修复 "no rules expected the token" 宏错误

Question

我正在尝试编写一个宏来解构 BSON 数据，如下所示：

let bson: Document = ...;
let (id, hash, name, path, modification_time, size, metadata, commit_data) = bson_destructure! {
    get id = from (bson), optional, name ("_id"), as ObjectId;
    get hash = from (bson), as String, through (|s| ContentHash::from_str(&s));
    get name = from (bson), as String;
    get path = from (bson), as Bson, through (PathBuf::from_bson);
    get modification_time = from (bson), as UtcDatetime, through (FileTime);
    get size = from (bson), as I64, through (|n| n as u64);
    get metadata = from (bson), as Document, through (Metadata::from_bson);
    get commit_data = from (bson), optional, as Document, through (CommitData::from_bson);
    ret (id, hash, name, path, modification_time, size, metadata, commit_data)
};

我已经为它写了下面的宏（相当大）：

macro_rules! bson_destructure {
    // required field
    (
        @collect req,
        [$target:ident, $source:expr, $field:expr, Bson, $f:expr],
        [];
        $($rest:tt)*
    ) => {{
        let $target = try!(match $source.remove($field) {
            Some(v) => $f(v),
            None => Err(BsonDestructureError::MissingField {
                field_name: $field,
                expected: "Bson"
            }),
        });
        bson_destructure!($($rest)*)
    }};
    (
        @collect req,
        [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr],
        [];
        $($rest:tt)*
    ) => {{
        let $target = try!(match $source.remove($field) {
            Some(v) => match v {
                ::ejdb::bson::Bson::$variant(v) => $f(v),
                v => Err(BsonDestructureError::InvalidType {
                    field_name: $field,
                    expected: stringify!($variant),
                    actual: v
                })
            },
            None => Err(BsonDestructureError::MissingField {
                field_name: $field,
                expected: stringify!($variant)
            }),
        });
        bson_destructure!($($rest)*)
    }};

    // optional field
    (
        @collect opt,
        [$target:ident, $source:expr, $field:expr, Bson, $f:expr],
        [];
        $($rest:tt)*
    ) => {{
        let $target = try!(match $source.remove($field) {
            Some(v) => $f(v).map(Some),
            None => Ok(None),
        });
        bson_destructure!($($rest)*)
    }};
    (
        @collect opt,
        [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr],
        [];
        $($rest:tt)*
    ) => {{
        let $target = try!(match $source.remove($field) {
            Some(v) => match v {
                ::ejdb::bson::Bson::$variant(v) => $f(v).map(Some),
                v => Err(BsonDestructureError::InvalidType {
                    field_name: $field,
                    expected: stringify!($variant),
                    actual: v
                })
            },
            None => Ok(None),
        });
        bson_destructure!($($rest)*)
    }};

    // change variant name
    (
        @collect $k:tt,
        [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr],
        [as $nv:ident, $($word:ident $arg:tt),*];
        $($rest:tt)*
    ) => {
        bson_destructure!(
            @collect $k,
            [$target, $source, $field, $nv, $f],
            [$($word $arg),*];
            $($rest)*
        )
    };

    // change final mapping function
    (
        @collect $k:tt,
        [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr],
        [through ($nf:expr), $($word:ident $arg:tt),*];
        $($rest:tt)*
    ) => {
        bson_destructure!(
            @collect $k,
            [$target, $source, $field, $variant, $nf],
            [$($word $arg),*];
            $($rest)*
        )
    };

    // change field name
    (
        @collect $k:tt,
        [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr],
        [name ($nn:expr), $($word:ident $arg:tt),*];
        $($rest:tt)*
    ) => {
        bson_destructure!(
            @collect $k,
            [$target, $source, $nn, $variant, $f],
            [$($word $arg),*];
            $($rest)*
        )
    };

    // main forms
    (get $target:ident = from ($source:expr), $($word:ident $arg:tt),*; $($rest:tt)*) => {
        bson_destructure!(
            @collect req,
            [$target, $source, stringify!($target), Bson, Ok],
            [$($word $arg),*];
            $($rest)*
        )
    };
    (get $target:ident = from ($source:expr), optional, $($word:ident $arg:tt),*; $($rest:tt)*) => {
        bson_destructure!(
            @collect opt,
            [$target, $source, stringify!($target), Bson, Ok],
            [$($word $arg),*];
            $($rest)*
        )
    };

    // final form
    (ret $e:expr) => { $e }
}

但是，上面的第一个示例会导致以下编译错误：

src/db/data.rs:345:22: 345:25 error: no rules expected the token `opt`
src/db/data.rs:345             @collect opt,
                                        ^~~

