let-else - Rust By Example
Description: Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries.

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# let-else

> 🛈 stable since: rust 1.65

> 🛈 you can target specific edition by compiling like this `rustc --edition=2021 main.rs`

With `let`-`else`, a refutable pattern can match and bind variables in the surrounding scope like a normal `let`, or else diverge (e.g. `break`, `return`, `panic!`) when the pattern doesn’t match.

    use std::str::FromStr;

    fn get_count_item(s: &str) -> (u64, &str) {
        let mut it = s.split(' ');
        let (Some(count_str), Some(item)) = (it.next(), it.next()) else {
            panic!("Can't segment count item pair: '{s}'");
        };
        let Ok(count) = u64::from_str(count_str) else {
            panic!("Can't parse integer: '{count_str}'");
        };
        (count, item)
    }

    fn main() {
        assert_eq!(get_count_item("3 chairs"), (3, "chairs"));
    }

The scope of name bindings is the main thing that makes this different from `match` or `if let`-`else` expressions. You could previously approximate these patterns with an unfortunate bit of repetition and an outer `let`:

    #![allow(unused)]
    fn main() {
    use std::str::FromStr;

    fn get_count_item(s: &str) -> (u64, &str) {
        let mut it = s.split(' ');
        let (count_str, item) = match (it.next(), it.next()) {
            (Some(count_str), Some(item)) => (count_str, item),
            _ => panic!("Can't segment count item pair: '{s}'"),
        };
        let count = if let Ok(count) = u64::from_str(count_str) {
            count
        } else {
            panic!("Can't parse integer: '{count_str}'");
        };
        (count, item)
    }

    assert_eq!(get_count_item("3 chairs"), (3, "chairs"));
    }

### Ver también

option, match, if let and the let-else RFC.