What does this Rust snippet output?

1struct Stack { items: Vec<i32> }
2impl Stack {
3    fn new() -> Self { Stack { items: vec![] } }
4    fn push(&mut self, x: i32) -> &mut Self {
5        self.items.push(x); self
6    }
7    fn len(&self) -> usize { self.items.len() }
8}
9fn main() {
10    let mut s = Stack::new();
11    s.push(1).push(2).push(3);
12    println!("{}", s.len());
13}