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Understanding Rust Error E0785

What Causes Rust Error E0785?

Rust Error E0785 occurs when an inherent implementation is written on a 'dyn auto trait'. This use of auto traits with a dyn object is considered erroneous because a dyn object can only have one non-auto trait as its principal trait.

The following example demonstrates an occurrence of Rust Error E0785:

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

fn main() {
    auto trait AutoTrait {}

    impl dyn AutoTrait {}
}

Fixing Rust Error E0785

To resolve Rust Error E0785, you must add a non-auto trait as the principal trait in the implementation. This allows the dyn object to have any number of auto traits in addition to the principal trait.

Here is a modified version of the previous example which corrects the error:

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

fn main() {
    trait PrincipalTrait {}

    auto trait AutoTrait {}

    impl dyn PrincipalTrait + AutoTrait + Send {}
}

Explanation of Changes

In the revised example, we introduced a new non-auto trait 'PrincipalTrait'. This trait is then used as the principal trait within the 'impl' block, followed by the auto traits 'AutoTrait' and 'Send'. The implementation is now coherent, resolving Rust Error E0785.

Conclusion

Rust Error E0785 is caused by having inherent implementations written on dyn auto traits. To resolve this error, you must specify a non-auto trait as the principal trait and allow dyn objects to have any number of auto traits in addition to the principal trait. This ensures coherence and proper functioning of your code.

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