Rust 替换字符串中的字符不起作用

我用 Rust 编写了一个小程序，但它不起作用。

use std::io;

fn main() {
    println!("Enter a string: ");
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();

    println!("you have entered:\n{}", input);

    println!("enter a char which you wish to replace: ");
    let mut char_to_be_replaced = String::new();
    io::stdin().read_line(&mut char_to_be_replaced).unwrap();
    println!("you have entered: {}", char_to_be_replaced);

    println!("enter a replacement char:");
    let mut replacement_char = String::new();
    io::stdin().read_line(&mut replacement_char).unwrap();
    println!("replacement char is: {}", replacement_char);

    let result = input.replace(&char_to_be_replaced, &replacement_char);
    println!("end result is: ");
    println!("{}", result);
}

输入是“asd，asd”

char_to_be_replaced 是“，”

替换字符是“。”

令我恐惧的结果是“asd，asd”而不是“asd。asd”