Rust for C-Programmers

21.15 if let Chains (Rust 2024+)

Stabilized in the Rust 2024 Edition, if let chains (previously known as let_chains) allow combining multiple if let patterns and regular boolean conditions within a single if statement using the logical AND operator (&&).

21.15.1 Motivation

Without let_chains, checking multiple patterns or conditions required nesting:

// Pre-Rust 2024: Nested structure
fn process_nested(opt_a: Option<i32>, opt_b: Option<&str>, flag: bool) {
    if let Some(a) = opt_a {
        if a > 10 {
            if let Some(b) = opt_b {
                 if b.starts_with("prefix") {
                    if flag {
                        println!("All conditions met: a={}, b={}", a, b);
                    }
                 }
            }
        }
    }
}

21.15.2 Example with if let Chains

The equivalent code becomes much flatter and arguably more readable:

// Assumes Rust 2024 edition or later, or enabling the feature explicitly
fn process_chained(opt_a: Option<i32>, opt_b: Option<&str>, flag: bool) {
    // Combine `if let` and boolean conditions with `&&`
    if let Some(a) = opt_a && a > 10 &&
       let Some(b) = opt_b && b.starts_with("prefix") &&
       flag
    {
        println!("All conditions met: a={}, b={}", a, b);
    } else {
        println!("Conditions not fully met.");
    }
}

fn main() {
    process_chained(Some(20), Some("prefix_data"), true); // Output: All conditions met...
    process_chained(Some(5), Some("prefix_data"), true);  // Output: Conditions not fully met. (a > 10 fails)
    process_chained(Some(20), Some("other_data"), true); // Output: Conditions not fully met. (b.starts_with fails)
    process_chained(Some(20), Some("prefix_data"), false);// Output: Conditions not fully met. (flag fails)
    process_chained(None, Some("prefix_data"), true);    // Output: Conditions not fully met. (opt_a is None)
}

The conditions are evaluated left-to-right. If any let pattern fails to match or any boolean expression is false, the entire if condition short-circuits to false, and the else block (if present) is executed.