How to write regression tests for a Rust binary crate?

Question

There's a lot of similar posts in the past 1 2 3 4 but they all seem outdated or irrelevant.

My question is: how do you write regression tests for Rust binaries?

I could set them up as "unit tests" in my src/main.rs file, but that's annoying. Ideally, it would be set up as

root
|---src
    |---main.rs
    |---foo.rs
    |---bar.rs
|---tests
    |---regress1.rs
    |---regress2.rs

score 4 · Accepted Answer · answered Apr 24 '21 at 23:39

Two options:

Split your code into a library and a binary: src/lib.rs and src/main.rs. Then you can write tests/ tests that can load the library part.

This option is best if you specifically want to take advantage of the fact that tests/ tests ("integration tests") are individual binaries on their own (e.g. if the code you want to test uses global variables or system calls that affect global state).
You can write #[test] tests in your binary's code without putting them directly in your src/main.rs file. Just write mod tests; or mod tests { mod regress1; } and put your tests in src/tests/regress1.rs, and in that file write #[test] functions as usual. (Or, if you really want them in a different directory, use the #[path] attribute on mod.)

This option allows faster test execution, because the tests aren't separate binaries and will be run parallel in threads by the Rust test harness.

1 Answers1