The bans check is used to deny (or allow) specific crates, as well as detect and handle multiple versions of the same crate.

cargo deny check bans

Use Case - Denying specific crates

Sometimes, certain crates just don't fit in your project, so you have to remove them. However, nothing really stops them from sneaking back in due to innocuous changes like doing a cargo update and getting it transitively, or even forgetting to set default-features = false, features = ["feature-without-the-thing"] when the crate is pulled in via the default features of a crate you already depend on, in your entire workspace.

For example, we previously depended on OpenSSL as it is the "default" for many crates that provide TLS. This was extremely annoying as it required us to have OpenSSL development libraries installed on Windows, for both individuals and CI. We moved all of our dependencies to use the much more streamlined native-tls or ring crates instead, and now we can make sure that OpenSSL doesn't return from the grave by accident.

Use Case - Duplicate version detection

The larger your project and number of external dependencies, the likelihood that you will have multiple versions of the same crate rises. This is due to two fundamental aspects of the Rust ecosystem.

  1. Cargo's dependency resolution tries to solve all the version constraints to a crate to the same version, but is totally ok with using multiple versions if it is unable to.
  2. Rust has a huge (ever growing) number of crates. Every maintainer has different amounts of time and energy they can spend on their crate, not to mention different philosophies on dependencies and how often (or not) they should be updated, so it is inevitable that crates will not always agree on which version of another crate they want to use.

This tradeoff of allowing multiple version of the same crate is one of the reasons that cargo is such a pleasant experience for many people new to Rust, but as with all tradeoffs, it does come with costs.

  1. More packages must be fetched, which especially impacts CI, as well as devs.
  2. Compile times increase, which impacts CI and devs.
  3. Target directory size increases, which can impact devs, or static CI environments.
  4. Final binary size will also tend to increase, which can impact users.

Normally, you will not really notice that you have multiple versions of the same crate unless you constantly watch your build log, but as mentioned above, it does introduce paper cuts into your workflows.

The intention of duplicate detection in cargo-deny is not to "correct" cargo's behavior, but rather to draw your attention to duplicates so that you can make an informed decision about how to handle the situation.

  • Maybe you want to open up a PR on a crate to use a version of the duplicate that is aligned with the rest of the ecosystem.
  • Maybe the crate has actually already been updated, but the maintainer hasn't published a new version yet, and you can ask if they can publish a new one.
  • Maybe, even though the versions are supposedly incompatible according to semver, they actually aren't, and you can temporarily patch 1 or more crates to use a different version requirement without actually change the crates' code itself.
  • Sometimes having the "latest and greatest" is not really that important for every version, and you can just specify a lower version in your own project that matches the transitive constraint(s).
  • And finally, you don't care about a particular case of multiple versions, so you just tell cargo-deny to ignore one or more of the specific versions, and the situation will eventually resolve itself.

Example output

bans output