Bazel usage

<!-- Part of the Carbon Language project, under the Apache License v2.0 with LLVM Exceptions. See /LICENSE for license information. SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception -->

This skill documents how best to use Bazel when building, testing, or manipulating the Carbon repository's Bazel in any way.

Bazel wrappers

Carbon uses Bazel for its build system. To ensure consistent versions, the project uses Bazelisk.

[!IMPORTANT] Always use bazelisk whenever you want to run Bazel. Never run bazel directly in the Carbon project. Anything you want to do with bazel can be done with the bazelisk command instead.

• Bazelisk: Try to use bazelisk in your existing $PATH if available.
  • run_bazelisk.py: If bazelisk isn't available, use ./scripts/run_bazelisk.py to run bazelisk without it being installed.

Essential commands

Building

• Build all: bazelisk build //...
• Build toolchain: bazelisk build //toolchain/...
• Build specific target: bazelisk build //toolchain:carbon

Testing

• Test all: bazelisk test //...:all
• Test toolchain: bazelisk test //toolchain/...
• Test examples: bazelisk test //examples/...

[!TIP] Running all of the tests can be slow, so try to narrowly test the immediately relevant parts of the project first, and only expand coverage as necessary to be confident in the changes.

[!TIP] For specialized instructions on testing and developing the Carbon toolchain, consult these skills:

• Toolchain tests: For authoring, structuring, and running file_test tests.
• Toolchain development: For architecture, essential commands, and debugging the toolchain.

Running binaries built by Bazel

[!IMPORTANT] Always manually run binaries built by Bazel using the bazelisk run command. Never run the binary directly from bazel-bin/.

You can run the Carbon driver or command line directly via Bazel:

• bazelisk run //toolchain -- compile --phase=parse toolchain/parse/testdata/basics/empty.carbon

Advanced configurations

AddressSanitizer (ASan)

To enable ASan for local testing:

• Pass --config=asan: bazelisk test --config=asan //...

Common pitfalls and troubleshooting

bazel clean

Changes to packages installed on your system (like changing LLVM versions or installing libc++) may not be noticed by Bazel.

• Run bazelisk clean to force cached state to be rebuilt when environment changes occur.