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diff --git a/mfbt/ThreadSafety.h b/mfbt/ThreadSafety.h
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+// Note: the file is largely imported directly from WebRTC upstream, so
+// comments may not completely apply to Mozilla's usage.
+//
+// Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the LICENSE file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS.  All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+//
+// Borrowed from
+// https://code.google.com/p/gperftools/source/browse/src/base/thread_annotations.h
+// but adapted for clang attributes instead of the gcc.
+//
+// This header file contains the macro definitions for thread safety
+// annotations that allow the developers to document the locking policies
+// of their multi-threaded code. The annotations can also help program
+// analysis tools to identify potential thread safety issues.
+
+#ifndef mozilla_ThreadSafety_h
+#define mozilla_ThreadSafety_h
+#include "mozilla/Attributes.h"
+
+#if defined(__clang__) && (__clang_major__ >= 11) && !defined(SWIG)
+#  define MOZ_THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))
+// Allow for localized suppression of thread-safety warnings; finer-grained
+// than MOZ_NO_THREAD_SAFETY_ANALYSIS
+#  define MOZ_PUSH_IGNORE_THREAD_SAFETY \
+    _Pragma("GCC diagnostic push")      \
+        _Pragma("GCC diagnostic ignored \"-Wthread-safety\"")
+#  define MOZ_POP_THREAD_SAFETY _Pragma("GCC diagnostic pop")
+
+#else
+#  define MOZ_THREAD_ANNOTATION_ATTRIBUTE__(x)  // no-op
+#  define MOZ_PUSH_IGNORE_THREAD_SAFETY
+#  define MOZ_POP_THREAD_SAFETY
+#endif
+
+// Document if a shared variable/field needs to be protected by a lock.
+// MOZ_GUARDED_BY allows the user to specify a particular lock that should be
+// held when accessing the annotated variable, while MOZ_GUARDED_VAR only
+// indicates a shared variable should be guarded (by any lock). MOZ_GUARDED_VAR
+// is primarily used when the client cannot express the name of the lock.
+#define MOZ_GUARDED_BY(x) MOZ_THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))
+#define MOZ_GUARDED_VAR MOZ_THREAD_ANNOTATION_ATTRIBUTE__(guarded_var)
+
+// Document if the memory location pointed to by a pointer should be guarded
+// by a lock when dereferencing the pointer. Similar to MOZ_GUARDED_VAR,
+// MOZ_PT_GUARDED_VAR is primarily used when the client cannot express the
+// name of the lock. Note that a pointer variable to a shared memory location
+// could itself be a shared variable. For example, if a shared global pointer
+// q, which is guarded by mu1, points to a shared memory location that is
+// guarded by mu2, q should be annotated as follows:
+//     int *q MOZ_GUARDED_BY(mu1) MOZ_PT_GUARDED_BY(mu2);
+#define MOZ_PT_GUARDED_BY(x) MOZ_THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_by(x))
+#define MOZ_PT_GUARDED_VAR MOZ_THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_var)
+
+// Document the acquisition order between locks that can be held
+// simultaneously by a thread. For any two locks that need to be annotated
+// to establish an acquisition order, only one of them needs the annotation.
+// (i.e. You don't have to annotate both locks with both MOZ_ACQUIRED_AFTER
+// and MOZ_ACQUIRED_BEFORE.)
+#define MOZ_ACQUIRED_AFTER(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(acquired_after(__VA_ARGS__))
+#define MOZ_ACQUIRED_BEFORE(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(acquired_before(__VA_ARGS__))
+
+// The following three annotations document the lock requirements for
+// functions/methods.
+
+// Document if a function expects certain locks to be held before it is called
+#define MOZ_REQUIRES(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(__VA_ARGS__))
+
+#define MOZ_REQUIRES_SHARED(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(shared_locks_required(__VA_ARGS__))
+
+// Document the locks acquired in the body of the function. These locks
+// cannot be held when calling this function (as google3's Mutex locks are
+// non-reentrant).
+#define MOZ_EXCLUDES(x) MOZ_THREAD_ANNOTATION_ATTRIBUTE__(locks_excluded(x))
+
+// Document the lock the annotated function returns without acquiring it.
+#define MOZ_RETURN_CAPABILITY(x) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(lock_returned(x))
+
+// Document if a class/type is a lockable type (such as the Mutex class).
+#define MOZ_CAPABILITY(x) MOZ_THREAD_ANNOTATION_ATTRIBUTE__(capability(x))
+
+// Document if a class is a scoped lockable type (such as the MutexLock class).
+#define MOZ_SCOPED_CAPABILITY MOZ_THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)
+
+// The following annotations specify lock and unlock primitives.
+#define MOZ_CAPABILITY_ACQUIRE(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(exclusive_lock_function(__VA_ARGS__))
+
+#define MOZ_EXCLUSIVE_RELEASE(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(release_capability(__VA_ARGS__))
+
+#define MOZ_ACQUIRE_SHARED(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(shared_lock_function(__VA_ARGS__))
+
+#define MOZ_TRY_ACQUIRE(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(exclusive_trylock_function(__VA_ARGS__))
+
+#define MOZ_SHARED_TRYLOCK_FUNCTION(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(shared_trylock_function(__VA_ARGS__))
+
+#define MOZ_CAPABILITY_RELEASE(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(unlock_function(__VA_ARGS__))
+
+// An escape hatch for thread safety analysis to ignore the annotated function.
+#define MOZ_NO_THREAD_SAFETY_ANALYSIS \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(no_thread_safety_analysis)
+
+// Newer capabilities
+#define MOZ_ASSERT_CAPABILITY(x) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(assert_capability(x))
+
+#define MOZ_ASSERT_SHARED_CAPABILITY(x) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(assert_shared_capability(x))
+
+// Additions from current clang assertions.
+// Note: new-style definitions, since these didn't exist in the old style
+#define MOZ_RELEASE_SHARED(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(release_shared_capability(__VA_ARGS__))
+
+#define MOZ_RELEASE_GENERIC(...) \
+  MOZ_THREAD_ANNOTATION_ATTRIBUTE__(release_generic_capability(__VA_ARGS__))
+
+// Mozilla additions:
+
+// AutoUnlock is supported by clang currently, but oddly you must use
+// MOZ_EXCLUSIVE_RELEASE() for both the RAII constructor *and* the destructor.
+// This hides the ugliness until they fix it upstream.
+#define MOZ_SCOPED_UNLOCK_RELEASE(...) MOZ_EXCLUSIVE_RELEASE(__VA_ARGS__)
+#define MOZ_SCOPED_UNLOCK_REACQUIRE(...) MOZ_EXCLUSIVE_RELEASE(__VA_ARGS__)
+
+#endif /* mozilla_ThreadSafety_h */