Review Diesel compile time error message

[weiznich]

Diesel is known for sometimes generating sub-optimal compile time error messages. With the upcoming stabilization of the #[diagnostic] attribute namespace and the #[diagnostic::on_unimplemented] attribute it's now the time to review such bad error messages and see where:

• we can use #[diagnostic::on_unimplemented] to improve the error message
• we might need extensions to the #[diagnostic] namespace

This post specifically asks for code examples that produce a bad error message. Ideally that would include a self contained example and an explanation which information would help you there in the compiler error message.