Bug 1843568 - Part 3: Annotate `nsIEventTarget` as rust_sync, r=xpcom-reviewers,barret

All event targets should be threadsafe and implemented in C++, and so should be
able to be used in `Sync` types in Rust code.

This also required annotating all interfaces deriving from `nsIEventTarget`, as
well as adding some associated constants to specific types to indicate to the
static assertion that they have threadsafe reference counts.

Differential Revision: https://phabricator.services.mozilla.com/D183592

mfbt/RefCounted.h

@@ -10,6 +10,7 @@
 #define mozilla_RefCounted_h
 
 #include <utility>
+#include <type_traits>
 
 #include "mozilla/AlreadyAddRefed.h"
 #include "mozilla/Assertions.h"
@@ -256,6 +257,9 @@ class RefCounted {
     }
   }
 
+  using HasThreadSafeRefCnt =
+      std::integral_constant<bool, Atomicity == AtomicRefCount>;
+
   // Compatibility with wtf::RefPtr.
   void ref() { AddRef(); }
   void deref() { Release(); }

mfbt/ThreadSafeWeakPtr.h

@@ -181,6 +181,8 @@ class SupportsThreadSafeWeakPtr : public detail::SupportsThreadSafeWeakPtrBase {
     return cnt;
   }
 
+  using HasThreadSafeRefCnt = std::true_type;
+
   // Compatibility with wtf::RefPtr
   void ref() { AddRef(); }
   void deref() { Release(); }

xpcom/threads/SharedThreadPool.h

@@ -8,6 +8,7 @@
 #define SharedThreadPool_h_
 
 #include <utility>
+#include <type_traits>
 #include "mozilla/AlreadyAddRefed.h"
 #include "mozilla/RefCountType.h"
 #include "nsCOMPtr.h"
@@ -52,6 +53,7 @@ class SharedThreadPool : public nsIThreadPool {
   NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
   NS_IMETHOD_(MozExternalRefCountType) AddRef(void) override;
   NS_IMETHOD_(MozExternalRefCountType) Release(void) override;
+  using HasThreadSafeRefCnt = std::true_type;
 
   // Forward behaviour to wrapped thread pool implementation.
   NS_FORWARD_SAFE_NSITHREADPOOL(mPool);

xpcom/threads/nsIEventTarget.idl

@@ -17,7 +17,7 @@ class nsITargetShutdownTask;
 native alreadyAddRefed_nsIRunnable(already_AddRefed<nsIRunnable>);
 [ptr] native nsITargetShutdownTask(nsITargetShutdownTask);
 
-[builtinclass, scriptable, uuid(a03b8b63-af8b-4164-b0e5-c41e8b2b7cfa)]
+[builtinclass, scriptable, rust_sync, uuid(a03b8b63-af8b-4164-b0e5-c41e8b2b7cfa)]
 interface nsIEventTarget : nsISupports
 {
   /* until we can get rid of all uses, keep the non-alreadyAddRefed<> version */

xpcom/threads/nsISerialEventTarget.idl

@@ -21,7 +21,7 @@
  * - However, one can "convert" a thread pool into an nsISerialEventTarget
  *   by putting a TaskQueue in front of it.
  */
-[builtinclass, scriptable, uuid(9f982380-24b4-49f3-88f6-45e2952036c7)]
+[builtinclass, scriptable, rust_sync, uuid(9f982380-24b4-49f3-88f6-45e2952036c7)]
 interface nsISerialEventTarget : nsIEventTarget
 {
 };

xpcom/threads/nsIThread.idl

@@ -36,7 +36,7 @@ native TimeDuration(mozilla::TimeDuration);
  *
  * See nsIThreadManager for the API used to create and locate threads.
  */
-[builtinclass, scriptable, uuid(5801d193-29d1-4964-a6b7-70eb697ddf2b)]
+[builtinclass, scriptable, rust_sync, uuid(5801d193-29d1-4964-a6b7-70eb697ddf2b)]
 interface nsIThread : nsISerialEventTarget
 {
   /**

xpcom/threads/nsIThreadInternal.idl

@@ -13,7 +13,7 @@ interface nsIThreadObserver;
  * The XPCOM thread object implements this interface, which allows a consumer
  * to observe dispatch activity on the thread.
  */
-[builtinclass, scriptable, uuid(a3a72e5f-71d9-4add-8f30-59a78fb6d5eb)]
+[builtinclass, scriptable, rust_sync, uuid(a3a72e5f-71d9-4add-8f30-59a78fb6d5eb)]
 interface nsIThreadInternal : nsIThread
 {
   /**

xpcom/threads/nsIThreadPool.idl

@@ -28,7 +28,7 @@ interface nsIThreadPoolListener : nsISupports
  * anonymous (unnamed) worker threads.  An event dispatched to the thread pool
  * will be run on the next available worker thread.
  */
-[uuid(76ce99c9-8e43-489a-9789-f27cc4424965)]
+[rust_sync, uuid(76ce99c9-8e43-489a-9789-f27cc4424965)]
 interface nsIThreadPool : nsIEventTarget
 {
   /**