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The reason to put tests in a separate module is so they are grouped.

Another advantage is that you can put helper functions that are only needed for tests, but are not themselves tests in the module as well:
```rust
#[cfg(test)]
mod tests {
    fn setup() -> Vec<i32> {
        vec![1, 2, 3]
    }

    #[test]
    fn test_length() {
        let v = setup();
        assert_eq!(v.len(), 3);
    }

    #[test]
    fn test_last() {
        let mut v = setup();
        assert_eq!(v.pop(), Some(3));
    }
}
```
It's *not* used to exclude test functions from the final binary, a `#[test]` annotation on it's own is sufficient to ensure that, i.e. it includes a `#[cfg(test)]` annotation. As [specified in the reference](https://doc.rust-lang.org/reference/attributes/testing.html#the-test-attribute)
> [`#[test]`] functions are only compiled when in test mode.