src/librustc_error_codes/error_codes/E0371.md

   1 A trait was implemented on another which already automatically implemented it.
   2
   3 Erroneous code examples:
   4
   5 ```compile_fail,E0371
   6 trait Foo { fn foo(&self) { } }
   7 trait Bar: Foo { }
   8 trait Baz: Bar { }
   9
  10 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
  11 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
  12 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
  13 impl Baz for Bar { } // Note: This is OK
  14 ```
  15
  16 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
  17 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
  18 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
  19 definition, so it is not useful to do this.