While the build system could be improved to actually support those
cases, because of all the legacy things involved, it would be a lot more
work for a low reward.

Instead, we forbid them, which allows to remove the non-unified builds
exception in xpcom/rust/gtest.

objdir/dist/include/mozilla/DeferredFinalize.h:30:1: error: unknown type name 'MOZ_NEVER_INLINE'
objdir/dist/include/mozilla/dom/BindingUtils.h:2776:5: error: no matching function for call to 'DeferredFinalize'
xpcom/base/AvailableMemoryTracker.cpp:55,40:: error: expected class name
xpcom/base/AvailableMemoryTracker.cpp:59,3:: error: 'AddRef' marked 'override' but does not override any member functions
xpcom/base/AvailableMemoryTracker.cpp:59,3:: error: 'QueryInterface' marked 'override' but does not override any member functions
xpcom/base/AvailableMemoryTracker.cpp:59,3:: error: 'Release' marked 'override' but does not override any member functions
xpcom/base/AvailableMemoryTracker.cpp:61,29:: error: unknown type name 'nsIHandleReportCallback'
xpcom/base/AvailableMemoryTracker.cpp:65,37:: error: use of undeclared identifier 'KIND_OTHER'
xpcom/base/AvailableMemoryTracker.cpp:65,49:: error: use of undeclared identifier 'UNITS_COUNT_CUMULATIVE'
xpcom/base/AvailableMemoryTracker.cpp:77,1:: error: invalid application of 'sizeof' to an incomplete type 'const QITableEntry []'
xpcom/base/AvailableMemoryTracker.cpp:77,1:: error: static_cast from '(anonymous namespace)::LowEventsReporter *' to 'nsISupports *', which are not related by inheritance, is not allowed
xpcom/base/AvailableMemoryTracker.cpp:77,38:: error: unknown type name 'nsIMemoryReporter'
xpcom/base/AvailableMemoryTracker.cpp:77,38:: error: use of undeclared identifier 'nsIMemoryReporter'
xpcom/base/AvailableMemoryTracker.cpp:187,3:: error: use of undeclared identifier 'RegisterLowMemoryEventsPhysicalDistinguishedAmount'
xpcom/base/AvailableMemoryWatcherWin.cpp:31,47:: error: base class has incomplete type
xpcom/base/AvailableMemoryWatcherWin.cpp:37,3:: error: unknown type name 'NS_DECL_NSIOBSERVER'
xpcom/base/AvailableMemoryWatcherWin.cpp:38,3:: error: 'virtual' can only appear on non-static member functions
xpcom/base/AvailableMemoryWatcherWin.cpp:38,3:: error: expected ';' at end of declaration list
xpcom/base/AvailableMemoryWatcherWin.cpp:38,3:: error: static_assert failed due to requirement 'std::is_convertible<mozilla::nsAvailableMemoryWatcher *, nsINamed *>::value' "nsITimerCallback implementations must also implement nsINamed"
xpcom/base/AvailableMemoryWatcherWin.cpp:39,3:: error: must use 'enum' tag to refer to type 'nsresult' in this scope
xpcom/base/AvailableMemoryWatcherWin.cpp:42,3:: error: must use 'enum' tag to refer to type 'nsresult' in this scope
xpcom/base/AvailableMemoryWatcherWin.cpp:102,1:: error: cannot initialize object parameter of type 'mozilla::nsAvailableMemoryWatcherBase' with an expression of type 'mozilla::nsAvailableMemoryWatcher'
xpcom/base/AvailableMemoryWatcherWin.cpp:102,1:: error: invalid application of 'sizeof' to an incomplete type 'const QITableEntry []'
xpcom/base/AvailableMemoryWatcherWin.cpp:102,1:: error: must use 'enum' tag to refer to type 'nsresult' in this scope
