Bug 1794012 - Add a static main thread capability for the thread safety analysis, r=xpcom-reviewers,kmag

This acts like the `EventTargetCapability` type, but is global and specific to
the main thread.

Differential Revision: https://phabricator.services.mozilla.com/D158872

xpcom/threads/EventTargetCapability.h

@@ -48,6 +48,10 @@ namespace mozilla {
 //   uint32_t mMediaCount MOZ_GUARDED_BY(mTargetCapability) = 0;
 //   EventTargetCapability<nsIEventTarget> mTargetCapability;
 // };
+//
+// NOTE: If you need a thread-safety capability for specifically the main
+// thread, the static `mozilla::sMainThreadCapability` capability exists, and
+// can be asserted using `AssertIsOnMainThread()`.
 
 template <typename T>
 class MOZ_CAPABILITY EventTargetCapability final {

xpcom/threads/MainThreadUtils.h

@@ -7,6 +7,7 @@
 #ifndef MainThreadUtils_h_
 #define MainThreadUtils_h_
 
+#include "mozilla/ThreadSafety.h"
 #include "nscore.h"
 
 class nsIThread;
@@ -27,10 +28,23 @@ bool NS_IsMainThread();
 
 namespace mozilla {
 
+/**
+ * A dummy static capability for the thread safety analysis which can be
+ * required by functions and members using `MOZ_REQUIRE(sMainThreadCapability)`
+ * and `MOZ_GUARDED_BY(sMainThreadCapability)` and asserted using
+ * `AssertIsOnMainThread()`.
+ *
+ * If you want a thread-safety-analysis capability for a non-main thread,
+ * consider using the `EventTargetCapability` type.
+ */
+class MOZ_CAPABILITY MainThreadCapability final {};
+constexpr MainThreadCapability sMainThreadCapability;
+
 #  ifdef DEBUG
-void AssertIsOnMainThread();
+void AssertIsOnMainThread() MOZ_ASSERT_CAPABILITY(sMainThreadCapability);
 #  else
-inline void AssertIsOnMainThread() {}
+inline void AssertIsOnMainThread()
+    MOZ_ASSERT_CAPABILITY(sMainThreadCapability) {}
 #  endif
 
 }  // namespace mozilla