Mutation Test: How does the cake work?

I’ve been a great fan of mutation test since I discovered a pit. When dived deeper, I wanted to check the status of the mutation test in the cake.

Starting with cargo-mutants

I found two casters for cake mutation test:

mutagen Cargo-Mutants is still active.

I gave the sample code pass from my previous Java code:

struct LowPassPredicate {
    threshold: i32,
}

impl LowPassPredicate {
    pub fn new(threshold: i32) -> Self {
        LowPassPredicate { threshold }
    }

    pub fn test(&self, value: i32) -> bool {
        value < self.threshold
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn should_return_true_when_under_limit() {
        let low_pass_predicate = LowPassPredicate::new(5);
        assert_eq!(low_pass_predicate.test(4), true);
    }

    #[test]
    fn should_return_false_when_above_limit() {
        let low_pass_predicate = LowPassPredicate::new(5);
        assert_eq!(low_pass_predicate.test(6), false);
    }
}

Use cargo-mutants It is a two -stage process:

1. Install, cargo install --locked cargo-mutants
2. Use, cargo mutants

Found 4 mutants to test
ok       Unmutated baseline in 0.1s build + 0.3s test
 INFO Auto-set test timeout to 20s
4 mutants tested in 1s: 4 caught

I waited for a mutant to survive because I did not test the limit when the test value was equal to the limit. Strangely, cargo-mutants did not detect.

Finding and correcting the problem

I searched the source code and found the place where the operators changed:

// We try replacing logical ops with == and !=, which are effectively
// XNOR and XOR when applied to booleans. However, they're often unviable
// because they require parenthesis for disambiguation in many expressions.
BinOp::Eq(_) => vec![quote! { != }],
BinOp::Ne(_) => vec![quote! { == }],
BinOp::And(_) => vec![quote! { || }],
BinOp::Or(_) => vec![quote! { && }],
BinOp::Lt(_) => vec![quote! { == }, quote! {>}],
BinOp::Gt(_) => vec![quote! { == }, quote! {<}],
BinOp::Le(_) => vec![quote! {>}],
BinOp::Ge(_) => vec![quote! {<}],
BinOp::Add(_) => vec![quote! {-}, quote! {*}],

Actually, < modified == And >but not <=. I bifurted the report and updated the code accordingly:

BinOp::Lt(_) => vec![quote! { == }, quote! {>}, quote!{ <= }],
BinOp::Gt(_) => vec![quote! { == }, quote! {<}, quote!{ => }],

I uploaded the new fork version:

cargo install --git https://github.com/nfrankel/cargo-mutants.git --locked

I revive the command:

cargo mutants

The output is this:

Found 5 mutants to test
ok       Unmutated baseline in 0.1s build + 0.3s test
 INFO Auto-set test timeout to 20s
MISSED   src/lib.rs:11:15: replace < with <= in LowPassPredicate::test in 0.2s build + 0.2s test
5 mutants tested in 2s: 1 missed, 4 caught

You can find the same information here missed.txt file. I thought I fixed it and I was ready to make a pull cargo-mutants Repo. I had to add the test to the limit:

#[test]
fn should_return_false_when_equals_limit() {
    let low_pass_predicate = LowPassPredicate::new(5);
    assert_eq!(low_pass_predicate.test(5), false);
}

cargo test

running 3 tests
test tests::should_return_false_when_above_limit ... ok
test tests::should_return_false_when_equals_limit ... ok
test tests::should_return_true_when_under_limit ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

cargo mutants

And all mutants were killed!

Found 5 mutants to test
ok       Unmutated baseline in 0.1s build + 0.2s test
 INFO Auto-set test timeout to 20s
5 mutants tested in 2s: 5 caught

Solution

Many blog posts do not end with a desire to draw, but this:

Unfortunately, I could not provide the tests to pass; Fortunately, the warehouse caregiver helped me a lot. The desire to pull is combined: Enjoy this light improvement.

I learned more about cargo-mutants and can improve the code in the process.

To go further:

Initially released on March 30, 2025 at A Java Geek