Mutation Testing: How Does it Work in Rust? | HackerNoon

I’ve been a big fan of Mutation Testing since I discovered PIT. As I dive deeper into Rust, I wanted to check the state of mutation testing in Rust.

Starting with cargo-mutants

I found two crates for mutation testing in Rust:

mutagen hasn’t been maintained for three years, while cargo-mutants is still under active development.

I’ve ported the sample code from my previous Java code to Rust:

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);
    }
}

Using cargo-mutants is a two-step process:

1. Install it, cargo install --locked cargo-mutants
2. Use it, 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 expected a mutant to survive, as I didn’t test the boundary when the test value equals the limit. Strangely enough, cargo-mutants didn’t detect it.

Finding and Fixing the Issue

I investigated the source code and found the place where it mutates operators:

// 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! {*}],

Indeed, < is changed to == and >, but not to <=. I forked the repo and updated the code accordingly:

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

I installed the new forked version:

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

I reran the command:

cargo mutants

The output is the following:

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 in the missed.txt file. I thought I fixed it and was ready to make a Pull Request to the cargo-mutants repo. I just needed to add the test at the boundary:

#[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 are 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

Conclusion

Not many blog posts end with a Pull Request, but this one does:

Unfortunately, I couldn’t manage to make the tests pass; fortunately, the repository maintainer helped me–a lot. The Pull Request is merged: enjoy this slight improvement.

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

To go further:

Originally published at A Java Geek on March 30th, 2025