xpcom/base/AvailableMemoryWatcherWin.cpp:102,1:: error: static_cast from 'mozilla::nsAvailableMemoryWatcher *' to 'nsINamed *', which are not related by inheritance, is not allowed
xpcom/base/AvailableMemoryWatcherWin.cpp:102,1:: error: static_cast from 'mozilla::nsAvailableMemoryWatcher *' to 'nsIObserver *', which are not related by inheritance, is not allowed
xpcom/base/AvailableMemoryWatcherWin.cpp:102,1:: error: static_cast from 'mozilla::nsAvailableMemoryWatcher *' to 'nsITimerCallback *', which are not related by inheritance, is not allowed
xpcom/base/AvailableMemoryWatcherWin.cpp:103,59:: error: 'nsIObserver' does not refer to a value
xpcom/base/AvailableMemoryWatcherWin.cpp:103,59:: error: incomplete type 'nsIObserver' named in nested name specifier
xpcom/base/AvailableMemoryWatcherWin.cpp:140,45:: error: cannot initialize a parameter of type 'nsIObserver *' with an rvalue of type 'mozilla::nsAvailableMemoryWatcher '
xpcom/base/AvailableMemoryWatcherWin.cpp:140,5:: error: must use 'enum' tag to refer to type 'nsresult' in this scope
xpcom/base/CycleCollectedJSRuntime.cpp:1605,7:: error: use of undeclared identifier 'PR_GetEnv'
xpcom/base/CycleCollectedJSRuntime.cpp:1605:7: error: use of undeclared identifier 'PR_GetEnv'
xpcom/base/DeferredFinalize.cpp:12,15:: error: out-of-line definition of 'DeferredFinalize' does not match any declaration in namespace 'mozilla'
xpcom/base/DeferredFinalize.cpp:12:15: error: out-of-line definition of 'DeferredFinalize' does not match any declaration in namespace 'mozilla'
xpcom/base/MemoryTelemetry.cpp:48,27:: error: unused variable 'kTotalMemoryCollectorTimeout' [-Werror,-Wunused-const-variable]
xpcom/base/MemoryTelemetry.cpp:48:27: error: unused variable 'kTotalMemoryCollectorTimeout' [-Werror,-Wunused-const-variable]
xpcom/base/nsErrorService.cpp:42,21:: error: use of function template name with no prior declaration in function call with explicit template arguments is a C++20 extension [-Werror,-Wc++20-extensions]
xpcom/base/nsErrorService.cpp:42,21:: error: use of undeclared identifier 'MakeUnique'; did you mean 'mozilla::MakeUnique'?
xpcom/base/nsErrorService.cpp:42:21: error: use of function template name with no prior declaration in function call with explicit template arguments is a C++20 extension [-Werror,-Wc++20-extensions]
xpcom/base/nsErrorService.cpp:42:21: error: use of undeclared identifier 'MakeUnique'; did you mean 'mozilla::MakeUnique'?
xpcom/base/nsMacPreferencesReader.mm:20:12: error: implicit instantiation of undefined template 'nsTSubstring<char16_t>'
xpcom/base/nsMacPreferencesReader.mm:24:60: error: use of undeclared identifier 'NSString'
xpcom/base/nsMacPreferencesReader.mm:24:69: error: expected expression
xpcom/base/nsMacPreferencesReader.mm:26:48: error: unknown type name 'NSArray'
xpcom/base/nsMacPreferencesReader.mm:28:30: error: use of undeclared identifier 'NSString'
xpcom/base/nsMacPreferencesReader.mm:29:30: error: no matching function for call to 'MakeStringSpan'
xpcom/base/nsMacPreferencesReader.mm:29:51: error: instance method '-UTF8String' not found (return type defaults to 'id') [-Werror,-Wobjc-method-access]
xpcom/base/nsMacPreferencesReader.mm:30:22: error: instance method '-isKindOfClass:' not found (return type defaults to 'id') [-Werror,-Wobjc-method-access]
