First integration test

As we stated previously, Rust expects all integration tests to live in the tests/ directory. Files within the tests/ directory are compiled as if they are separate binary crates while using our library under test. For the following example, we'll create a new crate by running  cargo new integration_test --lib, with the same function, sum ,as in the previous unit test, but now we have added a tests/ directory, which has an integration test function defined as follows:

// integration_test/tests/sum.rs

use integration_test::sum;

#[test]
fn sum_test() { 
    assert_eq!(sum(6, 8), 14); 
} 

We first bring the function sum in scope. Second, we have a function, sum_test , that calls sum and asserts on the return value. When we try to run cargo test, we are presented with the following error:

This error seems reasonable. We want the users of our crate to use the sum function, but in our crate we have it defined as a private function by default. So, after adding the pub modifier before the sum function and running cargo test, our test is green again:

Here's a view of the directory tree of our integration_test example crate:

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

As an example of an integration test, this was very trivial, but the gist of it is that when we write integration tests, we use the crate that's being tested, like any other user of a library would use it.