Why do these both structs differ in the way they are affected by lifetimes

问题

This is a follow up question to: How to fix: cannot infer an appropriate lifetime for automatic coercion.

I wonder why do these both structs differ in the way they are affected by lifetimes.

Example 1

use http;

pub struct Request<'a> {
    pub origin: &'a http::server::Request,
}

Example 2

use http;

pub struct Response<'a, 'b> {
    pub origin: &'a mut http::server::ResponseWriter<'b>,
}

They look pretty much similar to me except that the second one holds a mutable reference whereas the first one holds an immutable reference.

However, for Example 2 I can't just use a lifetime for the reference. I must give a lifetime for the struct as well.

So, I wonder is there something inside the struct that causes such behavior or is it really because the one in the second example is a mutable reference. And if so, why exactly does that cause that.

回答1:

&'a T means that you have a reference to a T object which is valid for the lifetime'a.

T<'b> means a T object containing an object inside itself valid for the lifetime 'b, as in struct T<'b> { t: &'b U }.

&'a T<'b>is thus a reference with lifetime 'a to a T<'b> object.

In the case of the ResponseWriter, it contains references to the Request and to the TcpStream, whereas the Request does not contain any references.

来源：https://stackoverflow.com/questions/24292831/why-do-these-both-structs-differ-in-the-way-they-are-affected-by-lifetimes