xpcom/base/nsMacPreferencesReader.mm:30:37: error: receiver type 'int ()(int)' is not 'id' or interface pointer, consider casting it to 'id' [-Werror,-Wreceiver-expr]
xpcom/base/nsMacPreferencesReader.mm:30:37: error: use of undeclared identifier 'NSNumber'; did you mean 'isnumber'?
xpcom/base/nsMacPreferencesReader.mm:30:46: error: instance method '-class' not found (return type defaults to 'id') [-Werror,-Wobjc-method-access]
xpcom/base/nsMacPreferencesReader.mm:31:27: error: cannot initialize a parameter of type 'int64_t' (aka 'long long') with an rvalue of type 'id'
xpcom/base/nsMacPreferencesReader.mm:31:33: error: instance method '-longLongValue' not found (return type defaults to 'id') [-Werror,-Wobjc-method-access]
xpcom/base/nsMacPreferencesReader.mm:32:37: error: use of undeclared identifier 'NSArray'
xpcom/base/nsMacPreferencesReader.mm:34:7: error: no matching function for call to 'EvaluateArray'
xpcom/base/nsMacPreferencesReader.mm:36:37: error: use of undeclared identifier 'NSDictionary'
xpcom/base/nsMacPreferencesReader.mm:38:7: error: no matching function for call to 'EvaluateDict'
xpcom/base/nsMacPreferencesReader.mm:44:60: error: use of undeclared identifier 'NSString'
xpcom/base/nsMacPreferencesReader.mm:44:69: error: expected expression
xpcom/base/nsSystemInfo.cpp:415,35:: error: no template named 'NotNull'; did you mean 'NonNull'?
xpcom/base/nsSystemInfo.cpp:419,25:: error: member reference type 'NonNull<IWSCProductList *>' is not a pointer; did you mean to use '.'?
xpcom/base/nsSystemInfo.cpp:419,27:: error: no member named 'get_Count' in 'mozilla::dom::NonNull<IWSCProductList *>'
xpcom/base/nsSystemInfo.cpp:426,19:: error: member reference type 'NonNull<IWSCProductList *>' is not a pointer; did you mean to use '.'?
xpcom/base/nsSystemInfo.cpp:426,21:: error: no member named 'get_Item' in 'mozilla::dom::NonNull<IWSCProductList *>'
xpcom/base/nsSystemInfo.cpp:484,17:: error: use of undeclared identifier 'ArrayLength'; did you mean 'mozilla::ArrayLength'?
xpcom/base/nsSystemInfo.cpp:484,47:: error: use of undeclared identifier 'ArrayLength'; did you mean 'mozilla::ArrayLength'?
xpcom/base/nsSystemInfo.cpp:487,36:: error: use of undeclared identifier 'ArrayLength'; did you mean 'mozilla::ArrayLength'?
xpcom/base/nsSystemInfo.cpp:500,10:: error: no matching function for call to 'EnumWSCProductList'
xpcom/base/nsSystemInfo.cpp:500,46:: error: use of undeclared identifier 'WrapNotNull'; did you mean 'mozilla::WrapNotNull'?
xpcom/base/nsTraceRefcnt.cpp:313,32:: error: use of undeclared identifier 'MakeUnique'
xpcom/base/nsTraceRefcnt.cpp:313:32: error: use of undeclared identifier 'MakeUnique'
xpcom/base/nsTraceRefcnt.cpp:461,19:: error: no matching member function for call to 'WithEntryHandle'
xpcom/base/nsTraceRefcnt.cpp:461:19: error: no matching member function for call to 'WithEntryHandle'
xpcom/base/nsTraceRefcnt.cpp:468,33:: error: use of function template name with no prior declaration in function call with explicit template arguments is a C++20 extension [-Werror,-Wc++20-extensions]
xpcom/base/nsTraceRefcnt.cpp:468,33:: error: use of undeclared identifier 'MakeUnique'
xpcom/base/nsTraceRefcnt.cpp:468:33: error: use of undeclared identifier 'MakeUnique'
xpcom/base/nsTraceRefcnt.cpp:736,20:: error: use of undeclared identifier '_write'

