Rust error E0371: the object type `(dyn Baz + 'static)` automatically implements the trait `Bar`

The compiler team's explanation

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 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:

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.

From rustc --explain E0371 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.