v26.0.0

• Changed: The default is now not to shuffle mutants: they run in the deterministic order they are generated in the source tree. This should give somewhat better locality of reference due to consecutively testing changes in each package or module. The previous behavior can be restored with --shuffle.

• New: --cargo-test-arg option to pass additional arguments to cargo test.

• New: --sharding option to control how mutants are distributed across multiple machines, with choices of slice or round-robin.

• Changed: The default sharding strategy is now slice; previously it was round-robin. Sliced sharding gives each worker better locality of reference due to testing changes to related packages, but may make the runtime more uneven between workers if some packages are slower to test than others.

• Changed: Tree copying now attempts to use reflinks (copy-on-write) for faster copying on supported filesystems (Btrfs, XFS, APFS), with automatic fallback to regular copying.

• Book: Recommend using the -Zunstable-options --fail-fast argument to test targets to speed up mutation testing, on recent nightly toolchains.

• Fixed: Don't error if the --in-diff patch file contains non-UTF-8 data in non-Rust files, or contains messages about binary files or git index changes.

• New: start_time and end_time fields in outcomes.json.

• New: Delete individual fields from struct literals that have a base (default) expression like ..Default::default() or ..base_value. This checks that tests verify each field is set correctly and not just relying on default values.

• New: cargo_mutants_version field in outcomes.json.

• Changed: Functions with attributes whose path ends with test are now skipped, not just those with the plain #[test] attribute. This means functions with #[tokio::test], #[sqlx::test], and similar testing framework attributes are automatically excluded from mutation testing.

• Changed: The bitwise assignment operators &= and |= are no longer mutated to ^=. In code that accumulates bits into a bitmap starting from zero (e.g., bitmap |= new_bits), |= and ^= produce the same result, making such mutations uninformative.