Refactor protobuf flag abstraction layer to custom flag targets

Instead of having rules depend on both native and Starlark versions of flags and reconciling them inside the rule implementation, this change moves the resolution logic into custom build setting flag targets (`compat_bool_flag` and `compat_string_list_flag`) defined in `third_party/protobuf/bazel/private/compat_flag.bzl`.

Key details:
- `compat_bool_flag`: Instantiates a custom string build setting flag with a "default" sentinel value. It checks for explicit Starlark flag settings vs. fallback to native `ctx.fragments.proto` fields or default values, providing a boolean `BuildSettingInfo`.
- `compat_string_list_flag`: Defines a string-list build setting flag that reconciles explicit Starlark build settings with `ctx.fragments.proto` and default values.
- Rules reference primary flag targets directly (without helper `_compat` targets or `config_setting` rules).
- Replaces `native_bool_flag.bzl` with `compat_flag.bzl`.

PiperOrigin-RevId: 952990495

Author: haberman

bazel/flags/BUILD

@@ -1,5 +1,6 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
-load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_flag", "string_list_flag")
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
+load("//bazel/private:compat_flag.bzl", "compat_bool_flag", "compat_string_list_flag")
 
 package(
     default_applicable_licenses = ["//:license"],
@@ -25,9 +26,10 @@ filegroup(
     ],
 )
 
-bool_flag(
+compat_bool_flag(
     name = "experimental_proto_descriptor_sets_include_source_info",
     build_setting_default = False,
+    fragment_field = "experimental_proto_descriptorsets_include_source_info",
     scope = "universal",
 )
 
@@ -36,9 +38,10 @@ label_flag(
     build_setting_default = "@bazel_tools//tools/proto:protoc",
 )
 
-string_list_flag(
+compat_string_list_flag(
     name = "protocopt",
     build_setting_default = [],
+    fragment_field = "experimental_protoc_opts",
     scope = "universal",
 )
 
@@ -75,39 +78,45 @@ config_setting(
 )
 
 # TODO: deprecate this flag.
-string_flag(
+compat_bool_flag(
     name = "strict_proto_deps",
-    build_setting_default = "error",
+    build_setting_default = True,
+    fragment_field = "strict_proto_deps",
     scope = "universal",
-    values = [
-        "off",
-        "OFF",
-        "warn",
-        "WARN",
-        "error",
-        "ERROR",
-        "strict",
-        "STRICT",
-        "default",
-        "DEFAULT",
-    ],
+    values = {
+        False: [
+            "off",
+            "OFF",
+        ],
+        True: [
+            "warn",
+            "WARN",
+            "error",
+            "ERROR",
+            "strict",
+            "STRICT",
+        ],
+    },
 )
 
 # TODO: deprecate this flag.
-string_flag(
+compat_bool_flag(
     name = "strict_public_imports",
-    build_setting_default = "off",
+    build_setting_default = False,
+    fragment_field = "strict_public_imports",
     scope = "universal",
-    values = [
-        "off",
-        "OFF",
-        "warn",
-        "WARN",
-        "error",
-        "ERROR",
-        "strict",
-        "STRICT",
-        "default",
-        "DEFAULT",
-    ],
+    values = {
+        False: [
+            "off",
+            "OFF",
+        ],
+        True: [
+            "warn",
+            "WARN",
+            "error",
+            "ERROR",
+            "strict",
+            "STRICT",
+        ],
+    },
 )

bazel/flags/cc/BUILD

@@ -1,5 +1,6 @@
-load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_list_flag")
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
+load("//bazel/private:compat_flag.bzl", "compat_string_list_flag")
 
 package(
     default_applicable_licenses = ["//:license"],
@@ -19,14 +20,16 @@ alias(
     deprecation = "Use //bazel/flags:protocopt instead.",
 )
 
-string_list_flag(
+compat_string_list_flag(
     name = "cc_proto_library_header_suffixes",
     build_setting_default = [".pb.h"],
+    fragment_field = "cc_proto_library_header_suffixes",
     scope = "universal",
 )
 
-string_list_flag(
+compat_string_list_flag(
     name = "cc_proto_library_source_suffixes",
     build_setting_default = [".pb.cc"],
+    fragment_field = "cc_proto_library_source_suffixes",
     scope = "universal",
 )

bazel/flags/flags.bzl

@@ -8,75 +8,22 @@ visibility([
     "//third_party/grpc/bazel",
 ])
 
