Code shared from the Rust Playground

alt.rs

fn main() {
  // If `option` is Some<T>, unwrap it and stick the value in `x`.
  if let Some(x) = option {
    foo(x);
  }
}

rust-if-let.rs

fn main() {
    let arg: Option<u8> = Some(42);
    //let arg: Option<u8> = None;
    let a: u8;
    
    // If the Option `arg` has Some() value, the value is bound to the
    // identifier in the pattern - in this case, `x`. In other words, if `arg`
    // has a value, `x` is bound to the result of unwrapping `arg`.
    // If `arg` is None, this branch isn't entered.
    if let Some(x) = arg {
        // If `arg` is None, this branch isn't entered.
        // `x` is the result of unwrapping `arg`.
        println!("x = {}", x);
        a = x;
    } else {
        // If `arg` is None, set a default value for `a`.
        a = 13;
    }
    println!("{}", a);
}