1 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
2 message for when a particular trait isn't implemented on a type placed in a
3 position that needs that trait. For example, when the following code is
7 #![feature(rustc_attrs)]
9 #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error!
10 trait BadAnnotation<A> {}
13 there will be an error about `bool` not implementing `Index<u8>`, followed by a
14 note saying "the type `bool` cannot be indexed by `u8`".
16 As you can see, you can specify type parameters in curly braces for
17 substitution with the actual types (using the regular format string syntax) in
18 a given situation. Furthermore, `{Self}` will substitute to the type (in this
19 case, `bool`) that we tried to use.
21 This error appears when the curly braces do not contain an identifier. Please
22 add one of the same name as a type parameter. If you intended to use literal
23 braces, use `{{` and `}}` to escape them.