clang: Add support for C++ modules in deps

This adds support for {{module_deps}}. This expansion is to be used by
the "cxx" tool. When the deps of a target include dependencies on
modules (defined as a target that includes a .modulemap file, which is
built to a .pcm), module_deps will be filled out with the required flags
to depend on those modules.

The targets that are depended upon must currently have a (handwritten)
.modulemap file in their sources.

The dependency semantics (as described in NinjaCBinaryTargetWriterTest)
- .modulemap are built to .pcm by cxx_module
- .pcm aren't linked against
- The .cc of a target that uses modules depend on the .pcm ("implicit"
dependency in ninja terminology)
- The .cc sources of targets that depend on a module have implicit
dependencies on the pcm of the dependent modules target as well.
- A .a or executable does not depend on the .pcm.

Bug: fuchsia:27276
Change-Id: I84c26975b93db71e5309ad607fce900fe2705f90
Commit-Queue: Scott Graham <>
Reviewed-by: Brett Wilson <>
Reviewed-by: Petr Hosek <>
13 files changed
tree: 6303562ae7f1953a23165f8f7b546b48d7fdc7d1
  1. build/
  2. docs/
  3. examples/
  4. infra/
  5. misc/
  6. src/
  7. tools/
  8. .clang-format
  9. .editorconfig
  10. .gitignore
  11. .style.yapf
  14. OWNERS


GN is a meta-build system that generates build files for Ninja.

Related resources:

Getting a binary

You can download the latest version of GN binary for Linux, macOS and Windows.

Alternatively, you can build GN from source:

git clone
cd gn
python build/
ninja -C out
# To run tests:

On Windows, it is expected that cl.exe, link.exe, and lib.exe can be found in PATH, so you'll want to run from a Visual Studio command prompt, or similar.

On Linux and Mac, the default compiler is clang++, a recent version is expected to be found in PATH. This can be overridden by setting CC, CXX, and AR.


There is a simple example in examples/simple_build directory that is a good place to get started with the minimal configuration.

To build and run the simple example with the default gcc compiler:

cd examples/simple_build
../../out/gn gen -C out
ninja -C out

For a maximal configuration see the Chromium setup:

and the Fuchsia setup:

Reporting bugs

If you find a bug, you can see if it is known or report it in the bug database.

Sending patches

GN uses Gerrit for code review. The short version of how to patch is:

Register at

... edit code ...
ninja -C out && out/gn_unittests

Then, to upload a change for review:

git commit
git push origin HEAD:refs/for/master

The first time you do this you'll get an error from the server about a missing change-ID. Follow the directions in the error message to install the change-ID hook and run git commit --amend to apply the hook to the current commit.

When revising a change, use:

git commit --amend
git push origin HEAD:refs/for/master

which will add the new changes to the existing code review, rather than creating a new one.

We ask that all contributors sign Google's Contributor License Agreement (either individual or corporate as appropriate, select ‘any other Google project’).


You may ask questions and follow along with GN‘s development on Chromium’s gn-dev@ Google Group.