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What is the purpose of grouping the tests in a `tests` module and is it possible to split them?
What is the purpose of grouping the tests in a tests module like this
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_function_1() {
// test code for function 1 goes here
}
#[test]
fn test_function_2() {
// test code for function 2 goes here
}
}
and is it possible to split them and have the test functions outside of the tests module?
#[test]
fn test_function_1() {
// test code for function 1 goes here
}
#[test]
fn test_function_2() {
// test code for function 2 goes here
}
I don't want to split them into their own directory.
3 answers
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Grouping related items into modules is of course generally good practice, but it serves a practical purpose as well. The important part here is the #[cfg(test)] annotation.
The
#[cfg(test)]annotation on the tests module tells Rust to compile and run the test code only when you runcargo test, not when you runcargo build. This saves compile time when you only want to build the library and saves space in the resulting compiled artifact because the tests are not included. You’ll see that because integration tests go in a different directory, they don’t need the#[cfg(test)]annotation. However, because unit tests go in the same files as the code, you’ll use#[cfg(test)]to specify that they shouldn’t be included in the compiled result.
As it says, it is just a way to separate tests from the library itself; the other way is to use a separate directory.
Creating tests outside of such a module actually does work, and as cafce25's answer mentions, #[test] already implies #[cfg(test)]. However, you might also have helper functions or mocks for your tests that you also don't want to include in your final non-test binary. By putting them all into one module, you not only group them semantically, but this also allows you to #[cfg(test)] all of them in one place rather than adding it to each of them individually.
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:
#[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
[
#[test]] functions are only compiled when in test mode.
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When you think about how to organize your test code, you should develop an understanding of your goals. Typical goals about test code organization (which typically includes helper code only needed for the tests) are:
-
Ensure that test code does not become part of production code - for some kinds of software this can even be a strict requirement (for example in safety critical software) - even if the test code within the production code would be dead code or unreachable.
-
Ensure that in deliveries to the customer only the agreed parts are included, for example production code without tests or vice versa - depending on the contract.
-
Make it easily possible to handle test code differently in tools (for example, test coverage tools, static code analysis tools), for example by excluding them by path or module name or the like. The possibility to exclude them during normal compilation (which the
#[cfg(test)]annotation achieves, as was mentioned by @Moshi) is just one example for this. -
Especially in contexts where different kinds of tests have to be performed (unit-tests, integration tests, performance tests, ...) it may become a practical necessity to have the different kinds of code clearly separated and individually structured to avoid confusion even during development. You may need to make additional distinctions between short running and long running tests and so on.
By grouping tests into a separated module or directory etc. you can fulfill the aforementioned goals - in some cases you will even have to go beyond that and introduce further substructuring.
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