Rust pushing to vector with borrow_mut

Question

Let's have a struct containing a vector of cities and a new_city function adding City to the vector. However, I got BorrowMutError which makes sense.

What should I do so I can call new_city multiple times (see below)? I would need to drop the borrow_mut reference in the new_city function but I don't know how.

//use std::rc::Rc;
use std::cell::RefCell;
use std::cell::Ref;

pub struct Simulation{
    cities: RefCell<Vec<City> >,
}


impl Simulation{

    pub fn new() -> Simulation
    {
        Simulation{
            cities: RefCell::new(Vec::new()),
        }
    }

    pub fn new_city(&self, name: &'static str) -> Ref<City> { 
        let city = City::new(name);
        self.cities.borrow_mut().push(city);

        Ref::map(self.cities.borrow(), |vec| vec.last().unwrap())
    }

}

#[derive(Debug, Copy, Clone)]
pub struct City {
    name: &'static str,
}

impl City{
    pub fn new(name: &'static str) -> City {
        City { name: name, }
    }
}

fn main(){

    let mut simulation = Simulation::new();

    let prg = simulation.new_city("Prague");
    let brn = simulation.new_city("Brno");
    let pls = simulation.new_city("Pilsen");


    println!("{:?}", prg);    

}

Answer 1

You could simply introduce a new scope at the end of which the BorrowMut gets dropped:

pub fn new_city(&self, name: &'static str) -> Ref<City> {
    {
        let city = City::new(name);
        self.cities.borrow_mut().push(city);
    }
    Ref::map(self.cities.borrow(), |vec| vec.last().unwrap())
}

But you can't hold on to the Refs across new_city calls either.

fn main() {
    let mut simulation = Simulation::new();
    let prg = simulation.new_city("Prague");
    drop(prg);
    let brn = simulation.new_city("Brno");
}