| 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,E0231 | |
| #![feature(rustc_attrs)] | |
| #![allow(internal_features)] | |
| #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error! | |
| trait BadAnnotation<A> {} | |
| ``` | |
| there will be an error about `bool` not implementing `Index<u8>`, followed by a | |
| note saying "the type `bool` cannot be indexed by `u8`". | |
| As you can see, you can specify type parameters in curly braces for | |
| instantiation with the actual types (using the regular format string syntax) in | |
| a given situation. Furthermore, `{Self}` will be instantiated to the type (in | |
| this case, `bool`) that we tried to use. | |
| This error appears when the curly braces do not contain an identifier. Please | |
| add one of the same name as a type parameter. If you intended to use literal | |
| braces, use `{{` and `}}` to escape them. |