Allow C++17's nested namespaces

This updates the style guide, adds a clang-tidy rule to help, and
applies the clang-tidy rule to the base/mac files.

Bug: none
Change-Id: Ie0cdf3e08adc6204e6c225d3843d69b1a081cbe9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3359337
Reviewed-by: danakj chromium <danakj@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#955950}

.clang-tidy

@@ -11,6 +11,7 @@
                     google-explicit-constructor,
                     google-readability-casting,
                     modernize-avoid-bind,
+                    modernize-concat-nested-namespaces,
                     modernize-loop-convert,
                     modernize-make-shared,
                     modernize-make-unique,

base/mac/authorization_util.h

@@ -30,8 +30,7 @@
 
 #include "base/base_export.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // Obtains an AuthorizationRef for the rights indicated by |rights|.  If
 // necessary, prompts the user for authentication. If the user is prompted,
@@ -76,7 +75,6 @@ OSStatus ExecuteWithPrivilegesAndWait(AuthorizationRef authorization,
                                       FILE** pipe,
                                       int* exit_status);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_AUTHORIZATION_UTIL_H_

base/mac/authorization_util.mm

@@ -21,8 +21,7 @@
 #include "base/strings/string_util.h"
 #include "base/threading/hang_watcher.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 AuthorizationRef GetAuthorizationRightsWithPrompt(
     AuthorizationRights* rights,
@@ -206,5 +205,4 @@ OSStatus ExecuteWithPrivilegesAndWait(AuthorizationRef authorization,
   return status;
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/backup_util.h

@@ -8,10 +8,10 @@
 #include "base/base_export.h"
 
 namespace base {
-
 class FilePath;
+}
 
-namespace mac {
+namespace base::mac {
 
 // Returns true if the file or directory at `file_path` is excluded from
 // OS-managed backups.
@@ -20,7 +20,6 @@ BASE_EXPORT bool GetBackupExclusion(const FilePath& file_path);
 // Excludes the file or directory given by `file_path` from OS-managed backups.
 BASE_EXPORT bool SetBackupExclusion(const FilePath& file_path);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_BACKUP_UTIL_H_

base/mac/backup_util.mm

@@ -15,8 +15,7 @@
 #include "base/threading/scoped_blocking_call.h"
 #include "build/build_config.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 bool GetBackupExclusion(const FilePath& file_path) {
   base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
@@ -73,5 +72,4 @@ bool SetBackupExclusion(const FilePath& file_path) {
 #endif
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/backup_util_unittest.mm

@@ -17,8 +17,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace {
 
@@ -68,5 +67,4 @@ TEST_F(BackupUtilTest, TestExcludeFileFromBackups_NotByPath) {
 
 }  // namespace
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/bundle_locations.h

@@ -15,10 +15,10 @@ class NSBundle;
 #endif  // __OBJC__
 
 namespace base {
-
 class FilePath;
+}
 
-namespace mac {
+namespace base::mac {
 
 // This file provides several functions to explicitly request the various
 // component bundles of Chrome.  Please use these methods rather than calling
@@ -60,7 +60,6 @@ BASE_EXPORT void SetOverrideFrameworkBundle(NSBundle* bundle);
 BASE_EXPORT void SetOverrideOuterBundlePath(const FilePath& file_path);
 BASE_EXPORT void SetOverrideFrameworkBundlePath(const FilePath& file_path);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_BUNDLE_LOCATIONS_H_

base/mac/bundle_locations.mm

@@ -8,8 +8,7 @@
 #include "base/mac/foundation_util.h"
 #include "base/strings/sys_string_conversions.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // NSBundle isn't threadsafe, all functions in this file must be called on the
 // main thread.
@@ -79,5 +78,4 @@ void SetOverrideFrameworkBundlePath(const FilePath& file_path) {
   AssignOverridePath(file_path, &g_override_framework_bundle);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/call_with_eh_frame.cc

@@ -9,8 +9,7 @@
 
 #include "build/build_config.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 #if defined(__x86_64__) || defined(__aarch64__)
 extern "C" _Unwind_Reason_Code __gxx_personality_v0(int,
@@ -49,5 +48,4 @@ void CallWithEHFrame(void (^block)(void)) {
   block();
 }
 #endif  // defined(__x86_64__) || defined(__aarch64__)
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/call_with_eh_frame.h