-# Maps flag names to their native reference
-_FLAGS = {
-    "protocopt": struct(
-        native = lambda ctx: getattr(ctx.fragments.proto, "experimental_protoc_opts"),
-        default = [],
-    ),
-    "experimental_proto_descriptor_sets_include_source_info": struct(
-        native = lambda ctx: getattr(ctx.attr, "_experimental_proto_descriptor_sets_include_source_info_native")[BuildSettingInfo].value,
-        default = False,
-    ),
-    "proto_compiler": struct(native = lambda ctx: getattr(ctx.attr, "_proto_compiler_native")[BuildSettingInfo].value, default = "@bazel_tools//tools/proto:protoc"),
-    "strict_proto_deps": struct(
-        native = lambda ctx: getattr(ctx.attr, "_strict_proto_deps_native")[BuildSettingInfo].value,
-        default = "error",
-    ),
-    "strict_public_imports": struct(
-        native = lambda ctx: getattr(ctx.attr, "_strict_public_imports_native")[BuildSettingInfo].value,
-        default = "off",
-    ),
-    "cc_proto_library_header_suffixes": struct(
-        native = lambda ctx: getattr(ctx.fragments.proto, "cc_proto_library_header_suffixes"),
-        default = [".pb.h"],
-    ),
-    "cc_proto_library_source_suffixes": struct(
-        native = lambda ctx: getattr(ctx.fragments.proto, "cc_proto_library_source_suffixes"),
-        default = [".pb.cc"],
-    ),
-    "proto_toolchain_for_java": struct(
-        native = lambda ctx: "//:java_toolchain",
-        default = "//:java_toolchain",
-    ),
-    "proto_toolchain_for_javalite": struct(
-        native = lambda ctx: "//:javalite_toolchain",
-        default = "//:javalite_toolchain",
-    ),
-    "proto_toolchain_for_cc": struct(
-        native = lambda ctx: "//:cc_toolchain",
-        default = "//:cc_toolchain",
-    ),
-}
-
 def get_flag_value(ctx, flag_name):
-    """Returns the value of the given flag in Starlark if it's set, otherwise reads the Java flag value, if the proto fragment exists.
+    """Returns the value of the given flag attribute from rule context.
 
     Args:
         ctx: The rule context.
         flag_name: The name of the flag to get the value for.
 
     Returns:
-        The value of the flag.
+        The value of the flag. If the value is a list, returns a mutable copy.
     """
-
-    # We probably got here from toolchains.find_toolchain. Leave the attribute alone.
-    if flag_name not in _FLAGS:
-        return getattr(ctx.attr, "_" + flag_name)
-
-    starlark_flag = getattr(ctx.attr, "_" + flag_name)
-
-    # Label flags don't have a BuildSettingInfo, just get the value.
-    if "toolchain" in flag_name:
-        starlark_flag_is_set = starlark_flag.label != _FLAGS[flag_name].default
-        starlark_value = starlark_flag
-    else:
-        starlark_flag_is_set = starlark_flag[BuildSettingInfo].value != _FLAGS[flag_name].default
-        starlark_value = starlark_flag[BuildSettingInfo].value
-
-    # Starlark flags take precedence over native flags.
-    # Also of course, use the Starlark value if the proto fragment no longer exists.
-    if starlark_flag_is_set or not hasattr(ctx.fragments, "proto"):
-        return starlark_value
-    else:
-        return _FLAGS[flag_name].native(ctx)
+    attr_val = getattr(ctx.attr, "_" + flag_name)
+    if BuildSettingInfo in attr_val:
+        val = attr_val[BuildSettingInfo].value
+        if type(val) == "list":
+            return list(val)
+        return val
+    if type(attr_val) == "list":
+        return list(attr_val)
+    return attr_val

bazel/private/BUILD

@@ -1,5 +1,4 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
-load(":native_bool_flag.bzl", "native_bool_flag")
 
 package(default_applicable_licenses = ["//:license"])
 
@@ -168,32 +167,9 @@ bzl_library(
     ],
 )
 
-native_bool_flag(
-    name = "experimental_proto_descriptor_sets_include_source_info",
-    flag = "experimental_proto_descriptor_sets_include_source_info",
-    match_value = "true",
-    visibility = ["//bazel:__subpackages__"],
-)
-
-native_bool_flag(
-    name = "strict_proto_deps",
-    flag = "strict_proto_deps",
-    match_value = "off",
-    result = False,
-    visibility = ["//bazel:__subpackages__"],
-)
-
-native_bool_flag(
-    name = "strict_public_imports",
-    flag = "strict_public_imports",
-    match_value = "off",
-    result = False,
-    visibility = ["//bazel:__subpackages__"],
-)
-
 bzl_library(
-    name = "native_bool_flag_bzl",
-    srcs = ["native_bool_flag.bzl"],
+    name = "compat_flag_bzl",
+    srcs = ["compat_flag.bzl"],
     visibility = ["//visibility:private"],
     deps = ["@bazel_skylib//rules:common_settings"],
 )
@@ -203,8 +179,8 @@ filegroup(
     testonly = True,
     srcs = [
         "BUILD",
+        ":compat_flag_bzl",
         ":java_proto_library_bzl",
-        ":native_bool_flag_bzl",
         ":toolchain_helpers_bzl",
         "//bazel:for_bazel_tests",
         "//bazel/private/oss/toolchains:for_bazel_tests",