xpcom/glue/FileUtils.cpp:130:10: error: 'return' will never be executed [-Werror,-Wunreachable-code-return]

There's nothing wrong there.

xpcom/ds/nsProperties.cpp:56:3: error: use of undeclared identifier 'AppendToArray'; did you mean 'mozilla::AppendToArray'?
xpcom/ds/PLDHashTable.cpp:324:3: error: use of undeclared identifier 'NS_ASSERTION'
xpcom/ds/PLDHashTable.cpp:395:3: error: use of undeclared identifier 'NS_ASSERTION'
xpcom/ds/PLDHashTable.cpp:679:7: error: use of undeclared identifier 'NS_ABORT_OOM'
xpcom/ds/PLDHashTable.cpp:685:7: error: use of undeclared identifier 'NS_ABORT_OOM'

xpcom/io/InputStreamLengthHelper.cpp:14:22: error: unused variable 'kStreamTransportServiceCID' [-Werror,-Wunused-const-variable]
xpcom/io/SeekableStreamWrapper.cpp:422:17: error: no member named 'InputStreamHelper' in namespace 'mozilla::ipc'
xpcom/io/nsSegmentedBuffer.cpp:117:11: error: no viable conversion from '(lambda at /tmp/central/xpcom/io/nsSegmentedBuffer.cpp:117:11)' to 'void *'
xpcom/io/nsSegmentedBuffer.cpp:128:11: error: no viable conversion from '(lambda at /tmp/central/xpcom/io/nsSegmentedBuffer.cpp:128:11)' to 'void *'
xpcom/io/nsSegmentedBuffer.cpp:131:25: error: out-of-line definition of 'FreeOMT' does not match any declaration in 'nsSegmentedBuffer'
xpcom/io/nsSegmentedBuffer.cpp:143:16: error: use of function template name with no prior declaration in function call with explicit template arguments is a C++20 extension [-Werror,-Wc++20-extensions]
xpcom/io/nsSegmentedBuffer.cpp:143:16: error: use of undeclared identifier 'MakeRefPtr'; did you mean 'mozilla::MakeRefPtr'?
xpcom/io/nsSegmentedBuffer.cpp:164:35: error: implicit instantiation of undefined template 'nsTArray<std::function<void ()>>'
xpcom/io/nsSegmentedBuffer.cpp:164:44: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
xpcom/io/nsSegmentedBuffer.cpp:165:14: error: use of undeclared identifier 'mTasks'
xpcom/io/nsSegmentedBuffer.h:90:25: error: member initializer 'mTasks' does not name a non-static data member or base class
xpcom/io/nsSegmentedBuffer.h:95:25: error: no template named 'function' in namespace 'std'
xpcom/io/nsSegmentedBuffer.h:96:20: error: use of undeclared identifier 'mTasks'
xpcom/io/nsSegmentedBuffer.h:104:24: error: use of undeclared identifier 'nsTArray'
xpcom/io/nsSegmentedBuffer.h:104:38: error: no member named 'function' in namespace 'std'
xpcom/io/nsSegmentedBuffer.h:104:55: error: expected member name or ';' after declaration specifiers
xpcom/io/nsSegmentedBuffer.h:104:57: error: private field 'mTasks' is not used [-Werror,-Wunused-private-field]
xpcom/io/nsSegmentedBuffer.h:108:21: error: no template named 'function' in namespace 'std'
xpcom/io/nsStorageStream.cpp:576:3: error: no template named 'DebugOnly'
xpcom/io/nsStorageStream.cpp:576:3: error: no template named 'DebugOnly'; did you mean 'mozilla::DebugOnly'?

xpcom/threads/CPUUsageWatcher.cpp:43:23: error: unused variable 'kCPUCheckInterval' [-Werror,-Wunused-const-variable]
xpcom/threads/CPUUsageWatcher.cpp:109:23: error: unused variable 'kCPUCheckInterval' [-Werror,-Wunused-const-variable]
xpcom/threads/InputEventStatistics.cpp:18:5: error: use of undeclared identifier 'ClearOnShutdown'
xpcom/threads/InputTaskManager.cpp:62:9: error: use of undeclared identifier 'InputEventStatistics'
xpcom/threads/InputTaskManager.cpp:89:3: error: use of undeclared identifier 'InputEventStatistics'
xpcom/threads/InputTaskManager.cpp:105:7: error: use of undeclared identifier 'nsRefreshDriver'
xpcom/threads/InputTaskManager.cpp:124:9: error: use of undeclared identifier 'InputEventStatistics'
xpcom/threads/MainThreadIdlePeriod.cpp:29:23: error: unused variable 'kMaxTimerThreadBoundClamp' [-Werror,-Wunused-const-variable]
xpcom/threads/VsyncTaskManager.h:15:67: error: implicit instantiation of undefined template 'mozilla::StaticRefPtr<mozilla::VsyncTaskManager>'
xpcom/threads/VsyncTaskManager.h:16:52: error: no viable overloaded '='
xpcom/threads/nsMemoryPressure.cpp:61:12: error: use of undeclared identifier 'nsIObserverService'
xpcom/threads/nsMemoryPressure.cpp:61:37: error: use of undeclared identifier 'services'
xpcom/threads/nsThreadManager.cpp:422:28: error: member access into incomplete type 'mozilla::ThreadEventTarget'
xpcom/threads/nsThreadManager.cpp:661:5: error: use of undeclared identifier 'CrashReporter'
xpcom/threads/nsThreadManager.cpp:662:59: error: expected ')'
xpcom/threads/nsThreadManager.cpp:662:9: error: use of undeclared identifier 'CrashReporter'
xpcom/threads/nsThreadManager.cpp:669:7: error: use of undeclared identifier 'CrashReporter'
xpcom/threads/nsThreadManager.cpp:670:11: error: use of undeclared identifier 'CrashReporter'
xpcom/threads/nsThreadManager.cpp:670:61: error: expected ')'
xpcom/threads/nsThreadManager.cpp:672:7: error: use of undeclared identifier 'CrashReporter'
xpcom/threads/nsThreadManager.cpp:673:11: error: use of undeclared identifier 'CrashReporter'
xpcom/threads/nsThreadUtils.cpp:636:13: error: member access into incomplete type 'mozilla::Task'

