自定义SqIterator迭代器兼容filter等函数时遇类型不匹配错误求助

我正尝试实现一个可与filter、map、flatmap等迭代器函数无缝配合的自定义迭代器SqIterator，但不确定当前实现是否正确。目前代码在Dasher的dash方法中出现类型不匹配错误：创建Sq2实例时，第二个参数是filter返回的Filter<SqIterator<T>, ...>类型，而Sq2要求参数为SqIterator<Iter<'_, Square>>类型。

相关代码 #

struct SqIterator<I>(I);

impl<I> Iterator for SqIterator<I>
  where I: Iterator<Item = Square>, {
  type Item  = I::Item;

  fn next(&mut self) -> Option<Self::Item> {
    self.0.next()
  }
}

impl<I: Iterator<Item = Square> + Clone> SqIterator<I> {

  fn new(iter: I) -> Self {
    SqIterator(iter)
  }

  fn into_a_pd(&self) -> impl Iterator<Item = (Square, BitboardPD)> {
    self.0.clone().flat_map(|a| Waypoint::a_square(a)
    .into_iter().map(move |pd| (a, pd)))
  }
}

struct Sq2<T>(SqIterator<T>, SqIterator<T>);


struct Dasher;

impl Dasher {
  /* P_k!!W */
  fn no_dash<'a, T: Iterator<Item = Square> + Clone>(bs: &'a Bs, a: &'a SqIterator<T>, b: &'a SqIterator<T>) -> Sq2<T> {
    todo!()
  }

  fn dash<'a, T: Iterator<Item = Square> + Clone>(bs: &'a Bs, a: &'a SqIterator<T>, b: &'a SqIterator<T>) -> impl Iterator<Item = Sq2<T>> {
    a.into_a_pd().map(|(a, pd)| {
      let aa = SqIterator([a].iter());
      let bb = b.filter(|&b| pd.dest.into() == b.pos);
      Sq2(aa, bb)
    })
  }
}

错误信息 #

error[E0308]: mismatched types
   --> src/gg.rs:106:15
    |
105 |       let bb = b.filter(|&b| pd.dest.into() == b.pos);
    |                         ---- the found closure
106 |       Sq2(aa, bb)
    |       ---     ^^ expected `SqIterator<std::slice::Iter<'_, bitboard::Square>>`, found `Filter<SqIterator<T>, ...>`
    |       |
    |       arguments to this struct are incorrect
    |
    = note: expected struct `SqIterator<std::slice::Iter<'_, bitboard::Square>>`
               found struct `Filter<SqIterator<T>, [closure@src/gg.rs:105:25: 105:29]>`
note: tuple struct defined here
   --> src/gg.rs:91:8
    |
91  | struct Sq2<T>(SqIterator<T>, SqIterator<T>);

补充定义 #

#[derive(Copy, Clone)]
struct Square {
    pos: u32,
    piece: u32
}

struct BitboardPD {
  path: u32,
  dest: u32
}

struct Bs;

struct Waypoint;

impl Waypoint {
    fn a_square(a: Square) -> Vec<BitboardPD> {
        vec![]
    }
}

问题根源 #

1. Sq2要求两个参数都是SqIterator<T>类型，但b.filter(...)返回的是标准库的Filter<SqIterator<T>, ...>迭代器适配器类型，并非自定义的SqIterator。
2. aa的类型是SqIterator<std::slice::Iter<'_, Square>>，而未包装的bb类型与aa不匹配，导致Sq2的两个参数类型无法统一。

修改方案 #

1. 为SqIterator实现From trait，方便将任意迭代器（包括标准库适配器）包装为SqIterator。
2. 调整Sq2的泛型约束，允许它接受两个不同底层类型的SqIterator，避免类型不匹配问题。

修正后代码 #

struct SqIterator<I>(I);

impl<I> Iterator for SqIterator<I>
where I: Iterator<Item = Square>, {
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.next()
    }
}

// 实现From trait，快速将任意迭代器转为SqIterator
impl<I: Iterator<Item = Square>> From<I> for SqIterator<I> {
    fn from(iter: I) -> Self {
        SqIterator(iter)
    }
}

impl<I: Iterator<Item = Square> + Clone> SqIterator<I> {
    fn new(iter: I) -> Self {
        SqIterator(iter)
    }

    fn into_a_pd(&self) -> impl Iterator<Item = (Square, BitboardPD)> {
        self.0.clone().flat_map(|a| Waypoint::a_square(a)
            .into_iter().map(move |pd| (a, pd)))
    }
}

// 修改Sq2，允许两个参数为不同底层类型的SqIterator
struct Sq2<A, B>(SqIterator<A>, SqIterator<B>)
where
    A: Iterator<Item = Square>,
    B: Iterator<Item = Square>;

struct Dasher;

impl Dasher {
    /* P_k!!W */
    fn no_dash<'a, T: Iterator<Item = Square> + Clone>(bs: &'a Bs, a: &'a SqIterator<T>, b: &'a SqIterator<T>) -> Sq2<T, T> {
        todo!()
    }

    fn dash<'a, T: Iterator<Item = Square> + Clone>(bs: &'a Bs, a: &'a SqIterator<T>, b: &'a SqIterator<T>) -> impl Iterator<Item = Sq2<std::slice::Iter<'_, Square>, impl Iterator<Item = Square>>> {
        a.into_a_pd().map(|(a, pd)| {
            let aa = SqIterator::from([a].iter());
            // 将filter返回的迭代器包装为SqIterator
            let bb = SqIterator::from(b.filter(|&b| pd.dest.into() == b.pos));
            Sq2(aa, bb)
        })
    }
}

// 补充定义保持不变
#[derive(Copy, Clone)]
struct Square {
    pos: u32,
    piece: u32
}

struct BitboardPD {
    path: u32,
    dest: u32
}

struct Bs;

struct Waypoint;

impl Waypoint {
    fn a_square(a: Square) -> Vec<BitboardPD> {
        vec![]
    }
}

额外说明 #

• 如果需要Sq2的两个参数类型严格一致，可以使用Box<dyn Iterator<Item = Square>>做类型擦除，但会带来轻微性能开销。
• 若要让SqIterator完全无缝适配标准库迭代器方法，可以为它实现Filter、Map等适配器的直接调用方法，或者直接在每次调用标准库适配器后手动包装回SqIterator。

内容的提问来源于stack exchange，提问作者eguneys