use std::iter::Iterator;

trait LineIter<'a> {
    type Iter: Iterator<Item = &'a mut String>;

    fn line_iter(&'a mut self) -> Self::Iter;
}

struct Page<'a> {
    lines: &'a mut Vec<String>,
}

impl<'a, 'b> LineIter<'a> for Page<'b>
where
    'a: 'b,
{
    type Iter = std::slice::IterMut<'a, String>;

    fn line_iter(&'a mut self) -> Self::Iter {
        self.lines.iter_mut()
    }
}

fn main() {
    let mut lines = Vec::new();
    lines.push("first line".to_string());
    lines.push("second line".to_string());
    lines.push("third line".to_string());

    let mut page = Page { lines: &mut lines };

    for line in page.line_iter() {
        line.push('!');
    }

    for line in page.line_iter() {
        println!("{}", line);
    }
}