struct Document<'a> {
    fields: Vec<Box<Field + 'a>>,
}

impl<'a> Document<'a> {
    fn new() -> Document<'a> {
        Document { fields: Vec::new() }
    }

    fn add_field<T>(&mut self, field: T)
    where
        T: Field + 'a,
    {
        self.fields.push(Box::new(field))
    }
}

struct TextField {
    name: String,
    value: String,
}

trait Field {
    fn name(&self) -> &str;

    fn string_value(&self) -> &str;
}

impl TextField {
    fn new(name: &str, value: &str) -> TextField {
        TextField {
            name: name.into(),
            value: value.into(),
        }
    }
}

impl Field for TextField {
    fn name(&self) -> &str {
        &self.name
    }

    fn string_value(&self) -> &str {
        &self.value
    }
}

fn main() {
    let mut doc = Document::new();
    doc.add_field(TextField::new("title", "Rust in Action"));
    doc.add_field(TextField::new("isbn", "12345678"));
}