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

Question

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.

"However, for Example 2 I can't just use a lifetime for the reference. I must give a lifetime for the struct as well." You mean that you can't omit <'b> after ResponseWriter? In this case it happens because ResponseWriter is parameterized by a lifetime, while Request is not. It is similar to that you have to write Option<int> with a parameter but String without one. — Vladimir Matveev
If you inspect the definition of ResponseWriter, you will find that it contains a reference. I'm guessing that it's this ResponseWriter: rust-ci.org/chris-morgan/rust-http/doc/http/server/response/… — A.B.
Ah, brilliant. It's parameterized by a lifetime in it's definition. So I have to take that into account. Makes sense! If anyone cares to add it as an answer, I'll be happy to accept it :) — Christoph
possible duplicate of How to fix: cannot infer an appropriate lifetime for automatic coercion — Chris Morgan
(As a general rule, if you have just a slight extra question—this was just a request for clarification—it’s best to keep it in the same question.) — Chris Morgan

Chris Morgan Chris Morgan · Accepted Answer · 2014-06-19T00:16:35

&'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.

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

1 Answers