Rust Playground: multiple mutable references to mutexed struct fields

playground.rs

use std::sync::{Arc, Mutex};

pub struct Foo {
   pub a: i32,
   pub b: i32,
}

pub fn function(foo: Arc<Mutex<Foo>>){
let foo = &mut *(foo.lock().unwrap());
inner(&mut foo.a, &mut foo.b);
}

fn inner(a: &mut i32, b:&mut i32){
 *a += 1;
 *b += *a;
}

fn main(){
  let x = Arc::new(Mutex::new(Foo{a :10, b: 20}));
  function(x.clone());
  function(x.clone());
  let x = x.lock().unwrap();
  println!("{} {}", x.a, x.b);

}