Function has_builtin_method 

pub fn has_builtin_method(recv: &Value, name: &str) -> bool

Whether a List or Dict has a built-in method named name — the gate a bound method read (such f = xs.append) checks before capturing the receiver. The method-name sets live beside each collection’s dispatch (list::LIST_METHODS, dict::DICT_METHODS), so binding and dispatch never disagree.