format: Fix not getting to fixed point in one step

In some styles of suffix
comments the formatter would wrap the suffix comments. But on reapplying
the formatter a second time, the parse tree is slightly different. The
second time through, the wrapped comment is the only thing on a line
separating two statements.

  a =                 1
    b + c  # x        2
           # y        3
  d = e               4

The ParseNode doesn't have a LocationRange that includes the suffix
comments, so when the formatter attempts to determine if there was a
blank line between two statements, it incorrectly decides there should
be an additional blank line added (because the end of the first
statement is not directly next to the start of the next one). That is,
the "end" of the "a=b+c" is line 2, not line 3.

I fiddled around with trying to amend the parse tree to make the
comments be included in the range of the node, but I didn't come up with
a reasonable way to do that. Instead, have the formatter calculate the
"real" range at formatting time, so instead of simply using line numbers
for A and B to determine if they're split, it walks down A looking for
suffix comments and finds the lowest down one and uses that instead. In
the example above, the walk will determine that "a=b+c" really ends on
line 3, so is adjacent to "d=e" on line 4 and no blank line should be

This makes the recent Chromium test reformat
"stick" in that no further changes are made if the formatter is run on
it again.

Bug: gn:141
Change-Id: If320a87feec9671d2368a799e9e396866698561d
Commit-Queue: Scott Graham <>
Reviewed-by: Brett Wilson <>
Reviewed-by: Nico Weber <>
4 files changed
tree: 7e79b62722b87cd25d3c61933e1fc09d22daac2b
  1. build/
  2. docs/
  3. examples/
  4. infra/
  5. misc/
  6. src/
  7. .clang-format
  8. .editorconfig
  9. .gitignore
  10. .style.yapf
  13. 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.

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