Hedron's Compile Commands Extractor for Bazel — User Interface

What is this project trying to do for me?

First, provide Bazel users cross-platform autocomplete for the C language family (C++, C, Objective-C, and Objective-C++), and thereby make development more efficient and fun!

More generally, export Bazel build actions into the compile_commands.json format that enables great tooling decoupled from Bazel.

Usage Visuals

▲ Extracts compile_commands.json, enabling clangd autocomplete in your editor ▼

Status

Pretty great with only very minor rough edges. We use this every day and love it.

If there haven't been commits in a while, it's because of stability, not neglect. This is in daily use inside Hedron.

For everyday use, we'd recommend using this rather than the platform-specific IDE adapters (like Tulsi or the ASwB/CLion plugin to the extent it works), except the times when you need some platform-editor-specific feature (e.g. Apple's NextStep Interface Builder) that's not ever going to be supported in a cross-platform editor.

Outside Testimonials

There are lots of people using this tool. That includes large companies and projects with tricky stacks, like in robotics.

We're including a couple of things they've said. We hope they'll give you enough confidence to give this tool a try, too!

"Thanks for an awesome tool! Super easy to set up and use." — a robotics engineer at Boston Dynamics

"Thank you for showing so much rigor in what would otherwise be just some uninteresting tooling project. This definitely feels like a passing the baton/torch moment. My best wishes for everything you do in life." — author of the previous best tool of this type

Usage

Basic Setup Time: 10m

Howdy, Bazel user 🤠. Let's get you set up fast with some awesome tooling for the C language family.

There's a bunch of text here but only because we're trying to spell things out and make them easy. If you have issues, let us know; we'd love your help making things even better and more complete—and we'd love to help you!

First, do the usual WORKSPACE setup.

Copy this into your Bazel WORKSPACE file to add this repo as an external dependency, making sure to update to the latest commit per the instructions below.

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")


# Hedron's Compile Commands Extractor for Bazel
# https://github.com/hedronvision/bazel-compile-commands-extractor
http_archive(
    name = "hedron_compile_commands",

    # Replace the commit hash in both places (below) with the latest, rather than using the stale one here.
    # Even better, set up Renovate and let it do the work for you (see "Suggestion: Updates" in the README).
    url = "https://github.com/hedronvision/bazel-compile-commands-extractor/archive/ed994039a951b736091776d677f324b3903ef939.tar.gz",
    strip_prefix = "bazel-compile-commands-extractor-ed994039a951b736091776d677f324b3903ef939",
    # When you first run this tool, it'll recommend a sha256 hash to put here with a message like: "DEBUG: Rule 'hedron_compile_commands' indicated that a canonical reproducible form can be obtained by modifying arguments sha256 = ..."
)
load("@hedron_compile_commands//:workspace_setup.bzl", "hedron_compile_commands_setup")
hedron_compile_commands_setup()

Suggestion: Updates

Improvements come frequently, so we'd recommend keeping up-to-date.

We'd strongly recommend you set up Renovate (or similar) at some point to keep this dependency (and others) up-to-date by default. [We aren't affiliated with Renovate or anything, but we think it's awesome. It watches for new versions and sends you PRs for review or automated testing. It's free and easy to set up. It's been astoundingly useful in our codebase, and we've worked with the wonderful maintainer to make things great for Bazel use. And it's used in official Bazel repositories.]

If not now, maybe come back to this step later, or watch this repo for updates. [Or hey, maybe give us a quick star, while you're thinking about watching.] Like Abseil, we live at head; the latest commit to the main branch is the commit you want. So don't rely on release notifications; use Renovate or poll manually for new commits.

Get the extractor running.

We'll generate a compile_commands.json file in the root of the Bazel workspace.

That file describes how Bazel is compiling all the (Objective-)C(++) files. With the compile commands in a common format, build-system-independent tooling (e.g. clangd autocomplete, clang-tidy linting etc.), can get to work.

We'll get it running and then move onto the next section while it whirrs away. But in the future, every time you want tooling (like autocomplete) to see new BUILD-file changes, rerun the command you chose below! Clangd will automatically pick up the changes.

