| 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. The attribute will let you filter on | |
| various types, with `on`: | |
| ```compile_fail,E0232 | |
| #![feature(rustc_attrs)] | |
| #![allow(internal_features)] | |
| #[rustc_on_unimplemented(on(blah, message = "foo"))] // error! | |
| trait BadAnnotation {} | |
| ``` | |
| For this to work a cfg-like predicate must be supplied. A malformed filter | |
| will not do anything. |