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RFC: labels -> metadata attribute

This RFC proposes to add new builtin per target attribute: metadata, as replacement for labels.

Context: labels​

In buck1 we have labels builtin rule attribute, which is a list of strings.

In buck2 we have labels attribute which is configured in prelude, it does not have special meaning.

Context: package values​

PACKAGE files have a function: write_package_value(key, value), where a key is a word-dot-word string, and value is arbitrary starlark value which should be serializable as JSON.

Context: metadata we use or we need​

There are several spaces where we use or need metadata to be stored in buck2 target graph.

  • fbcode uses per-package values to switch code to new clang (example)
  • testinfra wants to use PACKAGE values to mark a set of folders to a logical larger project
  • it is likely that per-target metadata attribute should be used in configuration factory function.
  • TD wants to declare CI trigger jobs per-target or per-package, and this logic is to be specified in BUCK or PACKAGE files — as metadata

Proposal: metadata attribute​

Add builtin metadata attribute to all the targets.

metadata has the same structure as package values: word-dot-word to arbitrary value serializable to JSON.

For example:

cxx_library(
    name = "mylib",
    metadata = {
        "td.run_on_windows": True,
    },
)

Metadata attribute is not configurable (means select cannot be used).