Function instr_set_label_callback 

pub unsafe extern "C" fn instr_set_label_callback(
    instr: *mut instr_t,
    func: instr_label_callback_t,
)

Set a function \p func which is called when the label instruction is freed. \p instr is the label instruction allowing \p func to free the label’s auxiliary data. \note This data field is not copied across instr_clone(). Instead, the clone’s field will be NULL (xref i#3962).