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")); }

See also:

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