rust - 如何在match语句分支中声明变量？

Question

我正在尝试解决一个任务
https://www.codewars.com/kata/591588d49f4056e13f000001/train/rust

Your task is to implement an simple interpreter for the notorious esoteric language HQ9+ that will work for a single character input:

• If the input is 'H', return 'Hello World!'
• If the input is 'Q', return the input
• If the input is '9', return the full lyrics of 99 Bottles of Beer.

由于某种原因，我无法使其正常运行。我不知道我在这里错过了什么。如何在match语句分支中声明变量？

fn hq9(code: &str) -> Option<String> {
    match code{
        "H" => Some("Hello World!".to_string()),
        "Q" => Some("Q".to_string()),
        "9" => let s = String::new();
        (0..=99).rev().for_each(|x|
                                match x {
                                    x @ 3..=99 => s.push_str("{} bottles of beer on the wall, {} bottles of beer.\nTake one down and pass it around, {} bottles of beer on the wall.\n",x,x,x-1),
                                    2 => s.push_str("2 bottles of beer on the wall, 2 bottles of beer.\nTake one down and pass it around, 1 bottle of beer on the wall.\n"),
                                    1 => s.push_str("1 bottle of beer on the wall, 1 bottle of beer.\nTake it down and pass it around, no more bottles of beer on the wall.\n"),
                                    0 => s.push_str("No more bottles of beer on the wall, no more bottles of beer.\nGo to the store and buy some more, 99 bottles of beer on the wall.\n"),
                                    _ => panic!(),
                                })
            Some(s),
        _  => None,
    }
}

Answer 1

匹配臂使用语法PATTERN => EXPRESSION，如果表达式需要多个语句，请使用block {}:

fn hq9(code: &str) -> Option<String> {
    match code {
        "H" => Some("Hello World!".to_string()),
        "Q" => Some("Q".to_string()),
        "9" => { // <----------------- start block
            let s = String::new();
            (0..=99).rev().for_each(|x|
                match x {
                    x @ 3..=99 => s.push_str("{} bottles of beer on the wall, {} bottles of beer.\nTake one down and pass it around, {} bottles of beer on the wall.\n",x,x,x-1),
                    2 => s.push_str("2 bottles of beer on the wall, 2 bottles of beer.\nTake one down and pass it around, 1 bottle of beer on the wall.\n"),
                    1 => s.push_str("1 bottle of beer on the wall, 1 bottle of beer.\nTake it down and pass it around, no more bottles of beer on the wall.\n"),
                    0 => s.push_str("No more bottles of beer on the wall, no more bottles of beer.\nGo to the store and buy some more, 99 bottles of beer on the wall.\n"),
                    _ => panic!(),
                });
            Some(s)
        } // <----------------- end block
        _  => None,
    }
}

解决此问题会暴露其他错误。