Rust community typically authors unit tests in a module placed in the same source file as the code being tested. This was covered earlier in the course and looks like this:

fn main() {
mod tests {
    fn my_test() {

In Chromium we place unit tests in a separate source file and we continue to follow this practice for Rust — this makes tests consistently discoverable and helps to avoid rebuilding .rs files a second time (in the test configuration).

This results in the following options for testing Rust code in Chromium:

  • Native Rust tests (i.e. #[test]). Discouraged outside of //third_party/rust.
  • gtest tests authored in C++ and exercising Rust via FFI calls. Sufficient when Rust code is just a thin FFI layer and the existing unit tests provide sufficient coverage for the feature.
  • gtest tests authored in Rust and using the crate under test through its public API (using pub mod for_testing { ... } if needed). This is the subject of the next few slides.

Mention that native Rust tests of third-party crates should eventually be exercised by Chromium bots. (Such testing is needed rarely — only after adding or updating third-party crates.)

Some examples may help illustrate when C++ gtest vs Rust gtest should be used:

  • QR has very little functionality in the first-party Rust layer (it’s just a thin FFI glue) and therefore uses the existing C++ unit tests for testing both the C++ and the Rust implementation (parameterizing the tests so they enable or disable Rust using a ScopedFeatureList).

  • Hypothetical/WIP PNG integration may need to implement memory-safe implementation of pixel transformations that are provided by libpng but missing in the png crate - e.g. RGBA => BGRA, or gamma correction. Such functionality may benefit from separate tests authored in Rust.