Providing a control resource to complete the closure

I'm going to get my head around Rust trying to implement a singly linked list. Here are my data structures:

struct Linked<T> {
    head: Option<Box<Node<T>>>
}

struct Node<T> {
    data: T,
    next: Option<Box<Node<T>>>
}

      

        
        
        
      

    

Now I would like to add an iterator to this:

struct LinkedIter<'a, T: 'a> {
    node: Option<&'a Node<T>>,
}

      

        
        
        
      

    

I wrote a method .iter()

for Linked<T>

that compiles and works fine.

impl<T> Linked<T> {
    fn iter(&self) -> LinkedIter<T> {
        LinkedIter { node: match self.head {
                Some(ref node) => Some(&**node),
                None           => None
            }
        }
    }
}

      

        
        
        
      

    

This block will now match

convert Option<Box<Linked<T>>>

to Option<&Linked<T>>

. This is just the method Option::map()

. So I reimplemented this method using head.as_ref()

instead head

to avoid ownership of the content Option

in the closure.

impl<T> Linked<T> {
    fn iter(&self) -> LinkedIter<T> {
        LinkedIter { node:
            self.head.as_ref().map(|b: &Box<Node<T>>| &**b)
        }
    }
}

      

        
        
        
      

    

Unfortunately, a reference created in a closure cannot survive the closure, as it refers to something passed to the closure as a parameter. The compiler complains (slightly paraphrased):

error: cannot deduce the appropriate lifetime for borrowing due to conflicting requirements:

firstly, the lifetime cannot survive the anonymous lifetime # 1 defined by [in close]

but the lifetime must be valid for [function lifetime iter

]

so that the expression [ node:

] is assigned

How can I explain to the compiler that the link will be valid after the closure is closed?

(Playground)