I’m having some issue figuring out a lifetime error I’m getting when trying to implement the Iterator trait on a struct internally storing a reference. I have simplified the code a bit, hopefully I haven’t omitted something important, but it looks like this:

pub trait QueryData {
    type Item<'a>;
    fn fetch<'r>(entity_registry: &'r mut EntityRegistry, idx1: usize, idx2: usize) -> Self::Item<'r>;
}

pub struct Query<'a, D>
where
    D: QueryData,
{
    entity_registry: &'a mut EntityRegistry,
    idx1: usize,
    idx2: usize,
    _pd: std::marker::PhantomData<D>,
}

impl<'a, D> Iterator for Query<'a, D>
where
    D: QueryData,
{
    type Item = D::Item<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        // Bunch of logic here to update the indices... 
        return Some(D::fetch(self.entity_registry, self.idx1, self.idx2));
    }
}

This fails to compile with the error:

172 | impl<'a, D> Iterator for Query<'a, D>
    |      -- lifetime `'a` defined here
...
178 |     fn next(&mut self) -> Option<Self::Item> {
    |             - let's call the lifetime of this reference `'1`
...
201 |                                 break Some(D::fetch(self.entity_registry, self.idx1, self.idx2));
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method was supposed to return data with lifetime `'a` but it is returning data with lifetime `'1`

I’m a bit puzzled because in my mind lifetimes should be properly setup:

• entity_registry has lifetime 'a, by the definition of the Query struct.
• Iterator::Item also has lifetime 'a, by the implementation of Iterator.
• QueryData::fetch requires entity_registry and the returned Item to have the same lifetime.

The moment I’m calling D::fetch within next, I would expect the compiler to figure out that self.entity_registry has lifetime 'a, by definition, which matches the lifetime of the returned data.

Clearly I’m missing something though. 😀 Is there some hidden lifetime issue, or maybe I’m just forgetting to annotate lifetimes somewhere?