通过了解`parse`的实现 does, 我们明白了`parse`的返回值被绑定到`Parser`的唯一原因是因为它引用了`Prser`中的`Context`, 也就是一个字符串切片的引用, 所以它的生命周期才是`parse_context`需要关心的字符串切片的真正的生命周期. We need a way to tell Rust that the string slice in `Context` and the
reference to the `Context` in `Parser` have different lifetimes and that the
return value of `parse_context` is tied to the lifetime of the string slice in