Rust Testing

1 minute read

Unit Testing

Unit test code is in the same file as the source under a test module

Say we had the following code in a file called

pub fn line(x : f64, m : f64, c : f64) -> f64 {
  x * m + c

mod tests {
  use super::*;

  fn test_line() {
    assert_eq!(line(0.0, 1.0, 2.0), 2.0);
    assert_eq!(line(1.0, 1.0, 2.0), 3.0);
    assert_eq!(line(2.0, 1.0, 2.0), 4.0);
    assert_eq!(line(-1.0, 1.0, 2.0), 1.0);

use super::* imports everything from the parent module

We can compile and run this with:

rustc --test

To give the binary a different name:

rustc --crate-name line_test --test

Test Asserts

assert!(line(1.0, 1.0, 2.0) == 3.0);
assert_eq!(line(1.0, 1.0, 2.0), 3.0);
assert_ne!(line(1.0, 1.0, 2.0), 4.0);

A trickier case is when we expect our code to panic:

fn test_that_panics() {

#[should_panic(expected = "Oh no!)"]
fn test_that_panics() {
  panic!("Oh no!");

Result-based asserts:

fn test_with_result() -> Result<(), String> {
  if line(1.0, 1.0, 2.0) == 3.0 {
  } else {

Ignoring tests

We can ignore tests using:

fn test_with_flakey_code() {
  // ...

If we want to check if ignored tests are working, run the binary with the ignored flag.

./line_test --ignored

Or run cargo test, but pass --ignored to the binary:

cargo test -- --ignored

Integration Testing

A suite of integration tests can be defined per crate. They are located in the tests directory right next to the src directory.

Integration tests only make use of a crate’s public interface.

To integration test line() in a package called line:

use line;

fn compute_line() {
    assert_eq!(line::line(1.5,1.0,-1.5), 0.0)

Each file in the tests directory is compiled as it’s own crate. This means scope cannot be shared between integration test files.

Integration tests are run the same way as unit tests.

Running Tests with Cargo

Cargo attempts to run unit, integration and doc tests.

In a cargo project, run:

cargo test

To filter only tests which contain line:

cargo test line

If line is part of a workspace package, run:

cargo test --workspace

This runs tests for all packages specified in the workspace.