TODO: Implement the functionality of `count_collection_for` but with an iterator instead of a `for` loop.

TODO: Create an empty cons list.

TODO: Create a non-empty cons list.

fetchUrlDemo 并发获取url内容

TODO: Fix the compiler error on this function.

TODO: Add another error conversion function here.

TODO: This function refuses to generate text to be printed on a nametag if you pass it an empty string. It'd be nicer if it explained what the problem

TODO: Add the correct return type to the function signature.

处理/hellow路由的控制器

HandlerFunc

init方法

TODO: Add the correct return type to the function signature.

TODO: Add a default implementation for `licensing_info` so that implementors like the two structs below can share that default behavior without repeat

TODO: Add the correct return type and complete the function body. Desired output: `[Ok(1), Ok(11), Ok(1426), Ok(3)]`

TODO: Fix the compiler error by updating the function signature.

定义一个中间件m1：统计请求处理函数的执行时间

定义一个中间件m2