"Unreachable" % can cause panics

[LHerskind]

Aim

In the process of testing a safe-math library (safe_u120.nr) I'm running into a strange thing while testing.

For my mul_div_up function as outlined below, I perform a mod operation against user provided input.

fn mul_div_up(
    self: Self,
    b: Self,
    divisor: Self
) -> Self {
    let c = self.mul(b);
    assert(!divisor.is_zero());
    let adder = ((self.value * b.value % divisor.value) as u120 > 0) as u120;
    c.div(divisor).add(Self {value: adder})
}

When running the test:

#[test(should_fail)]
fn test_mul_div_up_zero_divisor() {
    let a = SafeU120::new(6);
    let b = SafeU120::new(3);
    let c = SafeU120::new(0);
    let _d = SafeU120::mul_div_up(a, b, c);
}

It will fail due to a panic. If replacing % with / it will fail due to the assert.

Expected Behavior

Expect to fail due to the assert !divisor.is_zero() similarly to divisions.

Bug

When containing an unreachable modulo computation % divisor with divisor = 0, the program panics instead of failing due to the assert divisor != 0 which is checked earlier.

To Reproduce

1. Steal the tests in safe_u120.nr
2. Uncomment the test_mul_div_up_zero_divisor test
3. Run nargo test

Installation Method

None

Nargo Version

nargo 0.10.5 (git version hash: 5f78772, is dirty: false)

Additional Context

Installed nargo using noirup -v aztec.

Would you like to submit a PR for this Issue?

No

Support Needs

No response