# Error code E0232

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:

ⓘ

    #![feature(rustc_attrs)]

    #[rustc_on_unimplemented(lorem="")] // error!
    trait BadAnnotation {}

Run

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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