| 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`". |
| |
| As you can see, you can specify type parameters in curly braces for |
| substitution with the actual types (using the regular format string syntax) in |
| a given situation. Furthermore, `{Self}` will substitute to the type (in this |
| case, `bool`) that we tried to use. |
| |
| This error appears when the curly braces contain an identifier which doesn't |
| match with any of the type parameters or the string `Self`. This might happen |
| if you misspelled a type parameter, or if you intended to use literal curly |
| braces. If it is the latter, escape the curly braces with a second curly brace |
| of the same type; e.g., a literal `{` is `{{`. |
