Troubleshooting Cargo Panic With `cargo Rustdoc` And Dev Dependencies

Jul 19, 2025 by JurnalWarga.com 70 views

Troubleshooting Panic with `cargo rustdoc` on Dev Dependencies

Hey everyone! Have you ever run into a situation where you're trying to generate documentation for your Rust project using cargo rustdoc, and suddenly you're faced with a panic related to your dev dependencies or dependencies of proc-macros? It's a bit of a head-scratcher, but let's dive into the issue and see how we can tackle it.

Understanding the Panic

The problem arises when using cargo rustdoc on a dev dependency, or a dependency of a procedural macro. When this happens, Cargo might panic, giving you a stack backtrace that looks something like this:

$ RUST_BACKTRACE=1 cargo rustdoc -p syn
thread 'main' panicked at src/tools/cargo/src/cargo/core/resolver/features.rs:325:13:
did not find features for (PackageId { name: "syn", version: "2.0.104", source: "registry `crates-io`" }, NormalOrDev) within activated_features:
[
    (
        PackageId {
            name: "repro",
            version: "0.1.0",
            source: "/home/arvid/src/repro",
        },
        NormalOrDev,
    ),
    (
        PackageId {
            name: "serde",
            version: "1.0.219",
            source: "registry `crates-io`",
        },
        NormalOrDev,
    ),
]
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:75:14
   2: <cargo::core::resolver::features::ResolvedFeatures>::activated_features
   3: <cargo::ops::cargo_compile::unit_generator::UnitGenerator>::proposals_to_units
   4: <cargo::ops::cargo_compile::unit_generator::UnitGenerator>::generate_root_units
   5: cargo::ops::cargo_compile::create_bcx
   6: cargo::ops::cargo_compile::compile_ws
   7: cargo::ops::cargo_compile::compile_with_exec
   8: cargo::ops::cargo_compile::compile
   9: cargo::ops::cargo_doc::doc
  10: cargo::commands::rustdoc::exec
  11: <cargo::cli::Exec>::exec
  12: cargo::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

This panic typically indicates that Cargo couldn't find the features for a particular package within the activated features list. Specifically, it struggles with dev dependencies or dependencies of proc macros.

Keywords to Keep in Mind: When dealing with Rust projects, especially those involving macros and documentation, understanding how Cargo manages dependencies is crucial. The cargo rustdoc command is your go-to for generating documentation, but it sometimes stumbles with dev dependencies. Knowing how to diagnose and fix these issues can save you a lot of headache.

Reproducing the Issue: Step-by-Step

To better understand this issue, let's try to reproduce it. Follow these steps:

Create a new Rust project: cargo new repro
Navigate into the project directory: cd repro
Add serde or syn as a dev dependency: cargo add --dev syn (or cargo add serde)
Attempt to generate documentation for syn: cargo rustdoc -p syn

You'll likely encounter the panic described above. The interesting thing is that this doesn't happen if you add syn as a direct dependency or for indirect dependencies of normal dependencies. It seems to be specifically triggered by proc macros and other dev dependencies. This is a common pitfall when working with Rust projects, especially those that heavily rely on macros and advanced features.

Diving Deeper: Why Does This Happen?

So, why does this panic occur specifically with dev dependencies and proc macros? The answer lies in how Cargo resolves features and dependencies during the documentation generation process.

When you run cargo rustdoc, Cargo needs to build a dependency graph to understand which crates need to be documented and how they relate to each other. During this process, it activates features for each crate. However, when it comes to dev dependencies and proc macros, the feature resolution can sometimes go awry.

Key Concepts: Understanding Cargo's feature resolution is essential. Cargo uses features to conditionally compile code, and these features need to be correctly activated when generating documentation. Dev dependencies are often treated differently from regular dependencies, which can lead to discrepancies in feature activation. Proc macros, which are compiled and executed at compile time, add another layer of complexity to the dependency resolution process.

Possible Solutions and Workarounds

Now that we understand the problem, what can we do about it? Here are a few possible solutions and workarounds:

1. Better Error Reporting

The first and most immediate improvement would be to provide a more informative error message. Instead of a panic, Cargo could give a clear indication of what went wrong and how to fix it. A helpful error message might suggest checking feature flags or ensuring that all necessary dependencies are included.

2. Adjusting Feature Flags

Sometimes, the issue can be resolved by explicitly specifying the features you need. You can do this by using the --features flag with cargo rustdoc. For example:

cargo rustdoc -p syn --features full

This tells Cargo to activate the full feature for the syn crate, which might include the necessary dependencies for documentation generation. This is particularly useful when dealing with feature flags that control optional functionality in your crates.

3. Temporary Direct Dependency

As a workaround, you can temporarily add the problematic crate as a direct dependency instead of a dev dependency. Generate the documentation, and then revert the change. While not ideal, this can help you get your documentation generated in a pinch.

4. Cargo Bug Fixes

Ultimately, this issue might require a fix in Cargo itself. Reporting the bug to the Cargo team can help ensure that it's addressed in a future release. When reporting, be sure to include a minimal reproducible example and the steps to reproduce the panic. This helps the Cargo team quickly identify and fix the problem.

Best Practices for Handling Dependencies

To minimize the chances of running into issues like this, here are some best practices for handling dependencies in your Rust projects:

Keep Dependencies Organized: Clearly distinguish between regular dependencies and dev dependencies. Use dev dependencies only for crates that are needed for testing and development, not for the core functionality of your library.
Specify Features Explicitly: When a crate has multiple features, be explicit about which ones you need. This reduces ambiguity and can prevent unexpected dependency issues.
Regularly Update Dependencies: Keep your dependencies up to date to benefit from bug fixes and performance improvements. However, always test your code after updating dependencies to ensure that everything still works as expected.

Conclusion

Encountering panics when generating documentation can be frustrating, but understanding the underlying issues and potential workarounds can make the process much smoother. By being mindful of how Cargo handles dev dependencies and proc macros, you can avoid these pitfalls and keep your documentation generation on track.

So, next time you see a panic when running cargo rustdoc, remember to check your dev dependencies, feature flags, and consider whether a temporary workaround or a bug report is the best course of action. Happy documenting, folks! This proactive approach to dependency management is key to maintaining a healthy and well-documented Rust project.