如何在不返回的情况下传递 &mut str 并更改原始的 mut str？

问：

我正在从书中学习 Rust，我正在处理第 8 章末尾的练习，但我在将单词转换为猪拉丁语的练习中碰壁了。我特别想看看我是否可以将 a 传递给一个接受 a（也接受切片）的函数并修改其中引用的字符串，以便更改在不需要 a 的情况下反射回外部，就像在 C 中带有 a 一样。&mut String&mut strreturnchar **

我不太确定我是否只是搞砸了语法，或者由于 Rust 的严格规则，它是否比听起来更复杂，我还没有完全掌握。对于里面的终生错误，我记得读过一些解释如何正确处理这种情况的东西，但现在我找不到它，所以如果你也能为我指出来，将不胜感激。to_pig_latin()

另外，您如何看待我处理字符串内部字符和索引的方式？

use std::io::{self, Write};

fn main() {
    let v = vec![
        String::from("kaka"),
        String::from("Apple"),
        String::from("everett"),
        String::from("Robin"),
    ];

    for s in &v {
        // cannot borrow `s` as mutable, as it is not declared as mutable
        // cannot borrow data in a `&` reference as mutable
        to_pig_latin(&mut s);
    }

    for (i, s) in v.iter().enumerate() {
        print!("{}", s);

        if i < v.len() - 1 {
            print!(", ");
        }
    }

    io::stdout().flush().unwrap();
}

fn to_pig_latin(mut s: &mut str) {
    let first = s.chars().nth(0).unwrap();
    let mut pig;

    if "aeiouAEIOU".contains(first) {
        pig = format!("{}-{}", s, "hay");
        s = &mut pig[..]; // `pig` does not live long enough
    } else {
        let mut word = String::new();

        for (i, c) in s.char_indices() {
            if i != 0 {
                word.push(c);
            }
        }

        pig = format!("{}-{}{}", word, first.to_lowercase(), "ay");
        s = &mut pig[..]; // `pig` does not live long enough
    }
}

编辑：这是固定代码，其中包含以下建议。

fn main() {
    // added mut
    let mut v = vec![
        String::from("kaka"),
        String::from("Apple"),
        String::from("everett"),
        String::from("Robin"),
    ];

    // added mut
    for mut s in &mut v {
        to_pig_latin(&mut s);
    }

    for (i, s) in v.iter().enumerate() {
        print!("{}", s);

        if i < v.len() - 1 {
            print!(", ");
        }
    }

    println!();
}

// converted into &mut String
fn to_pig_latin(s: &mut String) {
    let first = s.chars().nth(0).unwrap();

    if "aeiouAEIOU".contains(first) {
        s.push_str("-hay");
    } else {
        // added code to make the new first letter uppercase
        let second = s.chars().nth(1).unwrap();

        *s = format!(
            "{}{}-{}ay",
            second.to_uppercase(),
            // the slice starts at the third char of the string, as if &s[2..]
            &s[first.len_utf8() * 2..],
            first.to_lowercase()
        );
    }
}