cargo nextest vs cargo test

cargo nextest or cargo test?

Use nextest when a workspace has many test binaries, when tests interfere with each other, or when CI needs retries and JUnit reports. Stay with cargo test for a small crate. Keep it for doctests either way: nextest does not run them.

How each runs your tests

cargo test runs one test binary at a time. Inside a binary, the tests share one process and run on threads.

nextest builds the same binaries, lists the tests in them, then “executes each individual test in a separate process, in parallel”.

cargo install cargo-nextest --locked
cargo nextest run
cargo test --doc

A test that leaks state

Our crate has a function that reads a process-wide flag, three unit tests and one doctest. One test turns the flag on and never turns it off. We ran each command 100 times on Rust 1.98.0 with nextest 0.9.138:

Under cargo test the three tests share a process. When the flag is set before another test reads it, that test fails, and whether that happens depends on thread timing. Under nextest each test has its own process, so the flag cannot leak.

The repeated cargo test runs used --lib, to leave the doctest out. The test is still wrong to leave the flag set. Fix it if the suite must also pass under cargo test.

What differs

nextest publishes its own benchmarks. The gain comes from running binaries in parallel, so measure it on your own workspace.

Coverage with either runner

cargo-llvm-cov works with both: cargo llvm-cov and cargo llvm-cov nextest. So does Supercov:

npx supercov -- cargo test
npx supercov -- cargo nextest run

On the crate above Supercov recorded coverage per test under both: 4 tests with cargo test, doctest included, and 3 with nextest, with 6/6 lines and 4/4 branches each time. It needs Rust 1.95 or newer and nextest 0.9.138 or newer, and keeps nextest’s retries as separate attempts.

For coverage tools, see Rust branch coverage on stable: cargo-llvm-cov, Tarpaulin, Supercov.