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 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` impl Baz for 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.