Empty trait doesn't compile anymore

[mahkoh]

<anon>:2:1: 2:12 error: parameter `Self` is never used
<anon>:2 trait X { }
         ^~~~~~~~~~~
<anon>:2:1: 2:12 help: consider removing `Self` or using a marker such as `core::marker::PhantomFn`
<anon>:2 trait X { }
         ^~~~~~~~~~~

[huonw]

As the error message indicates, this is by design.

[huonw]

(The spurious suggestion to remove Self is covered by #22598.)