Rust Brain Teasers

Puzzle 23

Constant Loops

const_loop/src/main.rs

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));
    }
}

Guess the Output
	Try to guess what the output is before moving to the next page.

The program will fail to compile with the following error message:

'for' is not allowed in a 'const fn'

Discussion

Marking a function as const causes the function to run at compile ...