0785

An inherent impl was written on a dyn auto trait.

Erroneous code example:

#![allow(unused)]
#![feature(auto_traits)]

fn main() {
auto trait AutoTrait {}

impl dyn AutoTrait {}
}

Dyn objects allow any number of auto traits, plus at most one non-auto trait. The non-auto trait becomes the “principal trait”.

When checking if an impl on a dyn trait is coherent, the principal trait is normally the only one considered. Since the erroneous code has no principal trait, it cannot be implemented at all.

Working example:

#![allow(unused)]
#![feature(auto_traits)]

fn main() {
trait PrincipalTrait {}

auto trait AutoTrait {}

impl dyn PrincipalTrait + AutoTrait + Send {}
}