Configuration modifiers

Modifiers (also referred to as configuration modifiers) are a feature that lets users add constraints to individual directories, target definitions and individual buck2 invocations.

They are the recommended to customize build configurations when building targets directly. If you need to customize parts of your build graph (e.g. always build some specific dependencies in release mode), configuration transitions are more appropriate.

(Open-Source Only) Getting started with modifiers

To add support for modifiers in your project, simply call set_cfg_constructor in your root PACKAGE file. Note this is not needed internally as modifiers are already enabled with all the right paramaters.

See the dedicated how-to for more information.

Per-PACKAGE modifiers

Modifiers can be specified in PACKAGE files. A per-PACKAGE modifier all targets in the directory of that PACKAGE file. For example, a modifier specified in foo/PACKAGE will cover all targets in foo/... and all targets in foo/bar/....

See the dedicated how-to for more information.

Per-target modifiers

Modifiers can be added to individual targets. See the dedicated how-to for more information.

CLI modifiers

Modifiers can be specified on the CLI to build a target with a specific constraint. For example, this can be used to build a target with a non-default compiler, or to run a test using a specific sanitizer.

See the dedicated how-to for more information.

More information

Official documentation on modifiers is sparse at the moment, so you might be interested in reading the original RFC for more details.