Testing Rust code

This document describes best practices for writing tests for Rust code, and paired with "Running tests as components" describes how to component-ize, package, and run these tests.

This document is targeted towards developers working inside of fuchsia.git, and the workflow described is unlikely to work for SDK consumers.

The source code for this tutorial is available at //examples/hello_world/rust.

Unit tests

Adding tests to code

The idiomatic way for adding Rust unit tests works just as well inside of Fuchsia as it does outside, and can be easily accomplished by dropping the following snippet into the bottom of whatever test you want to write:

mod tests {
    fn it_works() {
        assert_eq!(true, true);

This will cause a new mod named tests to be created, and this mod will only be included when building unit tests. Any functions annotated with #[test] will be run as a test, and if the function successfully returns then the test passes.

For tests exercising asynchronous code, use the #[fasync::run_until_stalled(test)] annotation as an alternative to using an asynchronous executor.

async fn my_test() {
    let some_future = async { 4 };
    assert_eq!(some_future.await, 4);

Building tests

These tests can be automatically built by the rustc_binary GN template by setting with_unit_tests = true. This would typically go in a BUILD.gn file next to the src directory containing the rust code.

rustc_binary("bin") {
  name = "hello_world_rust"
  with_unit_tests = true
  edition = "2018"

  deps = []
  test_deps = [ "//garnet/public/rust/fuchsia-async" ]

Setting with_unit_tests = true causes this build rule to generate two different executables, one with the provided and one with _bin_test appended to the provided name. In our example here, that means that one executable named hello_world_rust will be created and one executable named hello_world_rust_bin_test will be created.

Packaging and running tests

For the Hello world example, the test package needs to reference the generated targets, bin_test and hello_world_rust_bin_test:

test_package("hello_world_rust_tests") {
  # The bin_test target is generated by rustc_binary
  # when the with_unit_tests attribute is true.
  deps = [
  tests = [
      # The hello_world_rust_bin_test target is generated by rustc_binary
      # when the with_unit_tests attribute is true.
      name = "hello_world_rust_bin_test"

To run the tests run:

fx run-test hello_world_rust_tests

For information on packaging and running tests, please refer to the documentation on running tests as components.