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types - Rust 中这个奇怪的递归类型错误是怎么回事？

问题描述

我有一个名为的可迭代结构Join：

use std::iter::Peekable;

#[derive(Debug)]
pub struct Join<T, S> {
    container: T,
    separator: S,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum JoinItem<T, S> {
    Element(T),
    Separator(S),
}

pub struct JoinIter<Iter: Iterator, Sep> {
    iter: Peekable<Iter>,
    sep: Sep,
    next_sep: bool,
}

impl<Iter: Iterator, Sep> JoinIter<Iter, Sep> {
    fn new(iter: Iter, sep: Sep) -> Self {
        JoinIter {
            iter: iter.peekable(),
            sep,
            next_sep: false,
        }
    }
}

impl<I: Iterator, S: Clone> Iterator for JoinIter<I, S> {
    type Item = JoinItem<I::Item, S>;

    /// Advance to the next item in the Join. This will either be the next
    /// element in the underlying iterator, or a clone of the separator.
    fn next(&mut self) -> Option<Self::Item> {
        let sep = &self.sep;
        let next_sep = &mut self.next_sep;

        if *next_sep {
            self.iter.peek().map(|_| {
                *next_sep = false;
                JoinItem::Separator(sep.clone())
            })
        } else {
            self.iter.next().map(|element| {
                *next_sep = true;
                JoinItem::Element(element)
            })
        }
    }
}

Join对实现的引用IntoIterator：

impl<'a, T, S> IntoIterator for &'a Join<T, S>
where
    &'a T: IntoIterator,
{
    type IntoIter = JoinIter<<&'a T as IntoIterator>::IntoIter, &'a S>;
    type Item = JoinItem<<&'a T as IntoIterator>::Item, &'a S>;

    fn into_iter(self) -> Self::IntoIter {
        JoinIter::new(self.container.into_iter(), &self.separator)
    }
}

这将编译并通过使用测试。

我还在我的结构iter上定义了一个方法：Join

impl<T, S> Join<T, S>
where
    for<'a> &'a T: IntoIterator,
{
    pub fn iter(&self) -> JoinIter<<&T as IntoIterator>::IntoIter, &S> {
        self.into_iter()
    }
}

这编译得很好，但是当我实际尝试使用它时：

fn main() {
    // Create a join struct
    let join = Join {
        container: vec![1, 2, 3],
        separator: ", ",
    };

    // This works fine
    let mut good_ref_iter = (&join).into_iter();
    assert_eq!(good_ref_iter.next(), Some(JoinItem::Element(&1)));
    assert_eq!(good_ref_iter.next(), Some(JoinItem::Separator(&", ")));
    assert_eq!(good_ref_iter.next(), Some(JoinItem::Element(&2)));
    assert_eq!(good_ref_iter.next(), Some(JoinItem::Separator(&", ")));
    assert_eq!(good_ref_iter.next(), Some(JoinItem::Element(&3)));
    assert_eq!(good_ref_iter.next(), None);

    // This line fails to compile; comment out this section and it all works
    let bad_ref_iter = join.iter();
    assert_eq!(bad_ref_iter.next(), Some(JoinItem::Element(&1)));
    assert_eq!(bad_ref_iter.next(), Some(JoinItem::Separator(&", ")));
    assert_eq!(bad_ref_iter.next(), Some(JoinItem::Element(&2)));
    assert_eq!(bad_ref_iter.next(), Some(JoinItem::Separator(&", ")));
    assert_eq!(bad_ref_iter.next(), Some(JoinItem::Element(&3)));
    assert_eq!(bad_ref_iter.next(), None);
}

我得到某种奇怪的类型递归错误：

error[E0275]: overflow evaluating the requirement `&_: std::marker::Sized`
   --> src/join.rs:288:29
    |
 96 |         let mut iter = join.iter();
    |                             ^^^^
    |
    = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&_`
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&join::Join<_, _>`
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&join::Join<join::Join<_, _>, _>`
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&join::Join<join::Join<join::Join<_, _>, _>, _>`
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&join::Join<join::Join<join::Join<join::Join<_, _>, _>, _>, _>`
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&join::Join<join::Join<join::Join<join::Join<join::Join<_, _>, _>, _>, _>, _>`
    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `&join::Join<join::Join<join::Join<join::Join<join::Join<join::Join<_, _>, _>, _>, _>, _>, _>`
...

（我在...中修改了大约 100 行递归类型错误）

据我所知，它似乎是在尝试主动评估是否&Join<_, _>实现了IntoIterator，这需要检查是否&Join<Join<_, _>, _>满足 IntoIterator，等等。我无法弄清楚为什么它认为它必须这样做，因为我的实际类型完全限定为Join<Vec<{integer}, &'static str>. 我尝试过的一些事情：

为什么(&join).into_iter()工作，但join.iter()没有，即使只是在引擎盖下iter()调用？self.into_iter()

这个具有相同代码的完整示例也可以在Rust Playground中找到

有关更多上下文Join，请参阅我较早的Stack Overflow 问题和我的实际源代码。

标签： typesreferencerusttype-inference

解决方案

似乎编译器无法解决iter()返回类型所需的特征要求JoinIter<<&T as IntoIterator>::IntoIter, &S>。

我从rustc --explain E0275错误解释中收到了一个提示：

当递归特征要求在评估之前溢出时，会发生此错误。这通常意味着在解决某些类型边界时存在无限递归。

我不知道rust的推理过程的细节，我想会发生以下情况。

拿这个签名：

fn iter(&self) -> JoinIter<<&T as IntoIterator>::IntoIter, &S>

编译器尝试从以下位置推断返回类型：

JoinIter<<&T as IntoIterator>::IntoIter, &S>

但从impl<&T as IntoIterator>::IntoIter中推断：&'a Join

impl<'a, T, S> IntoIterator for &'a Join<T, S>
    where &'a T: IntoIterator
{
    type IntoIter = JoinIter<<&'a T as IntoIterator>::IntoIter, &'a S>;
    type Item = JoinItem<<&'a T as IntoIterator>::Item, &'a S>;

    fn into_iter(self) -> Self::IntoIter {
        JoinIter::new(self.container.into_iter(), &self.separator)
    }
}

又IntoIter是一个JoinIter<<&'a T as IntoIterator>::IntoIter, &'a S>无限循环的IntoIter无限循环。

使其编译的一种方法是使用turbofish帮助编译器：

let mut bad_ref_iter = Join::<Vec<i32>, &str>::iter(&join);

代替：

let bad_ref_iter = join.iter();

更新

该行：

type IntoIter = JoinIter<<&'a T as IntoIterator>::IntoIter, &'a S>;

生成递归，因为 Rust 在定义而不是使用特征时检查特征是否有效。

有关惰性标准化工作进展的更多详细信息和指针，请参阅此帖子，这可能会解决此问题。

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