Function only_referenced_within_cycle 

fn only_referenced_within_cycle(
    refs: &HashMap<FunctionId, RefInfo>,
    start: FunctionId,
) -> bool

Whether a function’s single reference only enters it through a reference cycle: the chain of single-reference sources loops back on start itself. A loop closing on a later node is someone else’s cycle, and start hangs off it as an inlineable tail.