Replace PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION macro with constexpr function for easier maintenance.

No semantic change expected.

PiperOrigin-RevId: 675981595

src/google/protobuf/compiler/cpp/message.cc

@@ -3276,10 +3276,9 @@ void MessageGenerator::GenerateArenaEnabledCopyConstructor(io::Printer* p) {
   auto force_allocation = [&] {
     if (ShouldForceAllocationOnConstruction(descriptor_, options_)) {
       p->Emit(R"cc(
-        //~ force alignment
-#ifdef PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION
-        $mutable_unknown_fields$;
-#endif  // PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION
+        if ($pbi$::DebugHardenForceAllocationOnConstruction()) {
+          $mutable_unknown_fields$;
+        }
       )cc");
     }
   };

src/google/protobuf/port.h

@@ -281,6 +281,10 @@ constexpr bool DebugHardenForceCopyInMove() {
   return false;
 }
 
+constexpr bool DebugHardenForceAllocationOnConstruction() {
+  return false;
+}
+
 constexpr bool DebugHardenFuzzMessageSpaceUsedLong() {
   return false;
 }

src/google/protobuf/port_def.inc

@@ -398,10 +398,6 @@ static_assert(PROTOBUF_ABSL_MIN(20230125, 3),
 #define PROTOBUF_RESTRICT
 #endif
 
-#ifdef PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION
-#error PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION was previously defined
-#endif
-
 // Specify memory alignment for structs, classes, etc.
 // Use like:
 //   class PROTOBUF_ALIGNAS(16) MyClass { ... }

src/google/protobuf/port_undef.inc

@@ -41,7 +41,6 @@
 #undef PROTOC_EXPORT
 #undef PROTOBUF_NODISCARD
 #undef PROTOBUF_RESTRICT
-#undef PROTOBUF_FORCE_ALLOCATION_ON_CONSTRUCTION
 #undef PROTOBUF_UNUSED
 #undef PROTOBUF_ASSUME
 #undef PROTOBUF_DEPRECATE_AND_INLINE