在实现返回可变引用的迭代器时，如何解决“无法为autoref推断适当的生存期”？

我正在尝试为Thread每个元素实现的链表编写一个可变的迭代器Block。

trait Block<'a> {
    fn next(&mut self) -> Option<&'a mut (dyn Block<'a> + 'a)> {
        None
    }
}

pub struct Thread<'a> {
    head: Box<dyn Block<'a> + 'a>,
}

impl<'a> Thread<'a> {
    fn iter_mut(&mut self) -> ThreadIterator<'a> {
        ThreadIterator {
            next: Some(self.head.as_mut()),
        }
    }
}

pub struct ThreadIterator<'a> {
    next: Option<&'a mut (dyn Block<'a> + 'a)>,
}

impl<'a> Iterator for ThreadIterator<'a> {
    type Item = &'a mut (dyn Block<'a> + 'a);

    fn next(&mut self) -> Option<&'a mut (dyn Block<'a> + 'a)> {
        self.next.take().map(|mut block| {
            self.next = block.next();
            block
        })
    }
}

编译将输出错误：

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  --> src/lib.rs:14:34
   |
14 |             next: Some(self.head.as_mut()),
   |                                  ^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 12:5...
  --> src/lib.rs:12:5
   |
12 | /     fn iter_mut(&mut self) -> ThreadIterator<'a> {
13 | |         ThreadIterator {
14 | |             next: Some(self.head.as_mut()),
15 | |         }
16 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/lib.rs:14:24
   |
14 |             next: Some(self.head.as_mut()),
   |                        ^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined on the impl at 11:6...
  --> src/lib.rs:11:6
   |
11 | impl<'a> Thread<'a> {
   |      ^^
note: ...so that the types are compatible
  --> src/lib.rs:14:24
   |
14 |             next: Some(self.head.as_mut()),
   |                        ^^^^^^^^^^^^^^^^^^
   = note: expected `dyn Block<'_>`
              found `dyn Block<'a>`

这就是为什么我需要'a所有Blocks的要求（它们都借用了Runtime）：

struct Runtime {}

struct ExampleBlock<'a> {
    runtime: &'a Runtime,
    next: Box<dyn Block<'a> + 'a>,
}

impl<'a> Block<'a> for ExampleBlock<'a> {
    fn next(&mut self) -> Option<&'a mut (dyn Block<'a> + 'a)> {
        Some(self.next.as_mut())
    }
}

我尝试的第一件事是从所有引用中删除可变需求。同样的错误。

我认为该错误告诉我该消息self.head.as_mut()已过期self.head，因此我必须确保该引用的生存期短于Thread<'a>。我以为我的'a生命达到了这一要求ThreadIterator<'a>。换句话说，您不可能掉到Thread之前ThreadIterator，对吗？

编辑：

我改变Block了结构以简化代码，尽管我最终需要将其作为特征。

struct Block {}

impl<'a> Block {
    fn next(&mut self) -> Option<&'a mut Block> {
        None
    }
}

pub struct Thread {
    head: Block,
}

impl<'a> Thread {
    fn iter_mut(&mut self) -> ThreadIterator<'a> {
        ThreadIterator {
            next: Some(&mut self.head),
        }
    }
}

pub struct ThreadIterator<'a> {
    next: Option<&'a mut Block>,
}

impl<'a> Iterator for ThreadIterator<'a> {
    type Item = &'a mut Block;

    fn next(&mut self) -> Option<&'a mut Block> {
        self.next.take().map(|mut block| {
            self.next = block.next();
            block
        })
    }
}

它基于https://rust-unofficial.github.io/too-many-lists/second-iter-mut.html。

“由于需求冲突而无法为autoref推断合适的生存期”但由于特征定义约束而不能改变任何内容的答案是Option为迭代器引入a ，我已经做到了。自定义迭代器中的可变参数上的生命周期参数问题以及LinkedList的重新实现：未编译的IterMut没有回答我的问题，尽管我很难将代码连接到他们的代码上。

我终于找到了行之有效的东西：

pub struct Block {}

impl<'a> Block {
    fn next(&mut self) -> Option<&'a mut Block> {
        None
    }
}

pub struct Thread {
    head: Block,
}

impl Thread {
    fn iter_mut(&mut self) -> ThreadIterator<'_> { // The lifetime here is changed
        ThreadIterator {
            next: Some(&mut self.head),
        }
    }
}

pub struct ThreadIterator<'a> {
    next: Option<&'a mut Block>,
}

impl<'a> Iterator for ThreadIterator<'a> {
    type Item = &'a mut Block;

    fn next(&mut self) -> Option<&'a mut Block> {
        self.next.take().map(|mut block| {
            self.next = block.next();
            block
        })
    }
}

我很难将其应用于原始代码，因为可能有两个不同的生存期，一个用于迭代器，一个用于特征。