why use a not age?

example.rs

// an example shows how rust make @ in match expression systax

match age {
  a @ 0..20 => println!("{} years old", age),
  _ => println!("older than 21")
}

// That's because we can

fn inc_age(mut age: int) -> int {
  age + 1
}

fn main() {
  let mut age = 10;
  match inc_age(age) {
    // we don't use `let a = inc_age(age)`, so convenient !
    a @ 0..20 => println!("{} years old", a),
    _ => println!("old than 21")
  }
}