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(lorem="")] // error!
10 trait BadAnnotation {}
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 For this to work, some note must be specified. An empty attribute will not do
17 anything, please remove the attribute or add some helpful note for users of the