Derive Macro Diagnostic 

#[derive(Diagnostic)]
{
    // Attributes available to this derive:
    #[diag]
    #[help]
    #[help_once]
    #[note]
    #[note_once]
    #[warning]
    #[primary_span]
    #[label]
    #[subdiagnostic]
    #[suggestion]
}

See https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-structs.html#derivediagnostic