C15: Writing Automated Tests

Question 1

Basic test

Answer 1

[cfg(test)]

Filename: src/lib.rs

mod tests {
#[test]
fn it_works() {
let result = 2 + 2;
assert_eq!(result, 4);
}

#[test]
fn larger_can_hold_smaller() {
    let larger = 1 > 8;

    assert!(larger);
} }

$ cargo test
Compiling adder v0.1.0 (file:///projects/adder)
Finished test [unoptimized + debuginfo] target(s) in 0.57s
Running unittests src/lib.rs (target/debug/deps/adder-92948b65e88960b4)

running 2 test
test tests::it_works … ok
test tests::larger_can_hold_smaller … ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Question 2

Adding Custom Failure Messages

Answer 2

[test]

fn greeting_contains_name() {
    let result = greeting("Carol");
    assert!(
        result.contains("Carol"),
        "Greeting did not contain name, value was `{}`",
        result
    );
}

Question 3

Checking for Panics with should_panic

Answer 3

[cfg(test)]

pub struct Guess {
value: i32,
}

impl Guess {
pub fn new(value: i32) -> Guess {
if value < 1 || value > 100 {
panic!(“Guess value must be between 1 and 100, got {}.”, value);
}

    Guess { value }
} }

mod tests {
use super::*;

#[test]
#[should_panic]
fn greater_than_100() {
    Guess::new(200);
} }

Question 4

Using Result<T, E> in Tests

Answer 4

[cfg(test)]

mod tests {
#[test]
fn it_works() -> Result<(), String> {
if 2 + 2 == 4 {
Ok(())
} else {
Err(String::from(“two plus two does not equal four”))
}
}
}

Question 5

Running Tests in Parallel or Consecutively

Answer 5

If you don’t want to run the tests in parallel or if you want more fine-grained control over the number of threads used, you can send the –test-threads flag and the number of threads you want to use to the test binary. Take a look at the following example:

$ cargo test – –test-threads=1

Question 6

Showing Function Output

Answer 6

cargo test – –show-output

Question 7

Running a Subset of Tests by Name

Answer 7

cargo test <test_name></test_name>

Question 8

Ignoring Some Tests Unless Specifically Requested

Answer 8

[test]

fn it_works() {
assert_eq!(2 + 2, 4);
}

#[ignore]
fn expensive_test() {
// code that takes an hour to run
}
cargo test – –ignored

Question 9

Show ignored tests

Answer 9

cargo test – –include-ignored

Question 10

[cfg(test)]

Answer 10

The #[cfg(test)] annotation on the tests module tells Rust to compile and run the test code only when you run cargo test, not when you run cargo build.

Question 11

Testing Private Functions

Answer 11

[cfg(test)]

pub fn add_two(a: i32) -> i32 {
internal_adder(a, 2)
}

fn internal_adder(a: i32, b: i32) -> i32 {
a + b
}

mod tests {
use super::*;

#[test]
fn internal() {
    assert_eq!(4, internal_adder(2, 2));
} }

Question 12

The tests Directory

Answer 12

adder
├── Cargo.lock
├── Cargo.toml
├── src
│ └── lib.rs
└── tests
└── integration_test.rs

Question 13

Helpers for tests

Answer 13

[test]

To avoid having common appear in the test output, instead of creating tests/common.rs, we’ll create tests/common/mod.rs. The project directory now looks like this:

├── Cargo.lock
├── Cargo.toml
├── src
│ └── lib.rs
└── tests
├── common
│ └── mod.rs
└── integration_test.rs

Filename: tests/integration_test.rs

use adder;

mod common;

fn it_adds_two() {
common::setup();
assert_eq!(4, adder::add_two(2));
}