Macro diagnostic_codes 

macro_rules! diagnostic_codes {
    ($(
        $(#[$attr:meta])*
        $name:ident = $code:literal, $severity:ident, $summary:literal
        $(, category = $category:ident)?
        $(, retired = $since:literal)? ;
    )*) => { ... };
}

Declare one crate’s diagnostic registry.

Each code literal appears once in the declaration and the generated ALL slice drives registry checks and reference generation.