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

this first day rust, i'm trying trivial, , i'm stuck.

what i'm trying add struct vector, , return result. i'm trying create simple rest service store data in memory when posting, , return data in json format when doing get.

this current code:

fn main() {     let mut server = nickel::new();     let mut reservations = vec::new();     server.post("/reservations/", middleware! { |request, response|         let reservation = request.json_as::<reservation>().unwrap();          reservations.push(reservation); // <-- error occurs here          format!("hello {} {}", reservation.name, reservation.email)      });      server.listen("127.0.0.1:3000"); } 

i tried this solution refcell, error trait sync not implemented vec<reservation::reservation>

this example of how rust protects thread unsafety.

if think it, in current code possible multiple threads try concurrently mutate reservations without kind of synchronization. data race , rust complain it.

a possible solution wrap reservations vector mutex synchronization. need arc (atomic reference counting), since rust cannot prove reservations live longer threads.

with these changes, code should 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 documentation additional info mutex , arc.