objdir/dist/include/mozilla/Queue.h:251:31: error: use of undeclared identifier 'moz_xcalloc'
xpcom/base/nsCOMPtr.h:436:5: error: static_assert failed due to requirement '1 < sizeof (TestForIID<nsIEventTarget>(nullptr))' "nsCOMPtr only works for types with IIDs. Either use RefPtr; add an IID to your type with NS_DECLARE_STATIC_IID_ACCESSOR/NS_DEFINE_STATIC_IID_ACCESSOR; or make the nsCOMPtr point to a base class with an IID."
xpcom/tests/gtest/TestDelayedRunnable.cpp:16:28: error: field has incomplete type '(anonymous namespace)::ReleaseDetector'
xpcom/tests/gtest/TestDelayedRunnable.cpp:16:3: error: 'explicit' can only appear on non-static member functions
xpcom/tests/gtest/TestDelayedRunnable.cpp:16:40: error: expected ')'
xpcom/tests/gtest/TestDelayedRunnable.cpp:16:61: error: use of undeclared identifier 'aActive'
xpcom/tests/gtest/TestDelayedRunnable.cpp:28:3: error: no template named 'Atomic'; did you mean 'mozilla::Atomic'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:34:3: error: no template named 'Atomic'; did you mean 'mozilla::Atomic'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:36:18: error: use of undeclared identifier 'TaskQueue'; did you mean 'taskQueue'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:36:18: error: variable 'taskQueue' declared with deduced type 'auto' cannot appear in its own initializer
xpcom/tests/gtest/TestDelayedRunnable.cpp:36:48: error: use of undeclared identifier 'MediaThreadType'
xpcom/tests/gtest/TestDelayedRunnable.cpp:36:48: error: use of undeclared identifier 'MediaThreadType'; did you mean 'mozilla::MediaThreadType'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:38:51: error: no matching conversion for functional-style cast from 'Atomic<bool> *' to '(anonymous namespace)::ReleaseDetector'
xpcom/tests/gtest/TestDelayedRunnable.cpp:54:3: error: no template named 'Atomic'; did you mean 'mozilla::Atomic'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:58:51: error: no matching conversion for functional-style cast from 'Atomic<bool> *' to '(anonymous namespace)::ReleaseDetector'
xpcom/tests/gtest/TestDelayedRunnable.cpp:88:3: error: use of undeclared identifier 'Unused'; did you mean 'mozilla::Unused'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:104:3: error: use of undeclared identifier 'Unused'; did you mean 'mozilla::Unused'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:109:10: error: unknown type name 'SharedThreadPool'; did you mean 'mozilla::SharedThreadPool'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:109:35: error: use of undeclared identifier 'SharedThreadPool'
xpcom/tests/gtest/TestDelayedRunnable.cpp:109:35: error: use of undeclared identifier 'SharedThreadPool'; did you mean 'mozilla::SharedThreadPool'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:110:25: error: use of function template name with no prior declaration in function call with explicit template arguments is a C++20 extension [-Werror,-Wc++20-extensions]
xpcom/tests/gtest/TestDelayedRunnable.cpp:110:36: error: unknown type name 'TaskQueue'; did you mean 'mozilla::TaskQueue'?
xpcom/tests/gtest/TestDelayedRunnable.cpp:112:36: error: unknown type name 'TaskQueue'; did you mean 'mozilla::TaskQueue'?
xpcom/tests/gtest/TestFileNTFSSpecialPaths.cpp:18:28: error: use of undeclared identifier 'do_CreateInstance'
xpcom/tests/gtest/TestFileNTFSSpecialPaths.cpp:26:28: error: use of undeclared identifier 'do_CreateInstance'
xpcom/tests/gtest/TestFileNTFSSpecialPaths.cpp:39:28: error: use of undeclared identifier 'do_CreateInstance'
xpcom/tests/gtest/TestFileNTFSSpecialPaths.cpp:279:28: error: use of undeclared identifier 'do_CreateInstance'
xpcom/tests/gtest/TestFilePreferencesWin.cpp:113:26: error: use of undeclared identifier 'do_CreateInstance'
xpcom/tests/gtest/TestFilePreferencesWin.cpp:140:26: error: use of undeclared identifier 'do_CreateInstance'
xpcom/tests/gtest/TestFilePreferencesWin.cpp:156:31: error: use of undeclared identifier 'NS_OS_TEMP_DIR'
