Unused Dependencies
After you have been working on your Rust project for a while, you notice complaints that the compilation times are slow. You look into it, and discover that in multiple crates of the project, several dependencies exist which are unused, driving up compilation times. You remove them manually for now, but you wonder if there is a way to ensure that your crates do not accumulate unused dependencies in the future.
Having unused dependencies in your crates means the compiler needs to do extra work to fetch and build them even though they will not be used. Ensuring that you don’t have any is therefore an important part in making sure your compile times are low.
If you have some some dependencies that you need only conditionally, consider using crate features1 or conditional dependencies2 to prevent them from being depended on unneccessarily. These let you express that dependencies are only needed if a certain feature is activated or on a specific platform.
There are two tools in the Rust ecosystem that you can use to detect
unused dependencies: cargo-udeps and cargo-machete. These differ in the way
they attempt to detect unused dependencies, and thereby the time they take
to make the determination.
cargo-machete
The cargo-machete tool should be your go-to to solve this3.
It aims to be very fast, at the expense of some precision. For that reason, it
is very suitable for running in CI for every merge request.
It can be installed like this:
cargo install cargo-machete
Once installed, it can be invoked in a project like this:
cargo machete
cargo-udeps
The cargo-udeps tool is more accurate, however it is also
slower as it relies on compiling the crate.
It can be installed like this:
cargo install cargo-udeps
Once installed, it can be invoked in a project like this:
cargo udeps
Examples
Having unused dependencies only impacts compile time but not the correctness of the project. Running these checks for every merge request might be too expensive for your project. For that reason, this check could be performed on a schedule (nightly or weekly, for example) rather than on every merge request, or manually by maintainers.
TODO
Reading
cargo machete, find unused dependencies quickly by Benjamin Bouvier
Benjamin introduces the cargo-machete tool, which lets you quickly find
unused dependencies. He explains how it works, and how it works differently to
cargo-udeps to be much faster, at the expense of some accuracy.
Item 25: Manage your dependency graph in Effective Rust
Finding unused dependencies with cargo-udeps by Amos Wenger
See chapter Optional Features.
A feature of Cargo to let you specify dependencies depending on the circumstance, for example the platform. See https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies for more information.
cargo-machete, find unused dependencies quickly: https://blog.benj.me/2022/04/27/cargo-machete/