There are four common paths:

1. Have a relatively simple codebase, where every target builds without needing any additional configuration or flags beyond what's in .bazelrc?

In that case, just bazel run @hedron_compile_commands//:refresh_all

Note: you have to bazel run this tool, not just bazel build it.

2. Are there Bazel flags, e.g., --config=my_important_flags_or_toolchains --compilation_mode=dbg, that you apply manually apply to all your builds while developing?

It's fairly important that you supply those same Bazel flags when running this tool, too, so we can accurately understand the build, where files are being generated, etc.

Append, e.g. -- --config=my_important_flags_or_toolchains --compilation_mode=dbg to the above, or whatever flags you normally build with while developing.

Note: the extra -- is not a typo, and functions to pass the flags to this tool when it runs rather than when it builds. Your command should look like:

bazel run @hedron_compile_commands//:refresh_all -- --config=my_important_flags_or_toolchains --compilation_mode=dbg

3. Often, though, you'll want to specify the top-level, output targets you care about and/or what flags they individually need. This avoids issues where some targets can't be built on their own; they need configuration on the command line or by a parent rule. An example of the latter is an android_library, which probably cannot be built independently of the android_binary that configures it.

In that case, you can easily specify the top-level, output targets you're working on and the flags needed to build them.

Open a BUILD file—we'd recommend using (or creating) //BUILD—and add something like:

load("@hedron_compile_commands//:refresh_compile_commands.bzl", "refresh_compile_commands")

refresh_compile_commands(
    name = "refresh_compile_commands",

    # Specify the targets of interest.
    # For example, specify a dict of targets and any flags required to build.
    targets = {
      "//:my_output_1": "--important_flag1 --important_flag2=true",
      "//:my_output_2": "",
    },
    # No need to add flags already in .bazelrc. They're automatically picked up.
    # If you don't need flags, a list of targets is also okay, as is a single target string.
    # Wildcard patterns, like //... for everything, *are* allowed here, just like a build.
      # As are additional targets (+) and subtractions (-), like in bazel query https://docs.bazel.build/versions/main/query.html#expressions
    # And if you're working on a header-only library, specify a test or binary target that compiles it.
)

(For more details on refresh_compile_commands, look at the docs at the top of refresh_compile_commands.bzl).

Finally, you'll need to bazel run :refresh_compile_commands

4. Using ccls or another tool that, unlike clangd, doesn't want or need headers in compile_commands.json?

Similar to the above, we'll use refresh_compile_commands for configuration, but instead of setting targets, set exclude_headers = "all".

If you've got a very large project and compile_commands.json is taking a while to generate:

Adding exclude_external_sources = True and exclude_headers = "external" can help, with some tradeoffs.

For now, we'd suggest continuing on to set up clangd (below). Thereafter, if you your project proves to be large enough that it stretches the capacity of clangd and/or this tool to index quickly, take a look at the docs at the top of refresh_compile_commands.bzl for instructions on how to tune those flags and others.

If you're using Gazelle v0.29 or older:

Please upgrade or add # gazelle:exclude external to the BUILD file in your workspace root. Gazelle had some problematic symlink handling in those versions that we fixed for them with a PR. (Conversely, if, at the time you're reading this, Gazelle v0.29 (January 2023) is so old that few would be using it, please file a quick PR to remove this section.)

Editor Setup — for autocomplete based on compile_commands.json

VSCode

Let's get clangd's extension installed and configured.

code --install-extension llvm-vs-code-extensions.vscode-clangd
# We also need make sure that Microsoft's C++ extension is not involved and interfering.
code --uninstall-extension ms-vscode.cpptools

Then, open VSCode user settings, so things will be automatically set up for all projects you open.

Search for "clangd".

Add the following three separate entries to "clangd.arguments":

--header-insertion=never
--compile-commands-dir=${workspaceFolder}/
--query-driver=**

(Just copy each as written; VSCode will correctly expand ${workspaceFolder} for each workspace.)

• They get rid of (overzealous) header insertion; locate the compile commands correctly, even when browsing system headers outside the source tree; and cause clangd to interrogate Bazel's compiler wrappers to figure out which system headers are included by default.
• If your Bazel WORKSPACE is a subdirectory of your project, change --compile-commands-dir to point into that subdirectory by overriding the flags in your workspace settings. You'll need to re-specify all the flags when you override, because the workspace settings replace all the flags in the user settings.

