rust - 如何在宏中的多个参数上编写嵌套循环？

Question

下面的代码

macro_rules! test {
    ( $( $x1:expr ),*; blub $( $x2:expr ),* ) => {
        $(
            println!("{} * {} = {}", $x1, $x2, $x1 * $x2);
        )*
    }
}

fn main() {
    test!{1, 2, 3; blub 4, 5, 6};
}

打印:

1 * 4 = 4
2 * 5 = 10
3 * 6 = 18

但是我想像嵌套循环一样分别遍历两个列表。它应该打印:

1 * 4 = 4
1 * 5 = 5
1 * 6 = 6
2 * 4 = 8
2 * 5 = 10
2 * 6 = 12
3 * 4 = 12
3 * 5 = 15
3 * 6 = 18

我该怎么做？

Answer 1

我发现的唯一方法是通过使用 token 树作为参数之一来作弊:

macro_rules! test {
    // secondary invocation with a `[]` delimited list of parameters
    // as the first arguments and a single second argument.
    ( [ $( $x1:expr),* ] ; $x2:expr ) => {
        $(
            println!("{:?} * {:?} = {:?}", $x1, $x2, $x1 * $x2);
        )*
    };

    // the main invocation of the macro, takes a token tree `x1`
    // and a `[]` delimited `,` separated list of arguments for
    // each of which it calls itself again with `x1` as first
    // parameter and the element of the list as the second
    ( $x1:tt [ $( $x2:expr ),* ] ) => {
        $(
            test!($x1; $x2);
        )*
    };
}

fn main() {
    test!{
        [1, 2, 3]
        [4, 5, 6]
    };
}