Can we share test utilites between crates?

Question

I have crate-0 that has src/lib.rs as follows:

#[cfg(test)]
pub mod test_utils {
    pub struct OnlyInTests(pub usize);
    pub fn helper() -> usize { 999 }

    #[test]
    fn test_0() { let _ = OnlyInTests(helper()); }
}

I also have crate-1 where I need the testing framework I defined in crate-0:

extern crate crate_0;

#[cfg(test)]
pub mod test_utils {
    // This will error out - cannot find mod test_utils in crate_0
    use crate_0::test_utils::{OnlyInTests, helper()};

    #[test]
    fn test_1() { let _ = OnlyInTests(helper()); }
}

The code here is trivial and can be copy-pasted, but in reality I have complex test utilities which I want to use while testing crate-1.

I cannot separate out the test utilities into a different crate as I would get cyclic dependency error: test_utils would depend on crate-0 create stuff and crate-0 would depend on test_utils for testing). Nor do I actually want to do this as there are further crates down the line whose testing utilities I would like to use in dependent crates.

Answer 1

Two solutions with sightly different trade-offs:

Separate Crate for utilities

Put your test utilities but not your tests in a new crate crate-0-testutils. Make crate-0 dev-depend on crate-0-testutils; crate-0-testutils depend on crate-0; crate-1 dev-depend on crate-0-testutils and depend on crate-0.

This does not create a circular dependency as the dev-dependencies do not get propagated. Because of the dev-dependency your tests in crate-0 can still use what is in crate-0-testutils.

This at most doubles the number of your lib crates if all your creates had such utilities.

Use a testing-feature across all crates

Create a feature in the Cargo.toml ofcrate-0 that does not depend on anything:

[features]
testing = []

And one in crate-1 that depends on the above:

testing = ["crate-0/testing"]

Then make the first tests on each of the crates fail when the feature is not enabled, to make for an easier to understand error:

#[cfg(all(not(feature = "testing"),test))]
mod testusage {
    #[test]
    fn panic_without_testing_feature() {
        panic!("Please use the feature testing when running tests.\n\nUse: cargo test --features testing\n\n");
    }
}

Only compile the utilities when the feature is enabled by adding a guard for it:

#[cfg(feature = "testing")]
fn testing_utility() {
}

This only adds one feature in each crate regardless of how many testing features it depends on but has the downside of needing a special invocation of the tests.

Avoid adding this feature in any crate wide dependency declaration. Including the testing-feature from crate-0 in the dev-dependency of crate-1 where crate-1 also has a normal dependency on crate-0 results in cargo building crate-0 with that feature even for the build target of crate-1 even in release mode.