Can I test just the code in a single module?

Question

I've got a Rust project that uses a fairly large framework. Compilation and macro expansion take a really long time. If I make a tiny change to the code, it takes a minute or more before before "cargo test" actually executes.

Is it possible to create a sub-project or sub-module within the same crate and test just the code in the module, assuming there are no dependencies on code outside the module?

Answer 1

You might be interested in "cargo workspaces" (https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html).

Essentially, instead of splitting your code into multiple mods, you split it into multiple crates. These crates can depend on each other via "path dependencies". For example, you could have something like:

[dependencies]
my_helper_crate = { path = "path/to/crate" }

The book has much more detail on this, but a nice feature of using workspaces is that your crates can have separate Cargo.tomls, but share a Cargo.lock, so you won't get issues around incompatible versions of crates.

With this setup, you can build one crate without building the rest of them, so you can cut down on a dev feedback loop.

However, if you have crate_a which depends on crate_b, building crate_a still requires building crate_b, there's not really any getting around that. The benefit is mainly for the leaves of your dependency graph.

Answer 2

Yeah, cargo test will take arguments which match specific tests that you want to run (Cargo book). For example, if you have modules foo and bar, you can run cargo test foo to run tests from that module, excluding all others.