Turn on: Clangd: Check Updates

• You always want the latest! New great features and fixes are always getting added to clangd.
• We'll assume you always have the latest and aren't using an old version nor Apple's clangd intended for Xcode. While we can and do make great efforts to workaround issues in the current version of clangd, we remove those workarounds when clangd fixes them upstream. This keeps the code simple and development velocity fast!

If turning on automatic updates doesn't prompt you to download the actual clangd server binary, hit (CMD/CTRL+SHIFT+P)->Download language Server.

You may need to subsequently reload VSCode [(CMD/CTRL+SHIFT+P)->reload] for the plugin to load. The clangd download should prompt you to do so when it completes.

If you work on your repository with others...

... and would like these settings to be automatically applied for your teammates, also add the settings to the VSCode workspace settings and then check .vscode/settings.json into source control.

Other Editors

If you're using another editor, you'll need to follow the same rough steps as above: get the latest version of clangd set up to extend the editor and then supply the same flags as VSCode. We know people have had an easy time setting up this tool with other editors, like Emacs and Vim+YouCompleteMe(YCM), for example.

Once you've succeeded in setting up another editor—or set up clang-tidy, or otherwise seen anything that might improve this readme—we'd love it if you'd give back and contribute what you know! Just edit this README.md on GitHub and file a PR :)

"Smooth Edges" — what we've enjoyed using this for

You should now be all set to go! Way to make it through setup.

There should be a compile_commands.json file in the root of your workspace, enabling your editor to provide great, clang-based autocomplete. And you should know what target to bazel run to refresh that autocomplete, when you make BUILD-file changes big enough to require a refresh.

Behind the scenes, that compile_commands.json file contains entries describing all the commands used to build every source file in your project. And, for now, there's also one entry per header, describing one way it is compiled. (This gets you great autocomplete in header files, too, so you don't have to think about clangd's biggest rough edge). Crucially, all these commands have been sufficiently de-Bazeled for clang tooling (or you!) to understand them.

Here's what you should be expecting, based on our experience:

We use this tool every day to develop a cross-platform library for iOS and Android on macOS. Expect Android completion in Android source, macOS in macOS, iOS in iOS, etc. People use it on Linux/Ubuntu and Windows, too.

All the usual clangd features should work. CMD/CTRL+click navigation (or option if you've changed keybindings), smart rename, autocomplete, highlighting etc. Everything you expect in an IDE should be there (because most good IDEs are backed by clangd). As a general principle: If you're choosing tooling that needs to understand a programming language, you want it to be based on a compiler frontend for that language, which clangd does as part of the LLVM/clang project.

Everything should also work for generated files, though you may have to run a build for the generated file to exist.

Rough Edges

We've self-filed issues for the rough edges we know about and are tracking. We'd love to hear from you there about what you're seeing, good and bad. Please add things if you find more rough edges, and let us know if you need help or more features.

On the other hand, if you've set things up and they're working well, we'd still love to hear from you. Please file a "non-issue" in the issues tab describing your success! We'd love to hear what you're working on, what platforms you're using, and what you're finding most useful. And maybe also toss a star our way so we know it was helpful to you.

We'd also love to work with you on contributions and improvements, of course! Development setup is easy, not onerous; we've got a great doc to guide you quickly into being able to make the changes you need. The codebase is super clean and friendly. Stepping into the code is a fun and efficient way to get the improvements you want.

Other Projects Likely Of Interest

If you're using Bazel for the C language family, you'll likely also want some of our other tooling, like...

1. A good way of making secure network requests: hedronvision/bazel-make-cc-https-easy
2. A way to use std::filesystem across platforms: hedronvision/bazel-cc-filesystem-backport

If so, please check out our other project, hedronvision/bazel-make-cc-https-easy.

Looking for implementation details instead? Want to dive into the codebase? See ImplementationReadme.md.

Bazel/Blaze maintainer reading this? If you'd be interested in integrating this into official Bazel tools, let us know in an issue or email, and let's talk! We love getting to use Bazel and would love to help.