df_unit_test.rs

// the internal function that is not exposed otherwise
fn generate_preview(image: &RgbaImage) -> RgbaImage {
  // ...
}

// ...

#[cfg(test)]    // this ensures that this module is only included if you build a `test` target
mod tests {     // the tests live in their own module

  // assert that the generated (landscape) image has the hardcoded maximum width
  #[test]
  fn test_preview_image_size() {

      let image = DynamicImage::new_rgb8(1920, 1080).to_rgba();
      let (_, _, preview) = PreviewService::new(image);

      //max width is 512
      assert_eq!(512, preview.preview_image.width());

      //9:16*512=288
      assert_eq!(288, preview.preview_image.height());        
      //...
  }
}