# Integration testing

[Unit tests][unit] are testing one module in isolation at a time: they're small
and can test private code. Integration tests are external to your crate and use
only its public interface in the same way any other code would. Their purpose is
to test that many parts of your library work correctly together.

Cargo looks for integration tests in `tests` directory next to `src`.

File `src/lib.rs`:

```rust,ignore
// Define this in a crate called `adder`.
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}
```

File with test: `tests/integration_test.rs`:

```rust,ignore
#[test]
fn test_add() {
    assert_eq!(adder::add(3, 2), 5);
}
```

Running tests with `cargo test` command:

```shell
$ cargo test
running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

     Running target/debug/deps/integration_test-bcd60824f5fbfe19

running 1 test
test test_add ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

   Doc-tests adder

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```

Each Rust source file in the `tests` directory is compiled as a separate crate. One
way of sharing some code between integration tests is making a module with public
functions, importing and using it within tests.

File `tests/common.rs`:

```rust,ignore
pub fn setup() {
    // some setup code, like creating required files/directories, starting
    // servers, etc.
}
```

File with test: `tests/integration_test.rs`

```rust,ignore
// importing common module.
mod common;

#[test]
fn test_add() {
    // using common code.
    common::setup();
    assert_eq!(adder::add(3, 2), 5);
}
```

Modules with common code follow the ordinary [modules][mod] rules, so it's ok to
create common module as `tests/common/mod.rs`.

[unit]: unit_testing.md
[mod]: ../mod.md
