Playground here. I had a variation of this code working, but now I’ve added some interior mutability to the Thing, so my implementation of iter_children for Thing is no longer good enough. The goal is to return from iter_children an abstract iterator over Rc<dyn Node> items.

Is there a way to adjust this code so it works with the borrowed state? Maybe with complicated iterators like this it would be easier to use an “internal” iterator – ie, pass a dyn Fn(&dyn Node) → () to the iter_children function, as opposed to trying to return in iterator?

use std::rc::Rc;
use std::cell::{Ref, RefCell};

type ChildIter<'a> = Box<dyn DoubleEndedIterator<Item = Rc<dyn Node + 'static>> + 'a>;

trait Node { 
    fn iter_children(&self) -> Option<ChildIter>;
}

struct Thing {
    state: RefCell<State>
}
struct ChildInfo {
    child: Rc<dyn Node>,
    other: u32,
}
struct State {
    nodes: Vec<ChildInfo>
}

impl Node for Thing {
    fn iter_children(&self) -> Option<ChildIter> {
        let b: Ref<State> = self.state.borrow();
        Some( Box::new( b.nodes.iter().map(|info| info.child.clone())) )
    }
}

The error:

error[E0515]: cannot return value referencing local variable `b`
  --> src/main.rs:26:9
   |
26 |         Some( Box::new( b.nodes.iter().map(|info| info.child.clone())) )
   |         ^^^^^^^^^^^^^^^^-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |         |               |
   |         |               `b` is borrowed here
   |         returns a value referencing data owned by the current function

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