Vec of Rng in struct field, cannot borrow immutable field as mutable

Question

I wanted to create a structwith a field containing a Vec<ThreadRng>.

So this works fine, I have a list of generators stored in Foo.generators:

extern crate rand;

use std::vec::Vec;
use rand::{Rng, ThreadRng};

struct Foo {
    generators: Vec<ThreadRng>,
}

impl Foo {
    fn new() -> Foo {
        Foo { generators: vec![rand::thread_rng(), rand::thread_rng()]}
    }
}

Now I would like to use it, say in a bar method:

impl Foo {
    fn bar(&self) -> i32 {
        self.generators[0].gen::<i32>()
    }
}

But that I can't, and I get a cannot borrow immutable field 'self.generators' as mutable.

As I understand I cannot use the gen method of ThreadRng because it requires a mutable reference to the RNG (gen<T: Rand>(&mut self)) and since the definition of my field Foo.generators "Vec<ThreadRng>" doesn't specify that the ThreadRng should be mutable, I can't do anything with it that requires mutability.

First question: is my understanding of what's happening correct, or am I completely wrong? If so, could someone explain to me what's really happening?

Second question: admitting that my understanding is correct, how am I supposed to allow actions that requires mutability on my ThreadRngs?

Thanks in advance!

Answer 1

You need to borrow self mutably:

impl Foo {
    fn bar(&mut self) -> i32 {
        self.generators[0].gen::<i32>()
    }
}