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 For this to work, some note must be specified. An empty attribute will not do
21 anything, please remove the attribute or add some helpful note for users of the