| 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,E0232 | |
| #![feature(rustc_attrs)] | |
| #![allow(internal_features)] | |
| #[rustc_on_unimplemented(lorem="")] // error! | |
| trait BadAnnotation {} | |
| ``` | |
| 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. |