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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,E0232
#![feature(rustc_attrs)]
#[rustc_on_unimplemented(lorem="")] // error!
trait BadAnnotation {}
```
there will be an error about `bool` not implementing `Index<u8>`, followed by a
note saying "the type `bool` cannot be indexed by `u8`".
For this to work, some note must be specified. An empty attribute will not do
anything, please remove the attribute or add some helpful note for users of the
trait.
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