Function apply_rules 

pub fn apply_rules<F>(diagnostics: &mut Vec<AnalysisDiagnostic>, lookup_rule: F)
where
    F: Fn(&str) -> Option<RuleConfig>,

Apply a [diagnostics.rules] configuration to a stream of analyser diagnostics in place. Drops diagnostics whose resolved severity is allow, and remaps the remaining diagnostics’ severity field:

• warn → the diagnostic’s intrinsic severity stays unchanged.
• deny → severity is upgraded to Error.

lookup_rule is the resolution function — typically LoadedLexConfig::lookup_diagnostic_rule, which consults the named built-in fields first and the extension-rules side-channel second. Diagnostics whose code has no matching entry on either surface pass through untouched at their intrinsic severity.