Rust - Passing Trait Objects to Threads

Question

I've been struggling with this for a while now and just going in circles, so I'm hoping someone can point out where I'm going wrong.

I'm playing with Actix Web, and setting up my first handlers - which is a simple Healthcheck of the system. So what I've got is:

• A Healthcheck trait defining a healthcheck
• A HealthcheckHandler struct that implements the Handler trait (This is an Actix Web concept) and contains a HashMap
• A function that builds an App instance for the healthcheck routes (This is an Actix Web concept) by taking a HashMap<String, &Healthcheck>

​

When I try to build this I get errors that the trait "cannot be sent between threads safely".

I've tried with &Healthcheck, Box<Healthcheck>, Box<Healthcheck + Send> based on another answer on here, and even Mutex<&Healthcheck> all without luck, but all with subtly different errors. It all seems to be around needing to implement some combinations of Sync, Send and/or Clone, but I'm not sure how to get around that here.

Any pointers to what I should to fix this?

​

Actual example code:

pub trait Healthcheck {
    fn check(&self) -> Result<String, String>;
}
struct HealthcheckHandler {
    handlers: HashMap<String, Box<Healthcheck>>,
}
pub fn build_app(handlers: HashMap<String, Box<Healthcheck>>) -> App<()> {
    let handler = HealthcheckHandler {
        handlers: handlers,
    };

    App::new()
        .prefix("/health")
        .resource("", |r| {
            r.get().h(handler);
        })
}
pub fn start(settings: HashMap<String, String>) {
    let mut healthchecks: HashMap<String, Box<Healthcheck>> = HashMap::new();

    let server = server::new(|| { // <-- This is where the errors happen. This closure is used to spawn threads.
        vec![
            build_app(healthchecks).middleware(middleware::Logger::default())
        ]
    });
}

Cheers