xpcom/tests/gtest/TestJSHolderMap.cpp:50:28: error: unknown type name 'JSHolderMap'
xpcom/tests/gtest/TestJSHolderMap.cpp:52:35: error: use of undeclared identifier 'i'
xpcom/tests/gtest/TestJSHolderMap.cpp:52:45: error: use of undeclared identifier 'i'
xpcom/tests/gtest/TestJSHolderMap.cpp:52:8: error: use of undeclared identifier 'JSHolderMap'
xpcom/tests/gtest/TestJSHolderMap.cpp:52:8: error: use of undeclared identifier 'JSHolderMap'; did you mean 'mozilla::JSHolderMap'?
xpcom/tests/gtest/TestJSHolderMap.cpp:53:24: error: use of undeclared identifier 'i'
xpcom/tests/gtest/TestJSHolderMap.cpp:54:24: error: use of undeclared identifier 'i'
xpcom/tests/gtest/TestJSHolderMap.cpp:68:3: error: unknown type name 'JSHolderMap'; did you mean 'mozilla::JSHolderMap'?
xpcom/tests/gtest/TestJSHolderMap.cpp:73:3: error: unknown type name 'JSHolderMap'; did you mean 'mozilla::JSHolderMap'?
xpcom/tests/gtest/TestJSHolderMap.cpp:99:3: error: unknown type name 'JSHolderMap'; did you mean 'mozilla::JSHolderMap'?
xpcom/tests/gtest/TestJSHolderMap.cpp:104:9: error: use of undeclared identifier 'JSHolderMap'
xpcom/tests/gtest/TestJSHolderMap.cpp:104:9: error: use of undeclared identifier 'JSHolderMap'; did you mean 'mozilla::JSHolderMap'?
xpcom/tests/gtest/TestJSHolderMap.cpp:125:41: error: use of undeclared identifier 'MakeUnique'
xpcom/tests/gtest/TestJSHolderMap.cpp:147:3: error: unknown type name 'JSHolderMap'; did you mean 'mozilla::JSHolderMap'?
xpcom/tests/gtest/TestJSHolderMap.cpp:151:41: error: use of undeclared identifier 'MakeUnique'
xpcom/tests/gtest/TestRWLock.cpp:92:3: error: no template named 'Maybe'; did you mean 'mozilla::Maybe'?
xpcom/tests/gtest/TestRWLock.cpp:95:23: error: use of undeclared identifier 'SyncRunnable'; did you mean 'mozilla::SyncRunnable'?
xpcom/tests/gtest/TestRWLock.cpp:103:5: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:107:5: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:111:7: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:120:7: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:130:5: error: unknown type name 'AutoWriteLock'; did you mean 'mozilla::AutoWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:133:5: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:137:7: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:142:3: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:146:5: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:155:5: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:159:5: error: unknown type name 'AutoTryReadLock'; did you mean 'mozilla::AutoTryReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:163:7: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:172:7: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:182:5: error: unknown type name 'AutoReadLock'; did you mean 'mozilla::AutoReadLock'?
xpcom/tests/gtest/TestRWLock.cpp:184:5: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:188:7: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:194:5: error: unknown type name 'AutoWriteLock'; did you mean 'mozilla::AutoWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:197:5: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?
xpcom/tests/gtest/TestRWLock.cpp:201:7: error: unknown type name 'AutoTryWriteLock'; did you mean 'mozilla::AutoTryWriteLock'?

There's nothing wrong there.

Note that xpcom/string is not fixed here, because it's a mess of template instantiation that requires a bunch of files to be compiled all together.

https://hg.mozilla.org/mozilla-central/rev/3bec2357ddfc
https://hg.mozilla.org/mozilla-central/rev/3c6dd4004f56
https://hg.mozilla.org/mozilla-central/rev/98948edc58ba
https://hg.mozilla.org/mozilla-central/rev/b1a882b94878
https://hg.mozilla.org/mozilla-central/rev/b597d44f08bf
https://hg.mozilla.org/mozilla-central/rev/b72ffa91b7d4
https://hg.mozilla.org/mozilla-central/rev/ea580bbec43e
https://hg.mozilla.org/mozilla-central/rev/ec9ecfe02fd0
https://hg.mozilla.org/mozilla-central/rev/47dbdd71d232