| 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,E0230 | |
| #![feature(rustc_attrs)] | |
| #![allow(internal_features)] | |
| #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"] // 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 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 `{{`. |