Underhanded Cairo, pt. 1

Here’s a pretty simple Cairo program:

use debug::PrintTrait;

fn calc(v1: u64, v2: u64) -> (u64, u64) {
    (v1 + v2, v2 * v2)
}

fn main() {
    let mut res: u64 = 0;
    let mut counter: u64 = 1;

    loop {
        if counter > 100 {
            break;
        }

        let (res, sq) = calc(res, counter);
        counter += sq;
    };

    res.print()
}

What is the value of res that’s printed at the end?

Spoiler: Hover over these words to find out.

The reason is that the let res on line 16 declares a new variable instead of reassigning to the mutable res from the outer scope (line 8). Once the loop terminates, the original res comes back into scope and its value of 0 is printed.

The example above is a distilled version to showcase the bug. However I’ve approved a pull request containing the same issue. My misunderstanding of let, mut and tuple destructuring lead me to believe it’s fine. Only a diligent test caught the issue.

Stay safe.