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Puzzle 23: Explanation

Explore how Rust's const functions enable calculations at compile time to enhance runtime performance. Understand the restrictions on const functions, how they differ from C++ constexpr, and scenarios where using const functions benefits your Rust programs. This lesson helps you grasp the trade-offs and evolving capabilities of compile-time function execution in Rust.

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Test it out

Press “Run” to see the output of the code.

C++
const fn fib(n: u128) -> u128 {
    let mut a = 1;
    let mut b = 1;
    for _ in 2..n {
        let tmp = a + b;
        a = b;
        b = tmp;
    }
    b
}
fn main() {
    for i in 0..5 {
        println!("Fib {} = {}", i, fib(i));
    }
}

Explanation

Marking a function as const causes the function to run at compile time rather than at runtime. When a function runs at compile time, the compiler calculates the results beforehand from constant inputs, which can help speed up complex calculations that we might need later.

Suppose our program requires a lot of Fibonacci numbers. Without a const function, our program would need to recalculate the numbers as needed, possibly more than once. However, by using a const function, we can store these numbers as constant values in our program, dramatically improving its performance. ...