@@ -7,8 +7,7 @@
 
 #include "base/base_export.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // Invokes the specified block in a stack frame with a special exception
 // handler. This function creates an exception handling stack frame that
@@ -20,7 +19,6 @@ namespace mac {
 // in such a way that disrupts the generation of useful stack traces.
 void BASE_EXPORT CallWithEHFrame(void (^block)(void));
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_CALL_WITH_EH_FRAME_H_

base/mac/call_with_eh_frame_unittest.mm

@@ -8,8 +8,7 @@
 
 #include "testing/gtest/include/gtest/gtest.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 namespace {
 
 class CallWithEHFrameTest : public testing::Test {
@@ -51,5 +50,4 @@ TEST_F(CallWithEHFrameTest, CatchExceptionLower) {
 }
 
 }  // namespace
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/foundation_util.h

@@ -47,10 +47,10 @@ typedef struct CF_BRIDGED_TYPE(id) __SecKey* SecKeyRef;
 typedef struct CF_BRIDGED_TYPE(id) __SecPolicy* SecPolicyRef;
 
 namespace base {
-
 class FilePath;
+}
 
-namespace mac {
+namespace base::mac {
 
 // Returns true if the application is running from a bundle
 BASE_EXPORT bool AmIBundled();
@@ -151,8 +151,7 @@ BASE_EXPORT const char* BaseBundleID();
 // make its own copy of new_base_bundle_id.
 BASE_EXPORT void SetBaseBundleID(const char* new_base_bundle_id);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #if !defined(__OBJC__)
 #define OBJC_CPP_CLASS_DECL(x) class x;
@@ -225,8 +224,7 @@ CF_TO_NS_CAST_DECL(CTFont, NSFont)
 #undef CF_TO_NS_MUTABLE_CAST_DECL
 #undef OBJC_CPP_CLASS_DECL
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // CFCast<>() and CFCastStrict<>() cast a basic CFTypeRef to a more
 // specific CoreFoundation type. The compatibility of the passed
@@ -373,8 +371,7 @@ BASE_EXPORT bool CFRangeToNSRange(CFRange range,
                                   NSRange* range_out) WARN_UNUSED_RESULT;
 #endif  // defined(__OBJC__)
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 // Stream operations for CFTypes. They can be used with NSTypes as well
 // by using the NSToCFCast methods above.

base/mac/foundation_util.mm

@@ -40,8 +40,7 @@ Boolean _CFIsObjC(CFTypeID typeID, CFTypeRef obj);
 #endif
 }  // extern "C"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace {
 
@@ -499,8 +498,7 @@ bool CFRangeToNSRange(CFRange range, NSRange* range_out) {
   return false;
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 std::ostream& operator<<(std::ostream& o, const CFStringRef string) {
   return o << base::SysCFStringRefToUTF8(string);

base/mac/foundation_util_unittest.mm

@@ -17,8 +17,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 #import "testing/gtest_mac.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 TEST(FoundationUtilTest, CFCast) {
   // Build out the CF types to be tested as empty containers.
@@ -422,5 +421,4 @@ TEST(FoundationLoggingTest, NSRange) {
   EXPECT_LOG_EQ("{0, 100}", NSMakeRange(0, 100));
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/launch_services_util.h

@@ -11,8 +11,7 @@
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // Launches the application bundle at |bundle_path|, passing argv[1..] from
 // |command_line| as command line arguments if the app isn't already running.
@@ -24,7 +23,6 @@ BASE_EXPORT NSRunningApplication* OpenApplicationWithPath(
     const CommandLine& command_line,
     NSWorkspaceLaunchOptions launch_options);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_LAUNCH_SERVICES_UTIL_H_

base/mac/launch_services_util.mm

@@ -7,8 +7,7 @@
 #include "base/logging.h"
 #include "base/strings/sys_string_conversions.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 NSRunningApplication* OpenApplicationWithPath(
     const base::FilePath& bundle_path,
@@ -49,5 +48,4 @@ NSRunningApplication* OpenApplicationWithPath(
   return app;
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/launchd.cc

@@ -12,8 +12,7 @@
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // MessageForJob sends a single message to launchd with a simple dictionary
 // mapping |operation| to |job_label|, and returns the result of calling
@@ -76,7 +75,6 @@ pid_t PIDForJob(const std::string& job_label) {
   return launch_data_get_integer(pid_data);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #pragma clang diagnostic pop

base/mac/launchd.h

@@ -12,8 +12,7 @@
 
 #include "base/base_export.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // MessageForJob sends a single message to launchd with a simple dictionary
 // mapping |operation| to |job_label|, and returns the result of calling
@@ -28,7 +27,6 @@ launch_data_t MessageForJob(const std::string& job_label,
 BASE_EXPORT
 pid_t PIDForJob(const std::string& job_label);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_LAUNCHD_H_

