`#[diagnostic::on_unimplemented]` without filters This commit adds support for a `#[diagnostic::on_unimplemented]` attribute with the following options: * `message` to customize the primary error message * `note` to add a customized note message to an error message * `label` to customize the label part of the error message The relevant behavior is specified in [RFC-3366](https://rust-lang.github.io/rfcs/3366-diagnostic-attribute-namespace.html)