Integration Tests

Integration tests check that all the different parts of your program actually work correctly together, from the outside.

In this page:

The tests Directory
Integration Tests Use the Public API Only
Why Integration Tests Matter
Running All Tests Together

Syntax

rust

// tests/name.rs
use crate_name::function;

#[test]
fn name() {
    assert_eq!(function(input), expected);
}

The tests Directory

Cargo recognizes a top-level tests/ directory as the home for integration tests, treating each .rs file inside it as a separate test crate.

Example: The tests Directory

bash

my_project/
  src/
    lib.rs
  tests/
    integration_test.rs

⚠️ Run this in your own terminal or Node.js environment.

Integration Tests Use the Public API Only

Because each file in tests/ is compiled as its own external crate, it can only call functions the library exposes publicly with pub, just like any other user of the library would.

Example: Integration Tests Use the Public API Only

markup

pub fn add_two(n: i32) -> i32 {
    n + 2
}

// In tests/integration_test.rs this would look like:
// use my_project::add_two;
// #[test]
// fn test_add_two() {
//     assert_eq!(add_two(3), 5);
// }

fn main() {
    println!("add_two(3) = {}", add_two(3));
}

Why Integration Tests Matter

Unit tests verify individual pieces in isolation, while integration tests confirm that those pieces genuinely work together correctly when used the way a real caller would use them.

Example: Why Integration Tests Matter

markup

// Define the function `parse_and_double` taking `input` and returning a `Option<i32>`
fn parse_and_double(input: &str) -> Option<i32> {
    input.parse::<i32>().ok().map(|n| n * 2)
}

// Entry point: execution of the program starts here
fn main() {
    // Print "{:?}" to the console, substituting in `parse_and_double("21")` (with a trailing newline)
    println!("{:?}", parse_and_double("21"));
    // Print "{:?}" to the console, substituting in `parse_and_double("oops")` (with a trailing newline)
    println!("{:?}", parse_and_double("oops"));
}

Running All Tests Together

cargo test compiles and runs unit tests from src/ alongside every integration test file from tests/, reporting a combined summary of the results.

Example: Running All Tests Together

bash

cargo test

⚠️ Run this in your own terminal or Node.js environment.

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