Fail-fast tests

When cargo-mutants runs the test suite, it only needs to find out if any tests fail, and so it's desirable that the test suite stop on the first failure.

In cargo test there are separate fail-fast configurations at two levels: the whole target and the individual test.

By default, cargo test will stop running test targets after the first one fails. Do not pass --no-fail-fast to cargo test under cargo-mutants.

Rust nightly releases after 1.92.0-nightly (2025-09-18) accept a --fail-fast option to the test harness. (This is distinct from the --fail-fast option to cargo test.) This causes the test target to stop after the first individual test fails. This can significantly improve cargo-mutants performance in the common case where there are many tests in a single target or some of them are slow.

If you have a sufficiently recent toolchain you can enable this in the cargo_test_args:

cargo mutants -- -- -Zunstable-options --fail-fast

Note: There are two -- separators: the first delimits the arguments from cargo mutants to be passed to cargo test and the second delimits the arguments from cargo test so they are passed to the test target.