compiler/rustc_error_codes/src/error_codes/E0371.md

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

Erroneous code examples:

```compile_fail,E0371
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:

```
# 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.