为什么我不能创建一个变量范围?[复制品]

我试图创造 pub fn sing(start: i32, end: i32) -> String 返回调用结果的连接字符串 pub fn verse(num: i32) -> String 在每个数字上重复 start 和 end .

我在谷歌上搜索了答案,似乎是 Rust String concatenation 回答我的问题,如果我把代码写进 playground 有效,但是:

我的代码:

pub fn verse(num: i32) -> String {
    match num {
        0 => "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".to_string(),
        1 => "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".to_string(),
        2 => "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".to_string(),
        num => format!("{0} bottles of beer on the wall, {0} bottles of beer.\nTake one down and pass it around, {1} bottles of beer on the wall.\n",num,(num-1)),
    }
}

pub fn sing(start: i32, end: i32) -> String {
    (start..end).fold(String::new(), |ans, x| ans+&verse(x))
}

问题是

#[test]
fn test_song_8_6() {
    assert_eq!(beer::sing(8, 6), "8 bottles of beer on the wall, 8 bottles of beer.\nTake one down and pass it around, 7 bottles of beer on the wall.\n\n7 bottles of beer on the wall, 7 bottles of beer.\nTake one down and pass it around, 6 bottles of beer on the wall.\n\n6 bottles of beer on the wall, 6 bottles of beer.\nTake one down and pass it around, 5 bottles of beer on the wall.\n");
}

失败了 beer::sing(8,6) 返回 "" .