Function link 

pub async fn link<'a, 'l, B, RB, F, RF>(
    arena: &'a ThreadLocal<Bump>,
    graph: &'l VarGraph<'a>,
    val: JsValue<'a>,
    early_visitor: &'l B,
    visitor: &'l F,
    fun_args_values: &Mutex<FxHashMap<u32, BumpVec<'a, JsValue<'a>>>>,
    var_cache: &Mutex<FxHashMap<Id, JsValue<'a>>>,
) -> Result<(JsValue<'a>, u32)>
where
    RB: 'l + Future<Output = Result<(JsValue<'a>, Modified)>> + Send,
    B: 'l + Fn(JsValue<'a>) -> RB + Sync,
    RF: 'l + Future<Output = Result<(JsValue<'a>, Modified)>> + Send,
    F: 'l + Fn(JsValue<'a>) -> RF + Sync,