error-handling - 宏匹配臂模式 “no rules expected the token ` if`”

Question

所以我有这个宏来匹配多种错误类型的Box<dyn error::Error>

#[macro_export]
macro_rules! dynmatch {
    ($e:expr, $(type $ty:ty {$(arm $pat:pat => $result:expr),*, _ => $any:expr}),*, _ => $end:expr) => (
        $(
            if let Some(e) = $e.downcast_ref::<$ty>() {
                match e {
                    $(
                        $pat => {$result}
                    )*
                    _ => $any
                }
            } else
        )*
        {$end}
    );
}

直到我尝试添加火柴为止，一切都很好。当我尝试在模式中使用“if”语句时，它给我错误 no rules expected the token 'if'

let _i = match example(2) {
    Ok(i) => i,
    Err(e) => {
        dynmatch!(e,                                                            
            type ExampleError1 {                                                
                arm ExampleError1::ThisError(2) => panic!("it was 2!"),  
                _ => panic!("{}",e)                                             
            },
            type ExampleError2 {
                arm ExampleError2::ThatError(8) => panic!("it was 8!"),
                arm ExampleError2::ThatError(9..=11) => 10,
                _ => panic!("{}",e)
            }, 
            type std::io::Error {                                               
                arm i if i.kind() == std::io::ErrorKind::NotFound => panic!("not found"), //ERROR no rules expected the token `if`
                _ => panic!("{}", e)
            },
            _ => panic!("{}",e)                                                 
        )
    }
};

有什么方法可以在我的模式匹配中使用匹配防护而不会出现 token 错误？

Answer 1

当然，即使我花了一个小时寻找解决方案，在我发布此问题后我也找到了答案。
正确的宏如下所示:

#[macro_export]
macro_rules! dynmatch {
    ($e:expr, $(type $ty:ty {$(arm $( $pattern:pat )|+ $( if $guard: expr )? => $result:expr),*, _ => $any:expr}),*, _ => $end:expr) => (
        $(
            if let Some(e) = $e.downcast_ref::<$ty>() {
                match e {
                    $(
                        $( $pattern )|+ $( if $guard )? => {$result}
                    )*
                    _ => $any
                }
            } else
        )*
        {$end}
    );
}

归功于 rust matches! source线244-251