Once you've fully migrated a codebase to bazel, one of the many advantages is that you can easily inspect your build graph using bazel query.

One of the many things you can do with queries is write scripts to enforce coding standards, or in today's example, find unused targets that can lead to discovering unused code.

The simplest version of this query starts like this:

let all_targets = //... in
let top_level_targets = tests($all_targets) union kind("(.*_binary) rule", $all_targets) in
$all_targets - deps($top_level_targets)

This initial version discovers all your first party targets, and then subtracts the dependencies of all "top level" targets, so that any remaining targets are considered unused.

The idea of "top level" targets is better defined as: anything that you consider to be important enough that its dependencies are used. Depending on your codebase you might want to exclude test targets from this so that targets only in the dependencies of test targets are diagnosed as unused (this won't work if you have intentional testonly dependencies, although you could special case those as shown below).

Once you have this initial query, you can start iterating in order to handle in edge cases. For example it's likely that you have some targets that aren't binaries or tests, but are considered used. There are 2 approaches I would recommend to handle this. First you can continue to build out the kind filter:

kind("(.*_binary|platform|test_suite) rule", $all_targets)

The downside with this approach is it can get unwieldy quickly. I like adding rules here that have many uses, but for other one off cases another approach you can use is to expand the query to look for special tags:

let all_targets = //... in
let top_level_targets = tests($all_targets) union kind("(.*_binary|platform|test_suite) rule", $all_targets) in
let allowed_unused = attr(tags, allow-unused, $all_targets) in
$all_targets - deps($top_level_targets) - $allowed_unused

Then you can add tags = ["allow-unused"] to whatever targets that you need.

Similarly if you want to special case targets that can be considered "top level" you can use a tag for that case as well:

let all_targets = //... in
let top_level_targets = tests($all_targets) union kind("(.*_binary|platform|test_suite) rule", $all_targets) union attr(tags, top-level, $all_targets) in
let allowed_unused = attr(tags, allow-unused, $all_targets) in
$all_targets - deps($top_level_targets) - $allowed_unused

Now adding tags = ["top-level"] to various targets will ensure their entire dependency tree is also considered used. This case is especially useful for targets like CC toolchains which are used in your MODULE.bazel but potentially don't appear as used by bazel query.

Hopefully this post was instructive and gives you some ideas for how you can write more advanced queries for your use cases.