Rust: error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements

Question

This is the minimal code:

struct Node {
    item: T,
    next: Link,
}

type Link = Option<         


        

Answer 1

We can rewrite your code as:

struct Node<T> {
    item: T,
    next: Link<T>,
}

type Link<T> = Option<Box<Node<T>>>;

pub struct IterMut<'a, T>(&'a mut Link<T>);

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;
    fn next(&mut self) -> Option<Self::Item> {
        if let Some(boxed_node) = self.0 {
            self.0 = &mut boxed_node.next;
            Some(&mut boxed_node.item)
        }
        else {
            None
        }
    }
}

You can see that boxed_node life end at the end of the function so you can't return a reference link to it.

The solution is to take a reference of the box and not a reference to the option:

struct Node<T> {
    item: T,
    next: Link<T>,
}

type Link<T> = Option<Box<Node<T>>>;

pub struct IterMut<'a, T>(Option<&'a mut Box<Node<T>>>);

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;
    fn next(&mut self) -> Option<Self::Item> {
        if let Some(boxed_node) = self.0.take() {
            self.0 = boxed_node.next.as_mut();
            Some(&mut boxed_node.item)
        }
        else {
            None
        }
    }
}

You can also remove the Box:

struct Node<T> {
    item: T,
    next: Link<T>,
}

type Link<T> = Option<Box<Node<T>>>;

pub struct IterMut<'a, T>(Option<&'a mut Node<T>>);

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;
    fn next(&mut self) -> Option<Self::Item> {
        if let Some(boxed_node) = self.0.take() {
            self.0 = boxed_node.next.as_mut().map(AsMut::as_mut);
            Some(&mut boxed_node.item)
        }
        else {
            None
        }
    }
}