借用vs变异借用生命中的奇怪失败

Question

While trying to implement an iterator which yields mutable refs to elements of a linked list, I stumbled upon a strange issue.

This works fine:

impl<'a, T> Iterator<&'a T> for LinkedListIterator<'a, T>{
    fn next(&mut self) -> Option<&'a T> {
        match self.current {
            &Cell(ref x, ref xs) => {self.current = &**xs; Some(x)},
            &End                 => None
        }
    }
}

But this doesn't work; the compiler says lifetime of self is too short to guarantee its contents can be safely reborrowed:

impl<'a, T> Iterator<&'a mut T> for LinkedListMutIterator<'a, T>{
    fn next(&mut self) -> Option<&'a mut T> {
        match self.current {
            &Cell(ref mut x, ref mut xs) => {self.current = &mut **xs; Some(x)},
            &End                         => None
        }
    }
}

I would expect that either both example work, or both do not, but I can't understand how borrowing something as mutable vs not-mutable would impact the way the compiler checks for lifetimes. Surely if something lives long enough to be safely borrowed, it lives long enough to be safely mutably borrowed?

EDIT: Here is the definition of both Iterators:

pub struct LinkedListIterator<'a, T> 
    current: &'a LinkedList<T>
}

pub struct LinkedListMutIterator<'a, T> {
    current: &'a mut LinkedList<T>
}

LinkedLisk:

#[deriving(Eq, Clone)]
pub enum LinkedList<T> {
    Cell(T, ~LinkedList<T>),
    End
}

For a complete view of the file, please see https://github.com/TisButMe/rust-algo/blob/mut_iter/LinkedList/linked_list.rs

Answer 1

Note that you've left out the definition(s) of LinkedListMutIterator for the two variant bits of code, which might be relevant to any real attempt to reproduce and dissect your problem.

---

So, I'll try to guess at what's going on.

The compiler error message here might be misleading you; there are other factors beyond the lifetime of self that may be relevant here.

In particular I suspect the borrow-checker is complaining because it is trying to ensure that you are not creating multiple mutable-borrows that alias the same state.

• It is sound to have multiple immutable-borrows to the same piece of state...

• ... but you cannot have multiple mutable-borrows to the same piece of state (because we want to ensure that if you have a &mut reference to some state, then that reference is the only way to mutate the state).