Add C++ toolchain selection enforcement infrastructure with user-friendly error messages

.bazelrc

@@ -56,6 +56,11 @@ build --action_env=BAZEL_VOLATILE_DIRTY --host_action_env=BAZEL_VOLATILE_DIRTY
 
 build --test_summary=terse
 
+# Enforce explicit toolchain selection
+# This flag causes the enforcement rule to run on every build, checking that
+# a toolchain has been explicitly selected via --config=gcc or --config=clang
+build --//tools/build_config:enforce_toolchain
+
 # TODO(keith): Remove once these 2 are the default
 build --incompatible_config_setting_private_default_visibility
 build --incompatible_enforce_config_setting_visibility
@@ -122,12 +127,14 @@ common:clang-common --action_env=CXX=clang++ --host_action_env=CXX=clang++
 common:clang --config=clang-common
 common:clang --config=libc++
 common:clang --action_env=LDFLAGS="-fuse-ld=lld"
+common:clang --//tools/build_config:toolchain_identifier=clang
 
 # Use gold linker for gcc compiler.
 build:gcc --config=libstdc++
 build:gcc --test_env=HEAPCHECK=
 build:gcc --action_env=BAZEL_COMPILER=gcc
 build:gcc --action_env=CC=gcc --action_env=CXX=g++
+build:gcc --//tools/build_config:toolchain_identifier=gcc
 # This is to work around a bug in GCC that makes debug-types-section
 # option not play well with fission:
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110885

tools/build_config/BUILD

@@ -0,0 +1,16 @@
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+load(":defs.bzl", "toolchain_enforcement")
+
+licenses(["notice"])  # Apache 2
+
+string_flag(
+    name = "toolchain_identifier",
+    build_setting_default = "",
+    visibility = ["//visibility:public"],
+)
+
+toolchain_enforcement(
+    name = "enforce_toolchain",
+    toolchain_identifier = ":toolchain_identifier",
+    visibility = ["//visibility:public"],
+)

tools/build_config/defs.bzl

@@ -0,0 +1,57 @@
+load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
+
+ERR_MSG_TOOLCHAIN = """
+╔═══════════════════════════════════════════════════════════════════════════════╗
+║                   C++ Toolchain Selection Required                            ║
+╚═══════════════════════════════════════════════════════════════════════════════╝
+
+ERROR: No C++ toolchain has been selected for this build.
+
+Automatic C++ toolchain detection is disabled in this repository.
+You must explicitly specify a toolchain using one of the following options:
+
+  • For GCC:   bazel build --config=gcc <target>
+  • For Clang: bazel build --config=clang <target>
+
+You can also set a default in your user.bazelrc file:
+  build --config=gcc
+
+For more information, see the Envoy developer documentation.
+"""
+
+DOC_TOOLCHAIN = """
+Rule that enforces explicit C++ toolchain selection.
+
+This rule reads the toolchain_identifier build setting and fails the build
+if no toolchain has been explicitly selected (i.e., the value is empty).
+
+This rule must be invoked as part of the build process (via
+--//tools/build_config:enforce_toolchain in .bazelrc) to provide enforcement.
+It does not produce any build artifacts.
+"""
+
+def _toolchain_enforcement_impl(ctx):
+    """Implementation of the toolchain enforcement rule.
+
+    This rule checks if a C++ toolchain has been explicitly selected.
+    If not, it fails with a helpful error message.
+    """
+    toolchain_id = ctx.attr.toolchain_identifier[BuildSettingInfo].value
+
+    if toolchain_id == "":
+        fail(ERR_MSG_TOOLCHAIN)
+
+    # Return an empty default info provider - this rule doesn't produce any outputs
+    return [DefaultInfo()]
+
+toolchain_enforcement = rule(
+    implementation = _toolchain_enforcement_impl,
+    attrs = {
+        "toolchain_identifier": attr.label(
+            providers = [BuildSettingInfo],
+            mandatory = True,
+            doc = "The toolchain_identifier build setting to check",
+        ),
+    },
+    doc = DOC_TOOLCHAIN,
+)