# Contribution tools The Carbon language project has a number of tools used to assist in preparing contributions. ## Table of contents - [Setup commands](#setup-commands) - [Debian or Ubuntu](#debian-or-ubuntu) - [Installing Bazelisk](#installing-bazelisk) - [Old `clang` versions](#old-clang-versions) - [macOS](#macos) - [Tools](#tools) - [Main tools](#main-tools) - [Running pre-commit](#running-pre-commit) - [Optional tools](#optional-tools) - [Manually building Clang and LLVM (not recommended)](#manually-building-clang-and-llvm-not-recommended) - [Troubleshooting build issues](#troubleshooting-build-issues) - [`bazel clean`](#bazel-clean) - [Old LLVM versions](#old-llvm-versions) - [Debugging](#debugging) - [Asking for help](#asking-for-help) ## Setup commands These commands should help set up a development environment on your machine. ### Debian or Ubuntu ```shell # Update apt. sudo apt update # Check that the `clang` version is at least 19, our minimum version. That needs # the number of the `:` in the output to be over 19. For example, `1:19.0-1`. apt-cache show clang | grep 'Version:' # Install tools. sudo apt install \ clang \ gh \ libc++-dev \ libc++abi-dev \ lld \ lldb \ python3 \ pipx # Install pre-commit. pipx install pre-commit # Set up git. # If you don't already have a fork: gh repo fork --clone carbon-language/carbon-lang cd carbon-lang pre-commit install # Run tests. ./scripts/run_bazelisk.py test //...:all ``` #### Installing Bazelisk Although the `run_bazelisk` script can make it easy to get started, if you're frequently building Carbon, it can be a bit much to type. Consider either aliasing `bazel` to the `run_bazelisk.py` script, or [downloading a bazelisk release](https://github.com/bazelbuild/bazelisk), adding it to your `$PATH`, and aliasing `bazel` to it. #### Old `clang` versions If the version of `clang` is earlier than 19, you may still have version 19 available. You can use the following install instead: ```shell # Install explicitly versioned Clang tools. sudo apt install \ clang-19 \ libc++-19-dev \ libc++abi-19-dev \ lld-19 \ lldb-19 # In your Carbon checkout, tell Bazel where to find `clang`. You can also # export this path as the `CC` environment variable, or add it directly to # your `PATH`. echo "build --repo_env=CC=$(readlink -f $(which clang-19))" >> user.bazelrc ``` And if it's not available directly from the distribution, you can install Clang tools on Debian/Ubuntu from . > NOTE: Most LLVM 19+ installs should build Carbon. If you're having issues, see > [troubleshooting build issues](#troubleshooting-build-issues). ### macOS ```shell # Install Homebrew. /bin/bash -c "$(curl -fsSL \ https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)" # IMPORTANT: Make sure `brew` is added to the PATH! # Install Homebrew tools. brew install \ bazelisk \ gh \ llvm \ python@3.10 \ pre-commit # IMPORTANT: Make sure `llvm` is added to the PATH! It's separate from `brew`. # Set up git. gh repo fork --clone carbon-language/carbon-lang cd carbon-lang pre-commit install # Run tests. Note homebrew makes `bazel` an alias to `bazelisk`. bazel test //...:all ``` > NOTE: On macOS, you should end up adding rc file lines similar to: > > ``` > # For `brew`, `gh`, and other tools: > export PATH="${HOME}/.brew/bin:${PATH}" > # For `llvm`: > export PATH="$(brew --prefix llvm)/bin:${PATH}" > ``` ## Tools ### Main tools These tools are essential for work on Carbon. - Package managers - `apt` (for Debian or Ubuntu) - To upgrade versions of `apt` packages, it will be necessary to periodically run `sudo apt update && sudo apt upgrade`. - [Homebrew](https://brew.sh/) (for macOS) - To upgrade versions of `brew` packages, it will be necessary to periodically run `brew upgrade`. - [Python](https://python.org) - Carbon requires Python 3.9 or newer. - To upgrade versions of pip-installed packages, it will be necessary to periodically run `pipx list --outdated`, then `pipx install -U ` to upgrade desired packages. - When upgrading, version dependencies may mean packages _should_ be outdated, and not be upgraded. - Main tools - [Bazel](https://www.bazel.build/) - [Bazelisk](https://docs.bazel.build/versions/master/install-bazelisk.html): Downloads and runs the [configured Bazel version](/.bazelversion). - [Clang](https://clang.llvm.org/) and [LLVM](https://llvm.org/) - NOTE: Most LLVM 19+ installs should build Carbon. If you're having issues, see [troubleshooting build issues](#troubleshooting-build-issues). - [gh CLI](https://github.com/cli/cli): Helps with GitHub. - [pre-commit](https://pre-commit.com): Validates and cleans up git commits. #### Running pre-commit [pre-commit](https://pre-commit.com) is typically set up using `pre-commit install`. When set up in this mode, it will check for issues when `git commit` is run. A typical commit workflow looks like: 1. `git commit` to try committing files. This automatically executes `pre-commit run`, which may fail and leave files modified for cleanup. 2. `git add .` to add the automatically modifications done by `pre-commit`. 