A trait was implemented on another which already automatically implemented it.
Erroneous code examples:
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 OKWhen 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.