Testing

Testing

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:

1. ```
   #![allow(unused)]
   ```
2. ```
   fn main() {
   ```
3. ```
   #[cfg(test)]
   ```
4. ```
   mod tests {
   ```
5. ```
       #[test]
   ```
6. ```
       fn my_test() {
   ```
7. ```
           todo!()
   ```
8. ```
       }
   ```
9. ```
   }
   ```
10. ```
    }
    ```

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.

43. Testing

Testing