Code shared from the Rust Playground

default-values-from-option.rs

fn main() {
    // If x has a value, unwrap it and set y to it's value.
    // Otherwise, set y to a default.
    // ------------------------------------------------------------------------
    // let x = Some(11); // Uncomment to test 
    let x = None;
    let y;
    if x.is_some() {
        y = x.unwrap();
    } else {
        y = 42;
    }  
    println!("y = {}", y);
    
    // If a has a value, set b to this value.
    // Otherwise, set b to a default.
    // ------------------------------------------------------------------------
    let a = Some(42);
    // let a = None; // Uncomment to test
    let b = match a {
        Some(b) => b,
        None => 13,
    };
    println!("b = {}", b);
  
    // If arg has a value, set val to this value.
    // Otherwise, set val to a default value of 42.
    // -------------------------------------------------------------------------
    let arg = Some(13);
    // let arg = None; // Uncomment to test
    let mut val = if let Some(val) = arg { val } else { 42 };
    val += 1;
    println!("val is {}", val);
}

See https://static.rust-lang.org/doc/master/book/if-let.html