Cannot borrow captured outer variable in an `Fn` closure as mutable

This is a very good example of how Rust protects you from thread unsafety.

If you think about it, in your current code it would be possible that multiple threads try to concurrently mutate reservations without any kind of synchronization. This is a data race and Rust will complain about it.

A possible solution would be to wrap the reservations vector into a Mutex to get synchronization. You will also need an Arc (atomic reference counting), since Rust cannot prove that reservations will live longer than the threads.

With these changes, your code should be like the following:

use std::sync::{Arc, Mutex};
fn main() {
    let mut server = Nickel::new();
    let reservations = Arc::new(Mutex::new(Vec::new()));

    server.post("/reservations/", middleware! { |request, response|
        let reservation = request.json_as::<Reservation>().unwrap();

        reservations.lock().unwrap().push(reservation); // <-- error occurs here

        format!("Hello {} {}", reservation.name, reservation.email)

    });

    server.listen("127.0.0.1:3000");
}

You can check the documentation for additional info about Mutex and Arc.