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# Error code E0231

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:

    #![allow(unused)]
    #![feature(rustc_attrs)]
    #![allow(internal_features)]

    fn main() {
    #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error!
    trait BadAnnotation<A> {}
    }

there will be an error about `bool` not implementing `Index<u8>`, followed by a note saying "the type `bool` cannot be indexed by `u8`".

As you can see, you can specify type parameters in curly braces for instantiation with the actual types (using the regular format string syntax) in a given situation. Furthermore, `{Self}` will be instantiated to the type (in this case, `bool`) that we tried to use.

This error appears when the curly braces do not contain an identifier. Please add one of the same name as a type parameter. If you intended to use literal braces, use `{{` and `}}` to escape them.