E0785 - Error codes index
Description: Book listing all Rust error codes

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# 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 {}
    }