As your rust project evolves, adding or changing the dependencies you use is quite common, leading to unused dependencies in your project. The cargo udeps add-on is here to help you.

After a while it’s easy to get to a point where (especially on a larger project) it’s hard to know which dependencies are still being used, and if the entries in your Cargo.toml file are all actually being used in your code.

Building those extra dependencies takes time in your local builds and in CI and unnecessarily slows everything down. Also, people installing/compiling your crate will also be downloading, installing and building those dependencies unnecessarily.

You can manually scan your Cargo.toml file and for each dependency, either search for the reference in your code, or try removing it and see if everything still compiles (and tests!) successfully.

But that’s a laborious process!

There is a cargo add-on called udeps that can help you out here.

Here is it’s crates.io entry

Find unused dependencies in Cargo.toml.

While compilation of this tool also works on Rust stable, it needs Rust nightly to actually run.

and you use it as described there:

cargo +nightly udeps

It’s unfortunate it still requires nightly to run (on my flow project I stick to stable, and so a udeps check requires a complete compile from scratch on nightly but I don’t run it so often…)

If there are no unused dependencies then the output will end with:

All deps seem to have been used

It there are any unused dependencies then they will be reported like this:

unused dependencies:
`flowr v0.34.3 (/Users/andrew/workspace/flow/flowr)`
└─── dependencies
     ├─── "ctrlc"
     └─── "toml"
Note: These dependencies might be used by other targets.
      To find dependencies that are not used by any target, enable `--all-targets`.
Note: They might be false-positive.
      For example, `cargo-udeps` cannot detect usage of crates that are only used in doc-tests.
      To ignore some of dependencies, write `package.metadata.cargo-udeps.ignore` in Cargo.toml.

You can then remove them from your list of dependencies in Cargo.toml and run again to check.

Not recursive

I am not sure if it does the check recursively on crates that you depend on, I suspect not. That is less useful as you couldn’t find the problem directly, you would have to raise an issue or a PR with the crate provider, but it still might be useful to raise awareness of unused dependencies in crates in general either to the author or others who can help remove them with issues or PRs…

Workspace projects

When running thus:

cargo +nightly udeps 

in a workspace project, it seems to only check the crates in the default-members list in the Cargo.toml file in the workspace root.

Check the entire workspace for unused dependencies using:

cargo +nightly udeps --workspace

This reports unused dependencies by crate in a tree view:

unused dependencies:
`flowr v0.34.3 (/Users/andrew/workspace/flow/flowr)`
└─── dependencies
     ├─── "ctrlc"
     └─── "toml"
`flowsamples v0.34.0 (/Users/andrew/workspace/flow/samples)`
└─── dependencies
     └─── "flowr"
`flowstdlib v0.34.3 (/Users/andrew/workspace/flow/flowstdlib)`
└─── build-dependencies
     └─── "flowc"
Note: These dependencies might be used by other targets.
      To find dependencies that are not used by any target, enable `--all-targets`.
Note: They might be false-positive.
      For example, `cargo-udeps` cannot detect usage of crates that are only used in doc-tests.
      To ignore some of dependencies, write `package.metadata.cargo-udeps.ignore` in Cargo.toml.

Once you have fixed all your unused dependencies, the output will return to the clean:

All deps seem to have been used.

Other Targets

If you have any dependencies that are only used on specific targets, as described in the output above you can check for them using the --all-targets option to cargo.

Optional Dependencies

The unused dependency check works correctly for optional dependencies, that are only used (or made dependencies) when specific features are enabled in Cargo.toml