Provide file-scoped pragma behind macro ABSL_POINTERS_DEFAULT_NONNULL to indicate the default nullability. This is a no-op for now (not understood by checkers), but does communicate intention to human readers. PiperOrigin-RevId: 668448175 Change-Id: I19802d19be9a310aa5f0dacf8b0567dd768fb830

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index fa3719d..06cd361 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel

@@ -649,6 +649,16 @@
 )
 
 cc_test(
+    name = "nullability_default_nonnull_test",
+    srcs = ["nullability_default_nonnull_test.cc"],
+    deps = [
+        ":nullability",
+        "@com_google_googletest//:gtest",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
     name = "raw_logging_test",
     srcs = ["raw_logging_test.cc"],
     copts = ABSL_TEST_COPTS,

diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index eed0afc..feb3e01 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt

@@ -95,6 +95,18 @@
     GTest::gtest_main
 )
 
+absl_cc_test(
+  NAME
+    nullability_default_nonnull_test
+  SRCS
+    "nullability_default_nonnull_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::nullability
+    GTest::gtest_main
+)
+
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME

diff --git a/absl/base/nullability.h b/absl/base/nullability.h
index 34dc083..6322c1d 100644
--- a/absl/base/nullability.h
+++ b/absl/base/nullability.h

@@ -161,10 +161,47 @@
 #include "absl/base/config.h"
 #include "absl/base/internal/nullability_impl.h"
 
+// ABSL_POINTERS_DEFAULT_NONNULL
+//
+// This macro specifies that all unannotated pointer types within the given
+// file are designated as nonnull (instead of the default "unknown"). This macro
+// exists as a standalone statement and applies default nonnull behavior to all
+// subsequent pointers; as a result, place this macro as the first non-comment,
+// non-`#include` line in a file.
+//
+// Example:
+//
+//     #include "absl/base/nullability.h"
+//
+//     ABSL_POINTERS_DEFAULT_NONNULL
+//
+//     void FillMessage(Message *m);                  // implicitly non-null
+//     absl::Nullable<T*> GetNullablePtr();           // explicitly nullable
+//     absl::NullabilityUnknown<T*> GetUnknownPtr();  // explicitly unknown
+//
+// The macro can be safely used in header files -- it will not affect any files
+// that include it.
+//
+// In files with the macro, plain `T*` syntax means `absl::Nonnull<T*>`, and the
+// exceptions (`Nullable` and `NullabilityUnknown`) must be marked
+// explicitly. The same holds, correspondingly, for smart pointer types.
+//
+// For comparison, without the macro, all unannotated pointers would default to
+// unknown, and otherwise require explicit annotations to change this behavior:
+//
+//     #include "absl/base/nullability.h"
+//
+//     void FillMessage(absl::Nonnull<Message*> m);  // explicitly non-null
+//     absl::Nullable<T*> GetNullablePtr();          // explicitly nullable
+//     T* GetUnknownPtr();                           // implicitly unknown
+//
+// No-op except for being a human readable signal.
+#define ABSL_POINTERS_DEFAULT_NONNULL
+
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-// absl::Nonnull
+// absl::Nonnull (default with `ABSL_POINTERS_DEFAULT_NONNULL`)
 //
 // The indicated pointer is never null. It is the responsibility of the provider
 // of this pointer across an API boundary to ensure that the pointer is never
@@ -197,7 +234,7 @@
 template <typename T>
 using Nullable = nullability_internal::NullableImpl<T>;
 
-// absl::NullabilityUnknown (default)
+// absl::NullabilityUnknown (default without `ABSL_POINTERS_DEFAULT_NONNULL`)
 //
 // The indicated pointer has not yet been determined to be definitively
 // "non-null" or "nullable." Providers of such pointers across API boundaries
@@ -208,9 +245,10 @@
 // migrated into one of the above two nullability states: `Nonnull<T>` or
 //  `Nullable<T>`.
 //
-// NOTE: Because this annotation is the global default state, unannotated
-// pointers are assumed to have "unknown" semantics. This assumption is designed
-// to minimize churn and reduce clutter within the codebase.
+// NOTE: For files that do not specify `ABSL_POINTERS_DEFAULT_NONNULL`,
+// because this annotation is the global default state, unannotated pointers are
+// are assumed to have "unknown" semantics. This assumption is designed to
+// minimize churn and reduce clutter within the codebase.
 //
 // Example:
 //

diff --git a/absl/base/nullability_default_nonnull_test.cc b/absl/base/nullability_default_nonnull_test.cc
new file mode 100644
index 0000000..bd5b483
--- /dev/null
+++ b/absl/base/nullability_default_nonnull_test.cc

@@ -0,0 +1,44 @@
+// Copyright 2024 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 <cassert>
+
+#include "gtest/gtest.h"
+#include "absl/base/nullability.h"
+
+ABSL_POINTERS_DEFAULT_NONNULL
+
+namespace {
+
+void FuncWithDefaultNonnullArg(int* /*arg*/) {}
+template <typename T>
+void FuncWithDeducedDefaultNonnullArg(T* /*arg*/) {}
+
+TEST(DefaultNonnullTest, NonnullArgument) {
+  int var = 0;
+  FuncWithDefaultNonnullArg(&var);
+  FuncWithDeducedDefaultNonnullArg<int>(&var);
+}
+
+int* FuncWithDefaultNonnullReturn() {
+  static int var = 0;
+  return &var;
+}
+
+TEST(DefaultNonnullTest, NonnullReturn) {
+  auto var = FuncWithDefaultNonnullReturn();
+  (void)var;
+}
+
+}  // namespace