ChatGPT generated code for a simple Diplomacy like game

diplo.rs

use std::collections::HashMap;

struct Square {
    name: String,
    pawn: Option<String>,
}

struct GameBoard {
    squares: HashMap<String, Square>,
}

impl GameBoard {
    fn new() -> Self {
        let mut squares = HashMap::new();
        // initialize squares with their names
        squares.insert("A1".to_string(), Square { name: "A1".to_string(), pawn: None });
        squares.insert("A2".to_string(), Square { name: "A2".to_string(), pawn: None });
        squares.insert("B1".to_string(), Square { name: "B1".to_string(), pawn: None });
        squares.insert("B2".to_string(), Square { name: "B2".to_string(), pawn: None });
        
        GameBoard { squares }
    }
    
    fn move_pawn(&mut self, pawn: &str, target: &str) {
        let current_square = self.squares.get_mut(pawn).unwrap();
        let target_square = self.squares.get_mut(target).unwrap();
        
        if target_square.pawn.is_none() {
            target_square.pawn = current_square.pawn.take();
        }
    }
}

fn main() {
    let mut game_board = GameBoard::new();
    // place pawns on the board
    game_board.squares.get_mut("A1").unwrap().pawn = Some("Pawn 1".to_string());
    game_board.squares.get_mut("B2").unwrap().pawn = Some("Pawn 2".to_string());
    
    // move Pawn 1 to B1
    game_board.move_pawn("A1", "B1");
    // move Pawn 2 to A2, but it fails because Pawn 1 is already on B1
    game_board.move_pawn("B2", "A2");
    
    println!("{:?}", game_board.squares);
}

Problem was:

let's say I have a game boards with named squares. A pawn on one square can only move to the adjacent square. Two pawns cannot be on the same square. If two paws decide to go on the same square, they do not move.
Please write a piece of Rust code that handle that.