if let / while let

fn main() {
    let some_value = Some(42);

    // Instead of full match:
    if let Some(n) = some_value {
        println!("got {n}");
    } else {
        println!("nothing");
    }

    // Drain a stack until empty
    let mut stack = vec![1, 2, 3, 4, 5];
    while let Some(top) = stack.pop() {
        println!("popped {top}");
    }

    // Combined with let-else (Rust 1.65+) for early-return shape
    fn parse_id(s: &str) -> Result<u64, String> {
        let Ok(n) = s.parse::<u64>() else {
            return Err(format!("not a number: {s}"));
        };
        Ok(n + 1)
    }
    println!("{:?}", parse_id("9"));            // Ok(10)
    println!("{:?}", parse_id("xyz"));          // Err("not a number: xyz")
}