I wanted to try replacing my Flask webserver for some local home projects with Rust. I decided to use Rocket, but I didn’t quite know how to keep some of the statefulness that I needed. Fortunately, in fewer than five minutes, I found the answer in Rocket’s docs: State.

Here’s a simple webserver that hosts a page at /hello/<name>/<age> (eg, /hello/dave/52), keeps track of the most recent person to visit, and displays messages accordingly:

#![feature(proc_macro_hygiene, decl_macro)]
extern crate rocket;
use rocket::State;
use std::sync::Mutex;

fn hello(last_person: State<Mutex<(String, u8)>>, name: String, age: u8) -> String {
    // Acquire the lock mutably,
    let guard = &mut last_person.lock().unwrap();

    // get the name from the lock,
    let prev_name = guard.0.clone();

    // then update it. One deref gets the MutexGuard from its &mut; another gets the values from the Guard
    **guard = (name, age);

    // We've moved name & age into the guard, so we have to use that instead
    if prev_name.is_empty() {
            "Hello, {} year old named {}! You're the first person to show up",
            guard.0, guard.1
    } else {
            "Hello, {} year old named {}! I just saw {}.",
            guard.0, guard.1, prev_name

fn main() {
        // Start with empty state:
        .manage(Mutex::new((String::new(), 0u8)))
        .mount("/", routes![hello])