Test runners

Rust comes with support for unit tests built-in. You can annotate a function with #[test] anywhere and it will be treated as a unit test.

#![allow(unused)]
fn main() {
#[test]
fn example_test() {
    assert_eq!(42, 40 + 2);
}
}

Often times, tests will be put into a separate module called tests, so that you can import things you need in the tests and not get warnings because those are not used in normal compilation.

You can also write integration tests by putting them into a tests/ folder in the crate root. Finally, code you put into crate documentation is also built, these are called doctests.

Cargo has a subcommnd called cargo test, which will build and run all of the tests in a crate. When you use a workspace, then using cargo test --all will run all tests from all crates in the workspace.

This subcommand works well, and you should use it. However, there are a few limitations with it that are especially noticeable when you are working in a large Rust workspace with a lot of crates (and thus, a lot of tests and integration tests): it runs a bit slow.

Cargo Nextest

Cargo Nextest is a tool that you can use as a drop-in replacement for cargo test. It has some useful features for running tests in CI, but the main advantage is that it is up to 3x faster according to their own benchmarks.

In my testing, I’ve observed a 10% speedup, but it depends on how many tests you have and what they are bottlenecked by. In my case, the tests were bottlenecked by external services, so there is not too much cargo nextest can do. But if you have a lot of tests and/or a large Cargo workspace, be sure to give cargo nextest a try, you may see a marked improvement in testing time and the output is also more terse, which I like.

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