Labels

The term "label" is a little overloaded in Buck2. It can refer to:

• A target identifier composed of its name, cell and path.
• One or more tag-like strings attached to a target.

For the first definition of label, the target label is determined by where it's defined, as well as its name attribute. Labels are exposed in the build API via the Label type, which can be used in rule implementations and BXL scripts.

For the second definition of label, rules expose a label attribute, which is a list of strings. Those labels could be used to filter targets in query expressions (e.g., by using attrfilter query function). Also specific labels can be targeted when running buck2 test by adding the --exclude/--include flags. See the test execution docs for more information.