| 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. |
