Rust宏分支无法匹配unfurl!宏的原因及展开方法问询

问题描述 #

以下是最小可复现代码：

fn testmatch() -> i32 {
    let dependant = 4;
    
    macro_rules! testmacro {
        () => { return 0 };
    
        ({ $cond:expr => $res:literal } $(, $rest:tt)*) => {
            if $cond {
                return $res;
            } else {
                return testmacro!($($rest)*);
            }
        };
        
        (unfurl!($furled:ident) $(, $rest:tt)*) => {
            furls!($furled);
            testmacro!($($rest)*);
        };
    }
    
    macro_rules! furls {
        (test_furl) => { testmacro! {
            { dependant == 4 => 4 },
            { dependant == 5 => 5 }
        } };
    }

    match 1 {
        1 => testmacro! {
            { dependant == 2 => 2 },
            { dependant == 3 => 3 },
            unfurl!(test_furl)
        },
        _ => 4000000
    }
}

fn main() {
    let x = testmatch();
    println!("{}", x);
}

运行代码时出现以下错误和警告：

error: no rules expected the token `!`
  --> src/main.rs:32:19
   |
4  |     macro_rules! testmacro {
   |     ---------------------- when calling this macro
...
32 |             unfurl!(test_furl)
   |                   ^ no rules expected this token in macro call
   |
   = note: while trying to match sequence end

warning: unused macro definition: `furls`
  --> src/main.rs:21:18
   |
21 |     macro_rules! furls {
   |                  ^^^^^
   |
   = note: `#[warn(unused_macros)]` on by default

问题原因 #

1. 宏匹配模式不匹配：testmacro!的第三个分支模式(unfurl!($furled:ident) $(, $rest:tt)*)期望匹配unfurl!test_furl（无括号）的写法，但实际调用时传入的是unfurl!(test_furl)（带括号）。Rust的macro_rules!按词法token严格匹配，模式中未包含括号，因此无法识别( token，导致匹配失败。
2. furls!未被调用：由于testmacro!的分支未匹配成功，furls!宏从未被执行，因此触发未使用警告。

解决方案 #

步骤1：修正宏匹配模式 #

修改testmacro!的第三个分支，让模式包含括号，匹配unfurl!(test_furl)的完整结构：

(unfurl!($furled:ident) $(, $rest:tt)*) => {
    let res = furls!($furled);
    if res != 0 {
        res
    } else {
        testmacro!($($rest)*)
    }
};

步骤2：调整宏的返回逻辑 #

原代码中使用return会直接终止函数，导致后续宏调用被忽略。改为让宏返回表达式，通过条件判断衔接后续逻辑：

macro_rules! testmacro {
    () => { 0 };

    ({ $cond:expr => $res:literal } $(, $rest:tt)*) => {
        if $cond {
            $res
        } else {
            testmacro!($($rest)*)
        }
    };
    
    (unfurl!($furled:ident) $(, $rest:tt)*) => {
        let res = furls!($furled);
        if res != 0 {
            res
        } else {
            testmacro!($($rest)*)
        }
    };
}

完整修改后的代码 #

fn testmatch() -> i32 {
    let dependant = 4;
    
    macro_rules! testmacro {
        () => { 0 };

        ({ $cond:expr => $res:literal } $(, $rest:tt)*) => {
            if $cond {
                $res
            } else {
                testmacro!($($rest)*)
            }
        };
        
        (unfurl!($furled:ident) $(, $rest:tt)*) => {
            let res = furls!($furled);
            if res != 0 {
                res
            } else {
                testmacro!($($rest)*)
            }
        };
    }
    
    macro_rules! furls {
        (test_furl) => { testmacro! {
            { dependant == 4 => 4 },
            { dependant == 5 => 5 }
        } };
    }

    match 1 {
        1 => testmacro! {
            { dependant == 2 => 2 },
            { dependant == 3 => 3 },
            unfurl!(test_furl)
        },
        _ => 4000000
    }
}

fn main() {
    let x = testmatch();
    println!("{}", x); // 输出4
}

运行结果 #

修改后的代码会正确展开宏，输出4，且不再有错误和警告。

内容的提问来源于stack exchange，提问作者MaxTretikov