Testing

101

Tests are identified by annotations:

pub fn add(left: usize, right: usize) -> usize {
    left + right
}

#[cfg(test)]
mod tests {
    use super::*;

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

Run them with:

$ cargo test

Tests from docs

Code from the documentation is also executed by tests.

Making things testable

We need to timplement PartialEq and Debug. Usually is enough with annotation:

#[derive(PartialEq, Debug)]

Macros

IMPORTANT: The assert macros accept additional parameters that are going to be passed to printf. Example: assert!(cond, "The user cond '{}' was supposed to be true", userId);

panic!(&str, &);: runtime error equals test failure.

assert!(boolean);: fails if provided argument is false.

assert_eq!<T>(T, T); and assert_ne!<T>(T, T);: fail if provided values are different or equal. They show the 2 values, so there are more convenient and that assert! for this use case.

Testing for `panic!`

Add #[should_panic] annotation to the test:

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic(expected = "optional error message fragment of the specific panic we want to trigger")]
    fn greater_than_100() {
        Guess::new(200);
    }
}

Tests returning `Result<T, E>!`

Tests can result Result<T, E>!. This is useful for tests where we want to use the ? operator. It cannot be used with #[should_panic]. If we want to check the Err, we need to use assert!(res.is_error()).

Running tests

There are arguments before and after --

cargo test --help

cargo test -- --help

Running specific tests

Run the add_a_number test:

cargo test add_a_nunber

Tests names are search patterns; run any tests that contains add in the name:

cargo test add

`#[ignore]`: Ignoring tests unless explicitly requested

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

Test Organization

Unit tests

The convention is to create a module named tests in each file to contain the test functions and to annotate the module with cfg(test).

#[cfg(xxxxx)] is an annotation that tells the compiler to only compile under a certain configuration option.

Integration tests

ONLY lib/ can be integration tested. This is why even binary crates have all the code in lib and as little code as possible in src/main.rs.

Create a tests directory next to src.
Create rust files. The convention is ending with _test.rs.
Add #[test] functions. No need for #[cfg(test)].

Each test is considered as a separate crate, so we need to bring our library into each test crate’s scope:

use adder; // Bring modules into scope on each test file

#[test]
fn it_adds_two() {
    assert_eq!(4, adder::add_two(2));
}

Shared code in tests

Only rust files that live in the tests directory are considered integration tests:

├── Cargo.lock
├── Cargo.toml
├── src
│   └── lib.rs
└── tests
    ├── common
    │   └── mod.rs           <-- old module structure is ignored by tests runner.
    ├── integration_test.rs  <-- contains test
    └── pipo
        └── piripo.rs        <-- files in subdirectories are ignored by tests runner.

Good for code shared by different tests.

use adder;

mod common;
mod pipo::piripo;

#[test]
fn it_adds_two() {
    common::setup();
    assert_eq!(4, adder::add_two(2));
    assert!(piripo());
}

Last modified: 23 d’agost 2024