我怎样才能使下面的代码编译，我做错了什么？

fn try_attack(game_state: &mut GameState) {
    let mut rng = rand::thread_rng();
    let length = game_state.kingdoms.len();

    let player = game_state.kingdoms.first_mut().unwrap();

    let target = game_state
        .kingdoms
        .get(rng.gen_range(1..length) as usize)
        .unwrap();

    if target.soldiers < player.soldiers || target.gold < player.gold {
        player.gold += (target.gold as f64 * 0.1) as u32;
        player.soldiers += (target.soldiers as f64 * 0.05) as u32;
    }
}

我得到的错误：

error[E0502]: cannot borrow `game_state.kingdoms` as immutable because it is also borrowed as mutable
  --> src\game_utils.rs:85:18
   |
83 |       let player = game_state.kingdoms.first_mut().unwrap();
   |                    ------------------------------- mutable borrow occurs here
84 |
85 |       let target = game_state
   |  __________________^
86 | |         .kingdoms
87 | |         .get(rng.gen_range(1..length) as usize)
   | |_______________________________________________^ immutable borrow occurs here
...
90 |       if target.soldiers < player.soldiers || target.gold < player.gold {
   |                            --------------- mutable borrow later used here

我知道编译器不喜欢我借用一些不可变的东西，当我借用一些可变的东西时，但是如果我想从 Realm 向量中得到一个随机目标，我需要这样做。
正确的方法是什么？rust试图在这里执行什么编程范式？