# Error code E0230

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},{B}>`"] // 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 contain an identifier which doesn't match with any of the type parameters or the string `Self`. This might happen if you misspelled a type parameter, or if you intended to use literal curly braces. If it is the latter, escape the curly braces with a second curly brace of the same type; e.g., a literal `{` is `{{`.