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Setup configuration modifiers

Note: this page is not relevant for Meta internal users.

To start using modifiers, all you need to do is call set_cfg_constructor from a PACKAGE file.

Start by adding the following to your root PACKAGE file:

load("@prelude//cfg/modifier:cfg_constructor.bzl", "cfg_constructor_post_constraint_analysis", "cfg_constructor_pre_constraint_analysis")
load("@prelude//cfg/modifier:common.bzl", "MODIFIER_METADATA_KEY")

native.set_cfg_constructor(
    stage0 = cfg_constructor_pre_constraint_analysis,
    stage1 = cfg_constructor_post_constraint_analysis,
    key = MODIFIER_METADATA_KEY,
    aliases = struct(),
    extra_data = struct(),
)

set_cfg_constructor is a Buck2 builtin used to setup configuration modifiers. It supports a few configuration points:

  • stage0 and stage1 are used to resolve modifiers from the target platform (which is now considered deprecated) and the modifiers set in the nearest PACKAGE file, on the target, and on the CLI. You can override those, but the functions provided by the prelude basically do what you would expect.
  • key is used to fetch modifiers defined on metadata attributes. It is only supported for legacy reasons, so you should not have to worry about it.
  • aliases contains modifier aliases to modifier modifiers. Populate it to make aliases available from the CLI.
  • extra_data is used for logging/validation by Meta's internal modifier implementation.

As you can see, aliases is the the only value that one would commonly want to configure, and it might make sense to call set_cfg_constructor with different aliases values in different PACKAGE files, if some aliases only make sense in specific projects (e.g. because they have custom configuration constraints).

The following is an example of exposing custom aliases for build constraints. We create a build_mode constraint with 2 values (debug and release), which would most likely be selected in a toolchain definition.

BUCK:

constraint_setting(name = "build_mode")

constraint_value(
    name = "debug",
    constraint_setting = ":build_mode",
)

constraint_value(
    name = "release",
    constraint_setting = ":build_mode",
)

PACKAGE:


load("@prelude//cfg/modifier:cfg_constructor.bzl", "cfg_constructor_post_constraint_analysis", "cfg_constructor_pre_constraint_analysis")
load("@prelude//cfg/modifier:common.bzl", "MODIFIER_METADATA_KEY")

native.set_cfg_constructor(
    stage0 = cfg_constructor_pre_constraint_analysis,
    stage1 = cfg_constructor_post_constraint_analysis,
    key = MODIFIER_METADATA_KEY,
    aliases = struct(
        debug = "//build:debug",
        release = "//build:release",
    ),
    extra_data = struct(),
)

Now, assuming that :my_target exists and is affected by the build_mode constraint, we can build it in debug and release mode from the command line using the -m/--modifier flag:

buck2 build :my_target -m debug
buck2 build :my_target -m release