Testing code changed in a diff

If you're working on a large project or one with a long test suite, you may not want to test the entire codebase every time you make a change. You can use cargo-mutants --in-diff to test only mutants generated from recently changed code.

The --in-diff DIFF_FILE option tests only mutants that overlap with regions changed in the diff.

The diff is expected to either have a prefix of b/ on the new filename, which is the format produced by git diff, or no prefix.

Some ways you could use --in-diff:

  1. Before submitting code, check your uncommitted changes with git diff.
  2. In CI, or locally, check the diff between the current branch and the base branch of the pull request.

Changes to non-Rust files, or files from which no mutants are produced, are ignored.

--in-diff is applied on the output of other filters including --package and --regex. For example, cargo mutants --in-diff --package foo will only test mutants in the foo package that overlap with the diff.


--in-diff makes tests faster by covering the mutants that are most likely to be missed in the changed code. However, it's certainly possible that edits in one region cause code in a different region or a different file to no longer be well tested. Incremental tests are helpful for giving faster feedback, but they're not a substitute for a full test run.

The diff is only matched against the code under test, not the test code. So, a diff that only deletes or changes test code won't cause any mutants to run, even though it may have a very material effect on test coverage.


In this diff, we've added a new function two to src/lib.rs, and the existing code is unaltered. With --in-diff, cargo-mutants will only test mutants that affect the function two.

--- a/src/lib.rs    2023-11-12 13:05:25.774658230 -0800
+++ b/src/lib.rs    2023-11-12 12:54:04.373806696 -0800
@@ -2,6 +2,10 @@

+pub fn two() -> String {
+    format!("{}", 2)
 mod test_super {
     use super::*;
@@ -10,4 +14,9 @@
     fn test_one() {
         assert_eq!(one(), "one");
+    #[test]
+    fn test_two() {
+        assert_eq!(two(), "2");
+    }