#[derive(Debug, Deserialize)]
struct Root {
    foo: Vec<Foo>,
}

#[derive(Debug, Deserialize)]
struct Foo {
    a: String,
    b: Option<String>,
}

fn main()
{
    let xml =
r#"
<root>
 <foo>
  <a>Hello</a>
  <b>World</b>
 </foo>
 <foo>
  <a>Hi</a>
  <b/>
 </foo>
</root>"#;

    let root: Root = xml::from_str(&xml).unwrap();
    println!("\n\n{:?}", root);
}