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:
- cargo-mutants
- and mutagen
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:
- Install it,
cargo install --locked cargo-mutants
- 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.
Also mutate `>` to `>=` and `<` to `<=`
#501
Most tests miss testing the boundary. Mutation testing can help a lot with this. The current code misses the change from > to >= and < and <=.
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
nfrankel
posted on