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 fn foo<T: Index<u8>>(x: T){}
11 #[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
12 trait Index<Idx> { /* ... */ }
14 foo(true); // `bool` does not implement `Index<u8>`
17 there will be an error about `bool` not implementing `Index<u8>`, followed by a
18 note saying "the type `bool` cannot be indexed by `u8`".
20 As you can see, you can specify type parameters in curly braces for
21 substitution with the actual types (using the regular format string syntax) in
22 a given situation. Furthermore, `{Self}` will substitute to the type (in this
23 case, `bool`) that we tried to use.
25 This error appears when the curly braces do not contain an identifier. Please
26 add one of the same name as a type parameter. If you intended to use literal
27 braces, use `{{` and `}}` to escape them.