No sources_assignment_filter

There is a strong consensus that the set_sources_assignment_filter feature from GN is a mis-feature and should be removed. This requires that Chromium's BUILD.gn file stop using the feature.

Why convert

When set_sources_assignment_filter is called, it configures a list of patterns that will be used to filter names every time a variable named “sources” is assigned a value.

As Chromium calls this function in build/BUILDCONFIG.gn, the patterns are applied to every BUILD.gn file in the project. This has multiple drawbacks:

1. the configuration of the list of patterns is located far from the point where they are applied and developer are usually confused when a file they add to a rule is not build due to those pattern

2. the filtering is applied to every assignment to a variable named “sources” after interpreting the string as a relative filename, thus build breaks if one of the forbidden pattern is used in unexpected location (like naming the build directory out/linux, or having mac/ in path to SDK, ...)

3. the filtering is applied to every assignment to a variable named “sources” in the whole project, thus it has significant negative impact on the performance of gn

Conversion pattern

To convert a BUILD.gn file it is neccessary to change the following:

  source_set("foo") {
    sources = [
      "foo.h",
      "foo_mac.mm",
      "foo_win.cc",
      "foo_linux.cc",
    ]
  }

to

  source_set("foo") {
    sources = [
      "foo.h",
    ]
    if (is_mac) {
      sources += [
        "foo_mac.mm",
      ]
    }
    if (is_win) {
      sources += [
        "foo_win.cc",
      ]
    }
    if (is_linux) {
      sources += [
        "foo_linux.cc",
      ]
    }
  }

Since the second pattern never assign a name that will be filtered out, then it is compatible whether the set_sources_assignment_filter feature is used or not.

Preventing regression

As said above, while the converted file are compatible with the feature, there is a risk of regression. To prevent such regression, the following is added at the top of every BUILD.gn file after it has been converted:

  # Reset sources_assignment_filter for the BUILD.gn file to prevent
  # regression during the migration of Chromium away from the feature.
  # See build/no_sources_assignment_filter.md for more information.
  # TODO(crbug.com/1018739): remove this when migration is done.
  set_sources_assignment_filter([])