Add an `Unreachable()` function to replace `PROTOBUF_ASSUME(false);` uses.

It shows intent more clearly.

PiperOrigin-RevId: 582822804

src/google/protobuf/BUILD.bazel

@@ -239,6 +239,7 @@ cc_test(
     deps = [
         ":port_def",
         ":varint_shuffle",
+        "@com_google_absl//absl/base:config",
         "@com_google_absl//absl/log:absl_check",
         "@com_google_googletest//:gtest",
         "@com_google_googletest//:gtest_main",
@@ -439,6 +440,7 @@ cc_library(
         "//src/google/protobuf/stubs:lite",
         "//third_party/utf8_range:utf8_validity",
         "@com_google_absl//absl/base",
+        "@com_google_absl//absl/base:config",
         "@com_google_absl//absl/container:btree",
         "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/hash",

src/google/protobuf/port.h

@@ -22,6 +22,7 @@
 #include <typeinfo>
 
 
+#include "absl/base/config.h"
 #include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
@@ -201,6 +202,18 @@ inline constexpr bool DebugHardenStringValues() {
 #endif
 }
 
+#if ABSL_HAVE_BUILTIN(__builtin_FILE) && ABSL_HAVE_BUILTIN(__builtin_LINE)
+[[noreturn]] inline void Unreachable(const char* file = __builtin_FILE(),
+                                     int line = __builtin_LINE()) {
+#else
+[[noreturn]] inline void Unreachable(const char* file = "", int line = 0) {
+#endif
+#if defined(NDEBUG) && ABSL_HAVE_BUILTIN(__builtin_unreachable)
+  __builtin_unreachable();
+#endif
+  protobuf_assumption_failed("Unreachable", file, line);
+}
+
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google

src/google/protobuf/port_def.inc

@@ -1030,8 +1030,9 @@ static_assert(PROTOBUF_ABSL_MIN(20230125, 3),
 namespace google {
 namespace protobuf {
 namespace internal {
-PROTOBUF_EXPORT void protobuf_assumption_failed(const char *pred,
-                                                const char *file, int line);
+[[noreturn]] PROTOBUF_EXPORT void protobuf_assumption_failed(const char *pred,
+                                                             const char *file,
+                                                             int line);
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google

src/google/protobuf/port_test.cc

@@ -5,10 +5,13 @@
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 //
+#include "google/protobuf/port.h"
+
 #include <stdio.h>
 #include <stdlib.h>
 
 #include <gtest/gtest.h>
+#include "absl/base/config.h"
 
 // Must be included last
 #include "google/protobuf/port_def.inc"
@@ -28,6 +31,16 @@ TEST(PortTest, ProtobufAssume) {
 #endif
 }
 
+TEST(PortTest, UnreachableTrapsOnDebugMode) {
+#ifdef GTEST_HAS_DEATH_TEST
+  EXPECT_DEBUG_DEATH(Unreachable(), "Assumption failed: 'Unreachable'");
+#if ABSL_HAVE_BUILTIN(__builtin_FILE)
+  EXPECT_DEBUG_DEATH(Unreachable(),
+                     "port_test\\.cc:.*Assumption failed: 'Unreachable'");
+#endif
+#endif
+}
+
 }  // namespace internal
 }  // namespace protobuf
 }  // namespace google