The fact that the pattern matches.

Here the type being matched is:

    enum Option<T> {
        None,
        Some(T)
    }

    let x = Some(42);
    let Some(y) = x else { panic!("None") };

if x is `Some`, then `y` is set to the inner value and the following block occurs. you can also use this with other enums - i don't remember off the top of my head if it works for all enums, but you can at least also use it with `Result`