Rust function return type with lifetime value

Question

I'd like to implement the Responder trait for my Hero structure, but the following method signature ( respond_to ):

use rocket::{http::Status, response::Responder, Request, Response};
use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
pub struct Hero {
    pub id: Option<i32>,
    pub name: String,
    pub identity: String,
    pub hometown: String,
    pub age: i32,
}

impl Hero {
    pub fn new(
        num: Option<i32>,
        name: String,
        identity: String,
        hometown: String,
        age: i32,
    ) -> Hero {
        Hero {
            id: num,
            name,
            identity,
            hometown,
            age,
        }
    }
}

impl<'r> Responder<'r> for Hero {
    fn respond_to(self, _request: &Request) -> Result<Response, Status> {
        unimplemented!()
    }
}

Throws a compilation error:

error[E0106]: missing lifetime specifier
  --> src/main.rs:32:55
   |
32 |     fn respond_to(self, _request: &Request) -> Result<Response, Status> {
   |                                                       ^^^^^^^^ expected lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the signature does not say which one of `_request`'s 2 lifetimes it is borrowed from

Dependencies:

[dependencies]
rocket = "0.4.2"
serde = {version = "1.0.99", features=["derive"]}

The documentation doesn't provide an example of how to provide lifetimes when a type is returned. How to specify lifetime when a type is returned?

Answer 1

The Responder trait is defined as:

pub trait Responder<'r> {
    fn respond_to(self, request: &Request) -> response::Result<'r>;
}

The response::Result<'r> is defined as:

pub type Result<'r> = ::std::result::Result<self::Response<'r>, ::http::Status>;

Your method signature is:

fn respond_to(self, request: &Request) -> Result<Response, Status>;

As you can see, you just forgot to specify lifetime for the Response . The correct method signature is:

impl<'r> Responder<'r> for Hero {
    fn respond_to(self, request: &Request) -> Result<Response<'r>, Status> {
        unimplemented!()
    }
}