base/mac/mac_util.h

@@ -14,10 +14,10 @@
 #include "base/base_export.h"
 
 namespace base {
-
 class FilePath;
+}
 
-namespace mac {
+namespace base::mac {
 
 // Returns an sRGB color space.  The return value is a static value; do not
 // release it!
@@ -214,7 +214,6 @@ BASE_EXPORT std::string GetOSDisplayName();
 // Returns the serial number of the macOS device.
 BASE_EXPORT std::string GetPlatformSerialNumber();
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_MAC_UTIL_H_

base/mac/mac_util.mm

@@ -30,8 +30,7 @@
 #include "base/threading/scoped_blocking_call.h"
 #include "build/build_config.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace {
 
@@ -487,5 +486,4 @@ std::string GetPlatformSerialNumber() {
   return base::SysCFStringRefToUTF8(serial_number_cfstring);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/mac_util_unittest.mm

@@ -21,8 +21,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace {
 
@@ -313,5 +312,4 @@ TEST_F(MacUtilTest, TestRemoveQuarantineAttributeNonExistentPath) {
 
 }  // namespace
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/objc_release_properties.h

@@ -45,8 +45,7 @@
 // properties. Distant subclasses might not expect it and over-release their
 // properties, so don't do that.
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace details {
 
@@ -59,7 +58,6 @@ void ReleaseProperties(Self* self) {
   details::ReleaseProperties(self, [Self class]);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_OBJC_RELEASE_PROPERTIES_H_

base/mac/objc_release_properties.mm

@@ -45,9 +45,7 @@ id ValueOf(id obj, objc_property_t property) {
 
 }  // namespace
 
-namespace base {
-namespace mac {
-namespace details {
+namespace base::mac::details {
 
 void ReleaseProperties(id self, Class cls) {
   unsigned int property_count;
@@ -61,6 +59,4 @@ void ReleaseProperties(id self, Class cls) {
   }
 }
 
-}  // namespace details
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac::details

base/mac/os_crash_dumps.cc

@@ -11,8 +11,7 @@
 #include "base/cxx17_backports.h"
 #include "base/logging.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace {
 
@@ -57,5 +56,4 @@ void DisableOSCrashDumps() {
   }
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/os_crash_dumps.h

@@ -7,8 +7,7 @@
 
 #include "base/base_export.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // On Mac OS X, it can take a really long time for the OS crash handler to
 // process a Chrome crash when debugging symbols are available.  This
@@ -16,7 +15,6 @@ namespace mac {
 // disables Apple Crash Reporter entirely.
 BASE_EXPORT void DisableOSCrashDumps();
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_OS_CRASH_DUMPS_H_

base/mac/scoped_aedesc.h

@@ -7,8 +7,7 @@
 
 #import <CoreServices/CoreServices.h>
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // The ScopedAEDesc is used to scope AppleEvent descriptors.  On creation,
 // it will store a NULL descriptor.  On destruction, it will dispose of the
@@ -45,7 +44,6 @@ class ScopedAEDesc {
   AEDescType desc_;
 };
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_AEDESC_H_

base/mac/scoped_authorizationref.h

@@ -13,8 +13,7 @@
 // ScopedAuthorizationRef maintains ownership of an AuthorizationRef.  It is
 // patterned after the unique_ptr interface.
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 class BASE_EXPORT ScopedAuthorizationRef {
  public:
@@ -84,7 +83,6 @@ class BASE_EXPORT ScopedAuthorizationRef {
   AuthorizationRef authorization_;
 };
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_AUTHORIZATIONREF_H_

base/mac/scoped_authorizationref.mm

@@ -4,12 +4,10 @@
 
 #include "base/mac/scoped_authorizationref.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 void ScopedAuthorizationRef::FreeInternal() {
   AuthorizationFree(authorization_, kAuthorizationFlagDestroyRights);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_block.h

