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. The attribute will let you filter on
various types, with `on`:

```compile_fail,E0232
#![feature(rustc_attrs)]
#![allow(internal_features)]

#[rustc_on_unimplemented(on(blah, message = "foo"))] // error!
trait BadAnnotation {}
```
For this to work a cfg-like predicate must be supplied. A malformed filter
will not do anything.
