Rust Error E0371: Trait Implementation Redundancy

Understanding Rust Error E0371

Rust error E0371 occurs when a trait is implemented on another, which already automatically implements it. In other words, this error occurs when you attempt to implement a trait for a type that implicitly implements it via another trait.

Erroneous Code Examples

Let's examine some code examples that cause the E0371 error:

#![allow(unused)]
fn main() {
    trait Foo { fn foo(&self) { } }
    trait Bar: Foo { }
    trait Baz: Bar { }

    impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
    impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
    impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
}

In this example, the first two impl blocks are unnecessary because `Baz` is defined to inherit from `Bar` and `Bar` is defined to inherit from `Foo`. The compiler raises error E0371 to prevent redundant trait implementations.

Resolving Rust Error E0371

To resolve Rust error E0371, eliminate redundant trait implementations by removing unnecessary impl blocks. In the example above, remove the impl blocks for Bar and Foo:

#![allow(unused)]
fn main() {
    trait Foo { fn foo(&self) { } }
    trait Bar: Foo { }
    trait Baz: Bar { }
}

Now the code compiles successfully without raising error E0371.

Additional Cases

Implementing a trait for itself is also considered redundant and will result in error E0371. For example:

#![allow(unused)]
fn main() {
    trait Baz { }

    impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
}

To resolve this case, simply remove the `impl Baz for Baz` block.

Conclusion

Rust error E0371 occurs when a trait is redundantly implemented on another trait that already automatically implements it. Identify and eliminate the impl blocks causing the error by removing the redundant trait implementations. This ensures that your code is efficient and free from unnecessary complexity.