Create an enable_wip_features Starlark build config. (#2806)

To be used for features still undergoing testing, or which will only be
used for testing for the foreseeable future.

(cherry picked from commit 795fdb4)

Co-authored-by: Nicholas Levin <nglevin@google.com>

Author: keith

apple/build_settings/BUILD

@@ -1,18 +1,36 @@
 # Build settings used throughout rules_apple build rules.
 
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
-load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_flag")
+load(
+    "@bazel_skylib//rules:common_settings.bzl",
+    "bool_flag",
+    "bool_setting",
+    "string_flag",
+)
 load(
     "//apple/build_settings:build_settings.bzl",
+    "build_flags",
     "build_settings",
 )
 
 package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])
 
+# Flags are configurable from the command line; pure "*_setting"s are not. Rely on this distinction
+# to keep Starlark custom build settings de-facto "private" vs public while keeping them accessible
+# to the toolchains.
 [
     bool_flag(
+        name = build_flag_name,
+        build_setting_default = build_flag_info.default,
+    )
+    for build_flag_name, build_flag_info in build_flags.items()
+    if type(build_flag_info.default) == "bool"
+]
+
+[
+    bool_setting(
         name = build_setting_name,
         build_setting_default = build_setting_info.default,
     )
@@ -22,11 +40,11 @@ licenses(["notice"])
 
 [
     string_flag(
-        name = build_setting_name,
-        build_setting_default = build_setting_info.default,
+        name = build_flag_name,
+        build_setting_default = build_flag_info.default,
     )
-    for build_setting_name, build_setting_info in build_settings.items()
-    if type(build_setting_info.default) == "string"
+    for build_flag_name, build_flag_info in build_flags.items()
+    if type(build_flag_info.default) == "string"
 ]
 
 bzl_library(
@@ -35,6 +53,9 @@ bzl_library(
     visibility = [
         "//:__subpackages__",
     ],
+    deps = [
+        "@bazel_skylib//lib:dicts",
+    ],
 )
 
 # Consumed by bazel tests.

apple/build_settings/build_settings.bzl

@@ -14,8 +14,11 @@
 
 """List of Bazel's rules_apple build settings."""
 
-# List of all registered build settings at `rules_apple/apple/build_settings/BUILD`.
-build_settings = {
+load("@bazel_skylib//lib:dicts.bzl", "dicts")
+
+# List of all registered build settings with command line flags at
+# `rules_apple/apple/build_settings/BUILD`.
+build_flags = {
     "parse_xcframework_info_plist": struct(
         doc = """
 Configuration for enabling XCFramework import rules use the xcframework_processor_tool to
@@ -50,14 +53,34 @@ physical devices) or `xcrun simctl list devices` (for simulators).
     ),
 }
 
-_BUILD_SETTING_LABELS = {
-    build_setting_name: str(Label("//apple/build_settings:{target_name}".format(
-        target_name = build_setting_name,
-    )))
-    for build_setting_name in build_settings
+# List of all registered build settings without command line flags at
+# `rules_apple/apple/build_settings/BUILD`.
+build_settings = {
+    "enable_wip_features": struct(
+        doc = """
+Enables functionality that is still a work in progress, with interfaces and output that can change
+at any time, that is only ready for automated testing now.
+
+This could indicate functionality intended for a future release of the Apple BUILD rules, or
+functionality that is never intended to be production-ready but is required of automated testing.
+""",
+        default = False,
+    ),
 }
 
+_all_build_settings = dicts.add(build_settings, build_flags)
+
 build_settings_labels = struct(
-    all_labels = _BUILD_SETTING_LABELS.values(),
-    **_BUILD_SETTING_LABELS
+    all_labels = [
+        str(Label("//apple/build_settings:{target_name}".format(
+            target_name = build_setting_name,
+        )))
+        for build_setting_name in _all_build_settings
+    ],
+    **{
+        build_setting_name: str(Label("//apple/build_settings:{target_name}".format(
+            target_name = build_setting_name,
+        )))
+        for build_setting_name in _all_build_settings
+    }
 )