Gazelle Feature Request: Always Keep Dependencies

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Gazelle Feature Request: Always Keep Dependencies

Hey guys! Let's dive into a cool feature request for Gazelle that could seriously streamline dependency management. This is about having more control over which dependencies Gazelle never removes, regardless of whether they appear to be used or not. Imagine the possibilities! This article explores the need for an "always keep" dependency feature in Gazelle, detailing why it’s important and how it could work.

The Need for 'Always Keep' Dependencies

When we talk about dependency management, it's crucial to understand that sometimes, certain dependencies serve purposes beyond just providing headers or directly used code. Think of them as the unsung heroes of your build process. In the context of Bazel and Gazelle, this becomes particularly relevant. Let's break down why.

Special Dependencies and Their Roles

In many projects, there are specific targets that, while they might not appear to be directly used in the conventional sense (like including a header file), they play a vital role in the build environment. A prime example of this is the //cc/runfiles target within the rules_cc repository.

Depending on //cc/runfiles is more than just linking a library; it's about defining the BAZEL_CURRENT_REPOSITORY macro. This macro is crucial because Bazel relies on it to understand the current repository's context during the build process. This is where things get interesting – and where Gazelle's current behavior can become a bit of a hurdle.

The Challenge with Automatic Dependency Management

Gazelle, being the awesome dependency management tool it is, automatically manages dependencies based on usage. It scans your files, identifies imports and includes, and then crafts the dependency graph accordingly. This works brilliantly most of the time, keeping your BUILD files clean and accurate. However, in scenarios like the //cc/runfiles example, Gazelle's automatic removal of seemingly unused dependencies can lead to problems.

If Gazelle doesn't detect a direct usage (like a header include) of //cc/runfiles, it might decide to remove it as an unused dependency. This is where the trouble begins. Removing this dependency means the BAZEL_CURRENT_REPOSITORY macro isn't defined, potentially breaking the build or causing unexpected behavior. We definitely don't want that, right?

The Importance of Manual Overrides

This is where the need for manual overrides comes into play. We need a way to tell Gazelle, “Hey, hold on! This dependency might look unused, but trust me, it's super important. Always keep it!” This level of control ensures that critical, yet implicitly used, dependencies aren't inadvertently removed.

Proposed Solution: A gazelle:keep Directive

So, how do we solve this? The proposition is to introduce a directive that explicitly instructs Gazelle to retain specific dependencies, regardless of their apparent usage. This directive could take the form of a comment within the BUILD file, making it clear and human-readable. Let’s explore this idea further.

The # gazelle:keep Directive

The suggested solution involves a new directive, # gazelle:keep <target>, which would be placed within a BUILD file. This directive acts as a safeguard, ensuring that the specified target is always included as a dependency, even if Gazelle's automatic analysis doesn't detect its direct usage.

For instance, in our //cc/runfiles example, you could add the following line to your BUILD file:

# gazelle:keep //cc/runfiles

This simple line tells Gazelle to always keep //cc/runfiles as a dependency for this target. It's straightforward, easy to understand, and incredibly effective. But why is this approach so beneficial?

Benefits of Explicitly Keeping Dependencies

The # gazelle:keep directive offers several key advantages:

  • Prevents Accidental Removal: It ensures that critical dependencies, like //cc/runfiles, are never inadvertently removed, preventing build breakages and unexpected issues.
  • Clarity and Documentation: The directive serves as a clear indication to anyone reading the BUILD file that this dependency is intentionally kept, providing valuable documentation.
  • Flexibility: It allows developers to handle special cases where dependencies have implicit or non-obvious uses, granting finer-grained control over dependency management.
  • Maintainability: By explicitly stating the intent to keep a dependency, it reduces the risk of future Gazelle updates or configuration changes accidentally removing it.

How It Differs from # keep

Now, you might be thinking, “Wait a minute, Gazelle already has a # keep directive. How is this different?” That’s a valid question! The existing # keep directive tells Gazelle not to modify a specific dependency. It preserves the dependency if it's already there, but it doesn't explicitly add it if Gazelle doesn't detect its usage. The # gazelle:keep directive, on the other hand, would go a step further and ensure the dependency is always present.

Real-World Use Cases and Examples

To really drive home the value of this feature, let's look at some practical scenarios where an “always keep” directive would be a game-changer. Beyond the //cc/runfiles example, there are numerous other cases where dependencies have non-obvious but crucial roles.

Scenario 1: Custom Macros and Build Logic

Imagine you have a custom macro that relies on a specific library for its internal workings. This library might not be directly included in any source file, but it's essential for the macro to function correctly. Without an “always keep” directive, Gazelle might remove this library, causing the macro to fail.

Scenario 2: Implicitly Linked Libraries

In some build systems, certain libraries are implicitly linked based on compiler flags or other configurations. These libraries might not be explicitly included in any source code, but they are necessary for the final binary to be built successfully. An “always keep” feature would ensure these libraries are retained.

Scenario 3: Tooling Dependencies

Consider a situation where a target depends on a specific tool for code generation or other build-time tasks. This tool might not be directly linked into the final binary, but it's crucial during the build process. The “always keep” directive would prevent Gazelle from removing this tool dependency.

Practical Implementation

To put this into practice, let's say you have a BUILD file for a library that uses a custom macro. This macro depends on a library called //utils/helpers, which isn't directly included in any source file. You could add the following line to your BUILD file:

# gazelle:keep //utils/helpers

load("//build_defs:custom_macro.bzl", "custom_macro")

custom_macro(
    name = "my_library",
    # ... other attributes ...
)

This ensures that //utils/helpers is always included as a dependency for my_library, even though it's not explicitly used in the source code.

Conclusion: Enhancing Gazelle's Power and Flexibility

In conclusion, the addition of an “always keep” dependency feature in Gazelle, like the proposed # gazelle:keep directive, would significantly enhance its power and flexibility. It addresses a critical need for handling dependencies that have implicit or non-obvious uses, preventing accidental removals and ensuring build stability. This feature would provide developers with greater control over their dependency management, making Gazelle an even more indispensable tool in the Bazel ecosystem. Let's hope the Gazelle team considers this awesome feature request!