我有点惊讶它没有像往常一样显示错误位置（即没有指示发生扩展的位置），但是，当我注释使用宏的代码段时错误消失了出。

我不明白为什么它说没有规则需要这个令牌，因为有这样的规则，但也许我不明白。

我很确定这是可能的，因为这正是 quick_error crate 所做的，但我的宏编写技巧似乎仍然不足。

我应该如何修复宏以使其按预期工作？

为了完整性，下面是BsonDestructureError的定义：

#[derive(Debug, Clone)]
pub enum BsonDestructureError {
    InvalidType {
        field_name: &'static str,
        expected: &'static str,
        actual: Bson
    },
    InvalidArrayItemType {
        index: usize,
        expected: &'static str,
        actual: Bson
    },
    MissingField {
        field_name: &'static str,
        expected: &'static str
    }
}

我也在使用从 ejdb 板条箱重新导出的 bson 板条箱。 Here is a minimal example, runnable with cargo script 在稳定的 Rust 上。

Answer 1

cargo script，一个递归的 muncher，和我最喜欢的内部规则语法；我怎么能不呢？

首先，确切的问题可以通过运行 cargo rustc -- -Z trace-macros来确定。这将在扩展时输出每个规则，给我们一个 "backtrace"，经过一些手动重新格式化后，它看起来像这样：

bson_destructure! {
    get id = from ( bson ) , optional , name ( "_id" ) , as ObjectId ;
    get hash = from ( bson ) , as String ;
    get name = from ( bson ) , as String ;
    get path = from ( bson ) , as Bson ;
    get modification_time = from ( bson ) , as UtcDatetime ;
    get size = from ( bson ) , as I64 , through ( | n | n as u64 ) ;
    get metadata = from ( bson ) , as Document ;
    get commit_data = from ( bson ) , optional , as Document ;
    ret ( id , hash , name , path , modification_time , size , metadata , commit_data )
}

bson_destructure! {
    @ collect opt ,
    [ id , bson , stringify ! ( id ) , Bson , Ok ] ,
    [ name ( "_id" ) , as ObjectId ] ;

    get hash = from ( bson ) , as String ;
    get name = from ( bson ) , as String ;
    get path = from ( bson ) , as Bson ;
    get modification_time = from ( bson ) , as UtcDatetime ;
    get size = from ( bson ) , as I64 , through ( | n | n as u64 ) ;
    get metadata = from ( bson ) , as Document ;
    get commit_data = from ( bson ) , optional , as Document ;
    ret ( id , hash , name , path , modification_time , size , metadata , commit_data )
}

bson_destructure! {
    @ collect opt ,
    [ id , bson , "_id" , Bson , Ok ] , [ as ObjectId ] ;

    get hash = from ( bson ) , as String ;
    get name = from ( bson ) , as String ;
    get path = from ( bson ) , as Bson ;
    get modification_time = from ( bson ) , as UtcDatetime ;
    get size = from ( bson ) , as I64 , through ( | n | n as u64 ) ;
    get metadata = from ( bson ) , as Document ;
    get commit_data = from ( bson ) , optional , as Document ;
    ret ( id , hash , name , path , modification_time , size , metadata , commit_data )
}

仔细阅读bson_destructure!中的规则可以看出问题所在：没有匹配第三个扩展的规则。坦率地说，macro_rules! 是垃圾在报告递归规则时报告合理的错误位置；它指向 opt 令牌是无关紧要的。 真正的问题是找不到匹配的规则。

特别是违规规则是：

// change variant name
(
    @collect $k:tt,
    [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr],
    [as $nv:ident, $($word:ident $arg:tt),*];
    $($rest:tt)*
) => {
    ...
};

请注意 $nv:ident 后紧跟的逗号 。另请注意，输入中没有这样的逗号。这可以通过将逗号移到重复中来解决，如下所示：

// change field name ( @collect $k:tt, [$target:ident, $source:expr, $field:expr, $variant:ident, $f:expr], [name ($nn:expr) $(, $word:ident $arg:tt)*]; $($rest:tt)* ) => { ... };

另一种选择（也是我通常使用的选择）是在第一次遇到输入时简单地改变输入，以确保始终有一个尾随逗号。

由于本机依赖性，代码实际上不会在我的机器上编译，但我确实验证了进行此更改（包括此处和其他规则）一个类似的问题）允许它完成宏扩展。您可以使用 cargo rustc -- -Z unstable-options --pretty=expanded.
检查输出是否正确

修复 "no rules expected the token" 宏错误

Fixing "no rules expected the token" macro error

macros

rust