Function custom_rule 

pub fn custom_rule<F>(
    code: impl Into<RuleCode>,
    severity: Severity,
    message: impl Into<String>,
    step_fn: F,
) -> CustomRule<F>
where
    F: Fn(&mut RuleState, u32, &Instruction, Option<&ParsedShell>) + Send + Sync,

Create a custom stateful rule.