Rust for C-Programmers

21.15 The let else Construct (Rust 1.65+)

Rust 1.65 introduced let else, which allows a refutable pattern in a let binding. If the pattern match fails, an else block runs and must diverge (e.g., via return or panic!). Otherwise, the matched bindings are available in the surrounding scope:

fn process_value(opt: Option<i32>) {
    let Some(val) = opt else {
        println!("No value provided!");
        return;
    };
    // If we reach this line, opt matched Some(val).
    println!("Got value: {}", val);
}

fn main() {
    process_value(None);
    process_value(Some(42));
}

Here, Some(val) is refutable. If opt is None, the else block executes and must end the current function (or loop). If opt is Some(...), the binding val is introduced into the parent scope.