pub struct A {
    pub val: Vec<u32>,
}

impl A {
    pub fn iter(&self) -> AIter {
        AIter {
            a: self,
            index: 0,
        }
    }
}

pub struct AIter<'a> {
    a: &'a A,
    index: usize,
}

pub struct AIterMut<'a> {
    a: &'a mut A,
    index: usize,
}


impl<'a> Iterator for AIter<'a> {
    type Item = &'a u32;

    fn next(&mut self) -> Option<Self::Item> {
        let item = if self.a.val.len() > self.index {
            Some(&self.a.val[self.index])
        } else {
            None
        };
        self.index += 1;
        item
    }
}

impl<'a> Iterator for AIterMut<'a> {
    type Item = &'a mut u32;

    fn next(&mut self) -> Option<Self::Item> {
        let item = if self.a.val.len() > self.index {
            Some(&mut self.a.val[self.index])
        } else {
            None
        };
        self.index += 1;
        item
    }
}

fn main() {

    let a: Vec<u32> = (0..10).collect();
    let struct_a = A { val: a };
    let iter = struct_a.iter();

    iter.for_each(|x| println!("{}", x));

}