Add ABSL_REQUIRE_EXPLICIT_INIT to Abseil to enable enforcing explicit…

… field initializations

Also include portable fallbacks for other compilers. They don't produce good error messages, but they do prevent silent breakages on other platforms/toolchains.

PiperOrigin-RevId: 723629074
Change-Id: I29ee8fbc679b70bb67d42577d0723b52268f7caf

absl/base/BUILD.bazel

@@ -340,6 +340,21 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "attributes_test",
+    srcs = [
+        "attributes_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":core_headers",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "c_header_test",
     srcs = ["c_header_test.c"],

absl/base/CMakeLists.txt

@@ -371,6 +371,19 @@ absl_cc_test(
     GTest::gtest_main
 )
 
+absl_cc_test(
+  NAME
+    attributes_test
+  SRCS
+    "attributes_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    GTest::gtest_main
+)
+
 absl_cc_test(
   NAME
     bit_cast_test

absl/base/attributes.h

@@ -759,6 +759,71 @@
 #define ABSL_CONST_INIT
 #endif
 
+// ABSL_REQUIRE_EXPLICIT_INIT
+//
+// ABSL_REQUIRE_EXPLICIT_INIT is placed *after* the data members of an aggregate
+// type to indicate that the annotated member must be explicitly initialized by
+// the user whenever the aggregate is constructed. For example:
+//
+//   struct Coord {
+//     int x ABSL_REQUIRE_EXPLICIT_INIT;
+//     int y ABSL_REQUIRE_EXPLICIT_INIT;
+//   };
+//   Coord coord = {1};  // warning: field 'y' is not explicitly initialized
+//
+// Note that usage on C arrays is not supported in C++.
+// Use a struct (such as std::array) to wrap the array member instead.
+//
+// Avoid applying this attribute to the members of non-aggregate types.
+// The behavior within non-aggregates is unspecified and subject to change.
+//
+// Do NOT attempt to suppress or demote the error generated by this attribute.
+// Just like with a missing function argument, it is a hard error by design.
+//
+// See the upstream documentation for more details:
+// https://clang.llvm.org/docs/AttributeReference.html#require-explicit-initialization
+#ifdef __cplusplus
+#if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_explicit_initialization)
+// clang-format off
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  [[clang::require_explicit_initialization]] = \
+    AbslInternal_YouForgotToExplicitlyInitializeAField::v
+#else
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  = AbslInternal_YouForgotToExplicitlyInitializeAField::v
+#endif
+// clang-format on
+#else
+// clang-format off
+#if ABSL_HAVE_ATTRIBUTE(require_explicit_initialization)
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  __attribute__((require_explicit_initialization))
+#else
+#define ABSL_REQUIRE_EXPLICIT_INIT \
+  /* No portable fallback for C is available */
+#endif
+// clang-format on
+#endif
+
+#ifdef __cplusplus
+struct AbslInternal_YouForgotToExplicitlyInitializeAField {
+  // A portable version of [[clang::require_explicit_initialization]] that
+  // never builds, as a last resort for all toolchains.
+  // The error messages are poor, so we don't rely on this unless we have to.
+  template <class T>
+#if !defined(SWIG)
+  constexpr
+#endif
+  operator T() const /* NOLINT */ {
+    // Infinite loop to prevent constexpr compilation
+    for (;;) {
+    }
+  }
+  // This is deliberately left undefined to prevent linking
+  static AbslInternal_YouForgotToExplicitlyInitializeAField v;
+};
+#endif
+
 // ABSL_ATTRIBUTE_PURE_FUNCTION
 //
 // ABSL_ATTRIBUTE_PURE_FUNCTION is used to annotate declarations of "pure"

absl/base/attributes_test.cc

@@ -0,0 +1,43 @@
+// Copyright 2025 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/attributes.h"
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace {
+
+TEST(Attributes, RequireExplicitInit) {
+  struct Agg {
+    int f1;
+    int f2 ABSL_REQUIRE_EXPLICIT_INIT;
+  };
+  Agg good1 ABSL_ATTRIBUTE_UNUSED = {1, 2};
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  Agg good2 ABSL_ATTRIBUTE_UNUSED(1, 2);
+#endif
+  Agg good3 ABSL_ATTRIBUTE_UNUSED{1, 2};
+  Agg good4 ABSL_ATTRIBUTE_UNUSED = {1, 2};
+  Agg good5 ABSL_ATTRIBUTE_UNUSED = Agg{1, 2};
+  Agg good6[1] ABSL_ATTRIBUTE_UNUSED = {{1, 2}};
+  Agg good7[1] ABSL_ATTRIBUTE_UNUSED = {Agg{1, 2}};
+  union {
+    Agg agg;
+  } good8 ABSL_ATTRIBUTE_UNUSED = {{1, 2}};
+  constexpr Agg good9 ABSL_ATTRIBUTE_UNUSED = {1, 2};
+  constexpr Agg good10 ABSL_ATTRIBUTE_UNUSED{1, 2};
+}
+
+}  // namespace