3. `git commit` again. You can also use `pre-commit run` to check pending changes without `git commit`, or `pre-commit run -a` to run on all files in the repository. > NOTE: Some developers prefer to run `pre-commit` on `git push` instead of > `git commit` because they want to commit files as originally authored instead > of with pre-commit modifications. To switch, run > `pre-commit uninstall && pre-commit install -t pre-push`. ### Optional tools These tools aren't necessary to contribute to Carbon, but can be worth considering if they fit your workflow. - [GitHub Desktop](https://desktop.github.com/): A UI for managing GitHub repositories. - `rs-git-fsmonitor` and Watchman: Helps make `git` run faster on large repositories. - **WARNING**: Bugs in `rs-git-fsmonitor` and/or Watchman can result in `pre-commit` deleting files. If you see files being deleted, disable `rs-git-fsmonitor` with `git config --unset core.fsmonitor`. - [vim-prettier](https://github.com/prettier/vim-prettier): A vim integration for [Prettier](https://prettier.io/), which we use for formatting. - [Visual Studio Code](https://code.visualstudio.com/): A code editor. - We provide [recommended extensions](/.vscode/extensions.json) to assist Carbon development. Some settings changes must be made separately: - Python › Formatting: Provider: `black` - **WARNING:** Visual Studio Code modifies the `PATH` environment variable, particularly in the terminals it creates. The `PATH` difference can cause `bazel` to detect different startup options, discarding its build cache. As a consequence, it's recommended to use **either** normal terminals **or** Visual Studio Code to run `bazel`, not both in combination. Visual Studio Code can still be used for other purposes, such as editing files, without interfering with `bazel`. - We also provide recommended setups for debugging in VS Code with either [LLDB](/toolchain/docs/debugging.md#debugging-with-lldb) or [GDB]((/toolchain/docs/debugging.md#debugging-with-gdb) - [clangd](https://clangd.llvm.org/installation): An LSP server implementation for C/C++. - To ensure that `clangd` reports accurate diagnostics. It needs a generated file called `compile_commands.json`. This can be generated by invoking the command below: ``` ./scripts/create_compdb.py ``` - **NOTE**: This assumes you have `python` 3 installed on your system. - [`uv`](https://docs.astral.sh/uv/): A fast Python package manager. - Notably, `uv` supports automatic management of even complex Python dependencies for scripts: https://docs.astral.sh/uv/guides/scripts/ - Installation: https://docs.astral.sh/uv/getting-started/installation/ ### Manually building Clang and LLVM (not recommended) We primarily test against [apt.llvm.org](https://apt.llvm.org) and Homebrew installations. However, you can build and install LLVM yourself if you feel more comfortable with it. The essential CMake options to pass in order for this to work reliably include: ``` -DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra;lld;lldb -DLLVM_ENABLE_RUNTIMES=compiler-rt;libcxx;libcxxabi;libunwind -DRUNTIMES_CMAKE_ARGS=-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF;-DCMAKE_POSITION_INDEPENDENT_CODE=ON;-DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON;-DLIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY=OFF;-DLIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY=ON;-DLIBCXX_USE_COMPILER_RT=ON;-DLIBCXXABI_USE_COMPILER_RT=ON;-DLIBCXXABI_USE_LLVM_UNWINDER=ON -DLLDB_ENABLE_PYTHON=ON ``` ## Troubleshooting build issues ### `bazel clean` Changes to packages installed on your system may not be noticed by `bazel`. This includes things such as changing LLVM versions, or installing libc++. Running `bazel clean` should force cached state to be rebuilt. ### Old LLVM versions Many build issues result from the particular options `clang` and `llvm` have been built with, particularly when it comes to system-installed versions. If you run `clang --version`, you should see at least version 19. If you see an older version, please update, or use the special `clang-19` instructions above. System installs of macOS typically won't work, for example being an old LLVM version or missing llvm-ar; [setup commands](#setup-commands) includes LLVM from Homebrew for this reason. Run [`bazel clean`](#bazel-clean) when changing the installed LLVM version. ### Debugging See the [toolchain documentation](/toolchain/docs/debugging.md) for guidance on how to debug problems with the toolchain itself. ### Asking for help If you're having trouble resolving issues, please ask on [#build-help](https://discord.com/channels/655572317891461132/824137170032787467), providing the output of the following diagnostic commands: ```shell echo $CC which clang which clang-19 clang --version grep llvm_bindir $(bazel info workspace)/bazel-execroot/external/+clang_toolchain_extension+bazel_cc_toolchain/clang_detected_variables.bzl # If on macOS: brew --prefix llvm ``` These commands will help diagnose potential build issues by showing which tooling is in use.