@@ -13,8 +13,7 @@
 #error "Cannot include base/mac/scoped_block.h in file built with ARC."
 #endif
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -32,7 +31,6 @@ struct ScopedBlockTraits {
 template <typename B>
 using ScopedBlock = ScopedTypeRef<B, internal::ScopedBlockTraits<B>>;
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_BLOCK_H_

base/mac/scoped_cffiledescriptorref.h

@@ -9,8 +9,7 @@
 
 #include "base/scoped_generic.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -33,7 +32,6 @@ using ScopedCFFileDescriptorRef =
     ScopedGeneric<CFFileDescriptorRef,
                   internal::ScopedCFFileDescriptorRefTraits>;
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_CFFILEDESCRIPTORREF_H_

base/mac/scoped_ionotificationportref.h

@@ -9,8 +9,7 @@
 
 #include "base/scoped_generic.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -27,7 +26,6 @@ using ScopedIONotificationPortRef =
     ScopedGeneric<IONotificationPortRef,
                   internal::ScopedIONotificationPortRefTraits>;
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_IONOTIFICATIONPORTREF_H_

base/mac/scoped_ioobject.h

@@ -9,8 +9,7 @@
 
 #include "base/mac/scoped_typeref.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -30,7 +29,6 @@ struct ScopedIOObjectTraits {
 template <typename IOT>
 using ScopedIOObject = ScopedTypeRef<IOT, internal::ScopedIOObjectTraits<IOT>>;
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_IOOBJECT_H_

base/mac/scoped_ioplugininterface.h

@@ -9,8 +9,7 @@
 
 #include "base/mac/scoped_typeref.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -32,7 +31,6 @@ template <typename T>
 using ScopedIOPluginInterface =
     ScopedTypeRef<T**, internal::ScopedIOPluginInterfaceTraits<T**>>;
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_IOPLUGININTERFACE_H_

base/mac/scoped_launch_data.h

@@ -14,8 +14,7 @@
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -30,8 +29,7 @@ struct ScopedLaunchDataTraits {
 using ScopedLaunchData =
     ScopedGeneric<launch_data_t, internal::ScopedLaunchDataTraits>;
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #pragma clang diagnostic pop  // -Wdeprecated-declarations
 

base/mac/scoped_mach_port.cc

@@ -6,8 +6,7 @@
 
 #include "base/mac/mach_logging.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 namespace internal {
 
 // static
@@ -72,5 +71,4 @@ ScopedMachSendRight RetainMachSendRight(mach_port_t port) {
   return {};
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_mach_port.h

@@ -11,8 +11,7 @@
 #include "base/scoped_generic.h"
 #include "third_party/abseil-cpp/absl/types/optional.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 namespace internal {
 
@@ -75,7 +74,6 @@ BASE_EXPORT bool CreateMachPort(
 // a new scoper to manage the additional right.
 BASE_EXPORT ScopedMachSendRight RetainMachSendRight(mach_port_t port);
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_MACH_PORT_H_

base/mac/scoped_mach_vm.cc

@@ -6,8 +6,7 @@
 
 #include "base/mac/mach_logging.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 void ScopedMachVM::reset(vm_address_t address, vm_size_t size) {
   DCHECK_EQ(address % PAGE_SIZE, 0u);
@@ -34,5 +33,4 @@ void ScopedMachVM::reset_unaligned(vm_address_t address, vm_size_t size) {
   size_ = size;
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_mach_vm.h

@@ -43,8 +43,7 @@
 //   }
 //   ScopedMachVM vm_owner(address, mach_vm_round_page(size));
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 class BASE_EXPORT ScopedMachVM {
  public:
@@ -96,7 +95,6 @@ class BASE_EXPORT ScopedMachVM {
   vm_size_t size_;
 };
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_MACH_VM_H_

base/mac/scoped_mach_vm_unittest.cc

@@ -17,8 +17,7 @@
 // allocation will report being part of the previously-deallocated large region.
 // That will cause the GetRegionInfo() expectations to fail.
 
-namespace base {
-namespace mac {
+namespace base::mac {
 namespace {
 
 void GetRegionInfo(vm_address_t* region_address, vm_size_t* region_size) {
@@ -230,5 +229,4 @@ TEST(ScopedMachVMTest, ResetMustBeAligned) {
 #endif  // DCHECK_IS_ON()
 
 }  // namespace
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_nsautorelease_pool.h

@@ -13,8 +13,7 @@
 class NSAutoreleasePool;
 #endif  // __OBJC__
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // ScopedNSAutoreleasePool allocates an NSAutoreleasePool when instantiated and
 // sends it a -drain message when destroyed.  This allows an autorelease pool to
@@ -41,7 +40,6 @@ class BASE_EXPORT ScopedNSAutoreleasePool {
   NSAutoreleasePool* autorelease_pool_;
 };
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_NSAUTORELEASE_POOL_H_

base/mac/scoped_nsautorelease_pool.mm

@@ -8,8 +8,7 @@
 
 #include "base/check.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 ScopedNSAutoreleasePool::ScopedNSAutoreleasePool()
     : autorelease_pool_([[NSAutoreleasePool alloc] init]) {
@@ -28,5 +27,4 @@ void ScopedNSAutoreleasePool::Recycle() {
   DCHECK(autorelease_pool_);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_objc_class_swizzler.h

