iterator - 将迭代器 Item 类型不匹配解析为具有显式生命周期的指针

Question

我正在尝试向 Iterator 添加函数，其中关联类型 Item 是对具有显式生命周期的结构的引用。

当我想修改迭代器状态或返回一个新值时，我没有遇到任何问题，但是当我尝试返回一个新的 Iterator where Item 时是一个具有显式生命周期的引用，编译器会提示。

例子

use std::marker::PhantomData;

/// First, an "Inner" struct to be contained in my custom iterator
pub struct Inner {
    text: String,
}

/// Then, the "CustomIterator" in question.  Notice that `Item` is `&'a Inner`.
pub struct CustomIterator<'a, I: Iterator<Item = &'a Inner>> {
    iter: I,
    _marker: PhantomData<&'a i8>,
}

/// Implementing Iterator for CustomIterator so as to define the `next()` function, as you do...
impl<'a, I: Iterator<Item = &'a Inner>> Iterator for CustomIterator<'a, I> {
    type Item = &'a Inner;
    fn next(&mut self) -> Option<Self::Item> {
        println!("Custom next called");
        self.iter.next()
    }
}

/// Now, creating a custom trait definition called IterateMore that:
///   1. inherits Iterator
///   2. includes a default method called `more` which returns a `CustomIterator`
pub trait IterateMore<'a>: Iterator {
    type Item;
    fn more(self) -> CustomIterator<'a, Self>
    where
        Self: Sized;
}

/// Implementing `IterateMore` for an iterator of the specific type `Iterator<Item=&'a Inner>`
impl<'a, I: Iterator<Item = &'a Inner>> IterateMore<'a> for I
where
    I: Iterator,
{
    type Item = &'a Inner;
    fn more(self) -> CustomIterator<'a, Self>
    where
        Self: Sized,
    {
        CustomIterator {
            iter: self,
            _marker: PhantomData,
        }
    }
}

fn main() {
    let inner = Inner {
        text: "Hello world".to_string(),
    };
    let inners = vec![&inner];
    inners.iter().more().next();
}

(在 repl.it 上查看)

错误

error[E0271]: type mismatch resolving `<Self as std::iter::Iterator>::Item == &'a Inner`
  --> src/main.rs:28:5
   |
28 | /     fn more(self) -> CustomIterator<'a, Self>
29 | |     where
30 | |         Self: Sized;
   | |____________________^ expected associated type, found reference
   |
   = note: expected type `<Self as std::iter::Iterator>::Item`
              found type `&'a Inner`
   = note: required by `CustomIterator`

为什么 Item 没有在这里解析？这有点令人沮丧，因为如果我尝试将 &'a Inner 设置为特征定义中的默认 Item 类型，编译器也会提示，说:

error: associated type defaults are unstable (see issue #29661)

如何解决这个问题或采取不同的方式？

Answer 1

我不清楚您为什么要将包装迭代器限制为某种自定义类型(假设您每次使用该类型时仍然必须写下限制，尽管这可能会改变)。但也许你的“真实”next函数做了一些有趣的事情。

• PhantomData在 where 中使用生命周期时，似乎不再需要(再)“使用”生命周期-子句。
• IterateMore不应该有 Item关联类型，给定 Iterator已经有了。 (如果您真的需要一种新类型，请选择不同的名称)
• 作为IterateMore使用 CustomIterator键入它需要重复要求，在本例中为 Iterator<Item = &'a Inner> (这就是类型不匹配错误的原因)；这与说 type Item = &'a Inner 不同在特征定义中。

Playground

/// an "Inner" struct to be contained in my custom iterator
pub struct Inner {
    text: String,
}

pub struct CustomIterator<'a, I>
where
    I: Iterator<Item = &'a Inner>,
{
    iter: I,
}

impl<'a, I> Iterator for CustomIterator<'a, I>
where
    I: Iterator<Item = &'a Inner>,
{
    type Item = I::Item;
    fn next(&mut self) -> Option<Self::Item> {
        println!("Custom next called");
        self.iter.next()
    }
}

pub trait IterateMore<'a>: Iterator<Item = &'a Inner> + Sized {
    fn more(self) -> CustomIterator<'a, Self>;
}

impl<'a, I> IterateMore<'a> for I
where
    I: Iterator<Item = &'a Inner>,
{
    fn more(self) -> CustomIterator<'a, Self> {
        CustomIterator { iter: self }
    }
}

fn main() {
    let inner = Inner {
        text: "Hello world".to_string(),
    };
    let inners = vec![inner];
    inners.iter().more().next();
}

你也可以像这样删除所有地方的类型限制(并且只将它添加回你实际需要/想要的地方):

Playground

pub struct CustomIterator<I> {
    iter: I,
}

impl<I> Iterator for CustomIterator<I>
where
    I: Iterator,
{
    type Item = I::Item;
    fn next(&mut self) -> Option<Self::Item> {
        println!("Custom next called");
        self.iter.next()
    }
}

pub trait IterateMore: Iterator + Sized {
    fn more(self) -> CustomIterator<Self>;
}

impl<I> IterateMore for I
where
    I: Iterator,
{
    fn more(self) -> CustomIterator<Self>
    {
        CustomIterator { iter: self }
    }
}

fn main() {
    let inners = vec!["Hello world".to_string()];
    inners.iter().more().next();
}