How to write Rocket FromRequest implementation? Getting "lifetimes do not match method in trait"

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05-10-2024

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Mastering Rocket FromRequest: Conquering the "Lifetimes do not match" Error

Problem: You're building a Rocket.rs web application, and you're trying to use the FromRequest trait to extract data from incoming HTTP requests. However, you're encountering the dreaded "lifetimes do not match method in trait" error, leaving you scratching your head.

Simplified Explanation: The "lifetimes do not match" error means your code is trying to use data that might not exist for the entire duration it needs to. This often happens when working with borrowed data from requests, as you need to ensure that the borrowed data lives as long as the code using it.

The Scenario:

Imagine you're building an API endpoint to retrieve a user's profile. You might define a User struct and want to extract the user ID from the request's path parameters:

use rocket::http::Status;
use rocket::request::{FromRequest, Request};
use rocket::serde::json::Json;

#[derive(serde::Serialize, Deserialize)]
struct User {
    id: u32,
    name: String,
}

#[get("/users/<id>")]
async fn get_user(id: UserId) -> Result<Json<User>, Status> {
    // Fetch user data based on the id 
    let user = fetch_user(id.0); 

    Ok(Json(user))
}

#[derive(Debug)]
pub struct UserId(u32);

#[rocket::async_trait]
impl<'r> FromRequest<'r> for UserId {
    type Error = ();

    async fn from_request(req: &'r Request<'_>) -> Result<Self, Self::Error> {
        let id: u32 = req.path().split('/').nth(2).unwrap().parse().unwrap(); // Extract ID from path
        Ok(UserId(id))
    }
}

This code might seem simple, but it can lead to the "lifetimes do not match" error due to the way data is borrowed and used.

Analysis:

• Borrowing: The FromRequest implementation borrows the req (request object) for its duration. This means the UserId struct created inside from_request can only use the borrowed data (req) as long as it lives.
• Lifetime Mismatch: The get_user function expects the id to live as long as the function itself. The problem arises because the UserId created in the FromRequest implementation is scoped to the from_request function, meaning it won't live as long as the get_user function.

The Solution:

To fix this, we need to make sure the UserId lives as long as the get_user function needs it. We can achieve this by using a lifetime annotation on the UserId struct:

#[derive(Debug)]
pub struct UserId<'r>(u32, &'r str); // Add lifetime annotation

#[rocket::async_trait]
impl<'r> FromRequest<'r> for UserId<'r> {
    type Error = ();

    async fn from_request(req: &'r Request<'_>) -> Result<Self, Self::Error> {
        let id: u32 = req.path().split('/').nth(2).unwrap().parse().unwrap(); 
        let id_str = req.path().split('/').nth(2).unwrap(); // Keep the string for lifetime

        Ok(UserId(id, id_str))
    }
}

#[get("/users/<id>")]
async fn get_user(id: UserId<'_, '_>) -> Result<Json<User>, Status> {
    // Fetch user data based on the id 
    let user = fetch_user(id.0); // Use the extracted ID

    Ok(Json(user))
}

By adding 'r to the UserId struct, we indicate that it borrows the lifetime of the request ('r). This allows the UserId to exist as long as the request does, which satisfies the lifetime requirement of the get_user function.

Additional Value:

• Understanding Lifetimes: This example highlights the importance of understanding Rust lifetimes. They ensure data is used safely by preventing dangling pointers and memory leaks.
• Error Handling: While we've focused on lifetimes, remember to handle errors gracefully in your FromRequest implementation. Consider using Result to communicate potential errors back to the caller.
• Generic Extraction: You can use the same pattern to extract other types of data from the request, such as headers, query parameters, or cookies.

References:

• Rocket.rs documentation
• Rust Book

By understanding lifetimes and applying these concepts, you can overcome the "lifetimes do not match" error and build robust and secure Rocket applications. Happy coding!