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.