Returning Iterator from trait method with lifetime bound

help
1

Hello,

The code below returns an iterator object and works fine

use std::marker::PhantomData;

struct MyContainer<'a> {
    records : Vec<String>,
    phantom : PhantomData<&'a ()>,
}

struct MyElementPointer<'a> {
    container : &'a MyContainer<'a>,
    record : &'a String
}

impl <'a>MyContainer<'a> {
    pub fn iter<'incoming>(&'incoming self) -> Box<dyn Iterator<Item=MyElementPointer<'incoming>> + 'incoming> {
        Box::new(self.records.iter().map(move |record| MyElementPointer{container : self, record : record}))
    }
}

fn main() {
    let my_container = MyContainer{phantom : PhantomData, records : vec!["one".to_string(), "two".to_string(), "three".to_string()]};
    
    for pointer in my_container.iter() {
        println!("{} of {}", pointer.record, pointer.container.records.len());
    }
}

But when I make the container into a dyn trait object, it fails.

use std::marker::PhantomData;

struct MyContainer<'a> {
    records : Vec<String>,
    phantom : PhantomData<&'a ()>,
}

struct MyElementPointer<'a> {
    container : &'a dyn ContainerTrait<'a>,
    record : &'a String
}

impl <'a>ContainerTrait<'a> for MyContainer<'a> {
    fn iter<'incoming>(&'incoming self) -> Box<dyn Iterator<Item=MyElementPointer<'incoming>> + 'incoming> {
        Box::new(self.records.iter().map(move |record| MyElementPointer{container : self, record : record}))
    }
}

trait ContainerTrait<'a> {
    fn iter<'incoming>(&'incoming self) -> Box<dyn Iterator<Item=MyElementPointer<'incoming>> + 'incoming>;
}

fn main() {
    let my_container = MyContainer{phantom : PhantomData, records : vec!["one".to_string(), "two".to_string(), "three".to_string()]};
    
    for pointer in my_container.iter() {
        println!("{} of {}", pointer.record, pointer.container.records.len());
    }
}

There is no problem if the container doesn't have a lifetime bound (hence the PhantomData). Somehow it's the interaction of the lifetime bound of the trait object with the variable capture of the closure.

Has anyone found a work-around for this?

Thank you.

2

Perhaps it is wrong-headed of me to try and bound the &dyn trait object. I guess that bound would be effectively accomplished by the lifetime bound of the reference to the object, which avoids the problem.