Integration Testing in Rust

Published on Joshleeb's blog

We all know testing is important. And, I hope, it might even be familiar. You’re coding away and catch yourself thinking “I should probably test this”. And before you know it you’ve mashed on your keyboard and there appears

#[cfg(test)]
mod tests {
    ...
}

These are unit tests which are super easy to setup. No added dependencies, no added tooling

But what about integration tests?

Rust supports those too, which you probably already know. They’re only slightly more involved to setup so they’re still fairly straightforward.

But there are some caveats that I personally ran into.

For one, The Rust Book explains that

each file in the tests directory is a separate crate.

Which on its own doesn’t sound too bad. But it has the implication that having code shared between your integration tests isn’t intuitive. I think it’s because the code structure goes back to the Rust 2015 edition of defining modules, using mod.rs.

And two, which is arguably the bigger issue, is that it’s not obvious how to run only all integration tests. Or only all unit tests for that matter, but we’ll get to that too.

A Better Setup

I went looking through docs and crates and eventually opened up the source for diesel-rs/diesel, which isn’t the first time it’s been a great reference.

Now it looks like diesel has a separate crate, internal to the workspace, called diesel_tests which appears to contain the integration tests. But I don’t want to setup a new crate and workspace just to extend my test suite. Instead let’s take a look at the Cargo.toml for that crate and borrow what we can borrow.

For my new crate, mycrate, I’ve setup the Cargo.toml as

[package]
name = "mycrate"
...
autotests = false

[[test]]
name = "integration"
path = "tests/lib.rs"

Specifically, we’ve added two sections.

Autotests

The autotests = false disables automatic test discovery. This sounds bad but isn’t needed because we’re manually telling Cargo where to find our tests with…

Test Target

This may look familiar if you’ve seen [[bin]] or [[example]] where we explicitly specify an additional target for Cargo to work with. This is no different. We are simply telling Cargo there’s a test target called “integration” which has an entry point at tests/lib.rs.

Single Integration Test Crate

This config solves the first caveat. We no longer have to deal with a crate per integration test file, and so the way we handle code shared between the tests is the same way we would handle sharing code in the implementation.

And everything is still in the tests directory, so no changes there either.

Running Tests

We’ve also made segmenting our tests a bit clearer.

# Run all tests
cargo test

# Run only unit tests
cargo test --lib

# Run only integration tests
cargo test --test integration

# Run only integration tests, single threaded
# (you’ll probably want this one)
cargo test --test integration -- --test-threads=1

Addendum