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.
Example:
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