E0371 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0371

A trait was implemented on another which already automatically implemented it.

Erroneous code examples:

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

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

This is okay, because `Bar` does not implement `Baz` by definition:

    #![allow(unused)]
    fn main() {
    trait Foo { fn foo(&self) { } }
    trait Bar: Foo { }
    trait Baz: Bar { }
    impl Baz for dyn Bar { } // Note: This is OK
    }

When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by definition, so it is not useful to do this.