Rust testing, data generation and const asserts

Ben McDonald
Jan 29 · 3 min read

Basic Rust test example

Luckily getting starting with testing Rust code is reasonably straightforward and no external libraries are needed. cargo test will run all test code in a Rust project. Test code is identified with the code attributes #[cfg(test)] and #[test]. For the placement of the test code, units tests are added at the end of a source code file and integration tests can be placed in their own tests directory.

// Minimal Rust tests examplefn double(x: u8) -> u8 {
x * 2
mod tests {
// import all from the parent scope into this scope
use super::*;
fn test_double() {
assert_eq!(double(2), 4);
fn test_error_and_none() {
assert!("not a number".parse::<u32>().is_err());
assert_eq!("not a number".parse::<u32>().ok(), None);
assert_eq!("4".parse::<u32>(), Ok(4));
$ cargo test
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Data generation

Using the right tools will help make your tests more effective. Below are data generation libraries that can increase the coverage of your tests.

Mock data

The fake-rs library contains an extensive list of functions to generate mock data (Lorem, Name, Internet, HTTP, Company, Address, Date/Time).

use fake::{Faker};
use fake::locales::{EN, ZH_TW};
assert!(test_person(FirstName(EN).fake(), LastName(EN).fake()));
assert!(test_ip_processing(IPv6(EN).fake(), MACAddress(EN).fake()));
let hash_map: HashMap<u8, u32> = Faker.fake();
println!("HashMap {:?}", hash_map);

Regex and arbitrary input tests

proptest tests that certain properties of rust code hold for arbitrary inputs and, if a failure is found, automatically finds the minimal test case to reproduce the problem.

proptest! {
fn parses_all_valid_dates(s in "[0-9]{4}-[0-9]{2}-[0-9]{2}") {

Machine learning generated tests

The test-data-generation library trains a model on sample data, and then can generate additional data matching the format of the samples. The library uses a Markov decision process to build a model of the data.

use test_data_generation::data_sample_parser::DataSampleParser;let mut dsp = DataSampleParser::new();
// train on data contained in a CSV file
// generate data from a trained model

Add these libraries under your [dev-dependencies] in the Cargo.toml file

# Cargo.toml[dev-dependencies]
fake = "2.0"
test-data-generation = "0.1.1"
proptest = "0.9.5"

Debug asserts

debug_asserts statements can check preconditions, postconditions and invariants in the body of your code. They are only included and executed when your code is run in debug mode and are removed for production builds.

fn double(x: u8) {
debug_assert!(x > 0, "Precondition - x must be positive");
let doubled = x * 2;
debug_assert_eq!(doubled, x * 2);
return doubled;

Doc tests

Built into the compiler is the ability to run tests in documentation comments. By default cargo test will execute tests in documentation and report failures

/// # Examples
/// ```
/// let x = 5;
/// assert_eq!(x, 5);
/// ```

Const asserts

static_assertions ensure conditions are met for const functions and const data. The const asserts will prevent code from compiling if they don’t pass.

// static_assertions = "1.1.0"
extern crate static_assertions;
const MY_CONFIG: Config = ConfigConfig {
foo: 10,
bar: 20,
const_assert!( >;

Other cargo commands

Run only tests matching a pattern

cargo test test_double

Run tests not marked with the #[ignore] attribute

cargo test -- --ignored

Watch the project and run cargo test after any code changes

cargo install cargo-watch
cargo watch -x test

Examples Tests

For an examples of well-done tests check out the Rust standard library code

About me

I’m a freelance developer from New Zealand. I spend my time teaching people about bitcoin or coding Rust. Find me on twitter

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Ben McDonald

Written by

Rust developer, freelancer, JavaScript, bitcoin education, StackOverflow contributor

The Startup

Medium's largest active publication, followed by +587K people. Follow to join our community.

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