@@ -9,8 +9,7 @@
 
 #include "base/base_export.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 // Within a given scope, swaps method implementations of a class interface, or
 // between two class interfaces. The argument and return types must match.
@@ -53,7 +52,6 @@ class BASE_EXPORT ScopedObjCClassSwizzler {
   Method new_selector_impl_;
 };
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_OBJC_CLASS_SWIZZLER_H_

base/mac/scoped_objc_class_swizzler.mm

@@ -8,8 +8,7 @@
 
 #include "base/check_op.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 ScopedObjCClassSwizzler::ScopedObjCClassSwizzler(Class target,
                                                  Class source,
@@ -67,5 +66,4 @@ void ScopedObjCClassSwizzler::Init(Class target,
   method_exchangeImplementations(old_selector_impl_, new_selector_impl_);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_objc_class_swizzler_unittest.mm

@@ -73,8 +73,7 @@
 }
 @end
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 TEST(ObjCClassSwizzlerTest, SwizzleInstanceMethods) {
   base::scoped_nsobject<ObjCClassSwizzlerTestOne> object_one(
@@ -155,5 +154,4 @@ TEST(ObjCClassSwizzlerTest, SwizzleViaInheritance) {
   EXPECT_EQ(3, [child method]);
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/mac/scoped_sending_event.h

@@ -24,8 +24,7 @@
 - (void)setHandlingSendEvent:(BOOL)handlingSendEvent;
 @end
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 class BASE_EXPORT ScopedSendingEvent {
  public:
@@ -43,7 +42,6 @@ class BASE_EXPORT ScopedSendingEvent {
   BOOL handling_;  // Value of -[app_ handlingSendEvent] at construction.
 };
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac
 
 #endif  // BASE_MAC_SCOPED_SENDING_EVENT_H_

base/mac/scoped_sending_event.mm

@@ -6,8 +6,7 @@
 
 #include "base/check.h"
 
-namespace base {
-namespace mac {
+namespace base::mac {
 
 ScopedSendingEvent::ScopedSendingEvent()
     : app_(static_cast<NSObject<CrAppControlProtocol>*>(NSApp)) {
@@ -20,5 +19,4 @@ ScopedSendingEvent::~ScopedSendingEvent() {
   [app_ setHandlingSendEvent:handling_];
 }
 
-}  // namespace mac
-}  // namespace base
+}  // namespace base::mac

base/win/.clang-tidy

@@ -4,6 +4,7 @@ Checks: '-*,
          google-build-namespaces,
          google-readability-namespace-comments,
          modernize-avoid-bind,
+         modernize-concat-nested-namespaces,
          modernize-make-shared,
          modernize-make-unique,
          modernize-redundant-void-arg,

styleguide/c++/c++11.md

@@ -382,6 +382,23 @@ Banned because `<chrono>` is banned.
 
 The following C++17 language features are allowed in the Chromium codebase.
 
+### Nested namespaces <sup>[allowed]</sup>
+
+```c++
+namespace A::B::C { ...
+```
+
+**Description:** Using the namespace resolution operator to create nested
+namespace definitions.
+
+**Documentation:**
+[Namespaces](https://en.cppreference.com/w/cpp/language/namespace)
+
+**Notes:**
+*** promo
+[Discussion thread](https://groups.google.com/a/chromium.org/g/cxx/c/gLdR3apDSmg/)
+***
+
 ### Template argument deduction for class templates <sup>[allowed]</sup>
 
 ```c++
@@ -685,23 +702,6 @@ variable, such that it does not need to be initialized in the source file.
 None
 ***
 
-### Nested namespaces <sup>[tbd]</sup>
-
-```c++
-namespace A::B::C { ...
-```
-
-**Description:** Using the namespace resolution operator to create nested
-namespace definitions.
-
-**Documentation:**
-[Namespaces](https://en.cppreference.com/w/cpp/language/namespace)
-
-**Notes:**
-*** promo
-None
-***
-
 ### Structured bindings <sup>[tbd]</sup>
 
 ```c++

ui/views/.clang-tidy

@@ -1,6 +1,7 @@
 ---
   Checks:          'google-build-namespaces,
                     google-readability-namespace-comments,
+                    modernize-concat-nested-namespaces,
                     modernize-replace-auto-ptr,
                     modernize-use-using'
   HeaderFilterRegex: 'ui/views/*'