Function grade_submission 

async fn grade_submission(
    conn: &mut PgConnection,
    user_id: Uuid,
    exercise: Exercise,
    submission: &StudentExerciseSlideSubmission,
    jwt_key: Arc<JwtKey>,
    file_store: &dyn FileStore,
    app_conf: &ApplicationConfiguration,
) -> Result<StudentExerciseSlideSubmissionResult, ControllerError>

Grades one slide submission and trims the result down to what the submitter may see. Runs inside process_submission’s transaction.