Solution

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

fn fib(n: u32) -> u32 {
    if n < 2 {
        return n;
    } else {
        return fib(n - 1) + fib(n - 2);
    }
}

fn main() {
    let n = 20;
    println!("fib({n}) = {}", fib(n));
}

We use the return syntax here to return values from the function. Later in the course, we will see that the last expression in a block is automatically returned, allowing us to omit the return keyword for a more concise style.

The if condition n < 2 does not need parentheses, which is standard Rust style.

Panic

The exercise asks when this function will panic. The Fibonacci sequence grows very rapidly. With u32, the calculated values will overflow the 32-bit integer limit (4,294,967,295) when n reaches 48.

In Rust, integer arithmetic checks for overflow in debug mode (which is the default when using cargo run). If an overflow occurs, the program will panic (crash with an error message). In release mode (cargo run --release), overflow checks are disabled by default, and the number will wrap around (modular arithmetic), producing incorrect results.