The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
message for when a particular trait isn't implemented on a type placed in a
position that needs that trait. For example, when the following code is
compiled:

```compile_fail
#![feature(rustc_attrs)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`
```

there will be an error about `bool` not implementing `Index<u8>`, followed by a
note saying "the type `bool` cannot be indexed by `u8`".

For this to work, some note must be specified. An empty attribute will not do
anything, please remove the attribute or add some helpful note for users of the
trait.