Bug 1299384 - Use MOZ_MUST_USE with NS_warn_if_impl(). r=erahm.

This change avoids lots of false positives for Coverity's CHECKED_RETURN
warning, caused by NS_WARN_IF's current use in both statement-style and
expression-style.

In the case where the code within the NS_WARN_IF has side-effects, I made the
following change.

> NS_WARN_IF(NS_FAILED(FunctionWithSideEffects()));
> -->
> Unused << NS_WARN_IF(NS_FAILED(FunctionWithSideEffects()));

In the case where the code within the NS_WARN_IF lacks side-effects, I made the
following change.

> NS_WARN_IF(!condWithoutSideEffects);
> -->
> NS_WARNING_ASSERTION(condWithoutSideEffects, "msg");

This has two improvements.
- The condition is not evaluated in non-debug builds.
- The sense of the condition is inverted to the familiar "this condition should
  be true" sense used in assertions.

A common variation on the side-effect-free case is the following.

> nsresult rv = Fn();
> NS_WARN_IF_(NS_FAILED(rv));
> -->
> DebugOnly<nsresult rv> = Fn();
> NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "Fn failed");

devtools/shared/heapsnapshot/AutoMemMap.cpp

@@ -5,6 +5,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/devtools/AutoMemMap.h"
+
+#include "mozilla/Unused.h"
 #include "nsDebug.h"
 
 namespace mozilla {
@@ -13,17 +15,17 @@ namespace devtools {
 AutoMemMap::~AutoMemMap()
 {
   if (addr) {
-    NS_WARN_IF(PR_MemUnmap(addr, size()) != PR_SUCCESS);
+    Unused << NS_WARN_IF(PR_MemUnmap(addr, size()) != PR_SUCCESS);
     addr = nullptr;
   }
 
   if (fileMap) {
-    NS_WARN_IF(PR_CloseFileMap(fileMap) != PR_SUCCESS);
+    Unused << NS_WARN_IF(PR_CloseFileMap(fileMap) != PR_SUCCESS);
     fileMap = nullptr;
   }
 
   if (fd) {
-    NS_WARN_IF(PR_Close(fd) != PR_SUCCESS);
+    Unused << NS_WARN_IF(PR_Close(fd) != PR_SUCCESS);
     fd = nullptr;
   }
 }

devtools/shared/heapsnapshot/HeapSnapshot.cpp

@@ -29,6 +29,7 @@
 #include "mozilla/dom/HeapSnapshotBinding.h"
 #include "mozilla/RangedPtr.h"
 #include "mozilla/Telemetry.h"
+#include "mozilla/Unused.h"
 
 #include "jsapi.h"
 #include "jsfriendapi.h"
@@ -1473,7 +1474,7 @@ class DeleteHeapSnapshotTempFileHelperChild
   constexpr DeleteHeapSnapshotTempFileHelperChild() { }
 
   void operator()(PHeapSnapshotTempFileHelperChild* ptr) const {
-    NS_WARN_IF(!HeapSnapshotTempFileHelperChild::Send__delete__(ptr));
+    Unused << NS_WARN_IF(!HeapSnapshotTempFileHelperChild::Send__delete__(ptr));
   }
 };
 

devtools/shared/heapsnapshot/ZeroCopyNSIOutputStream.cpp

@@ -6,6 +6,7 @@
 #include "mozilla/devtools/ZeroCopyNSIOutputStream.h"
 
 #include "mozilla/DebugOnly.h"
+#include "mozilla/Unused.h"
 
 namespace mozilla {
 namespace devtools {
@@ -24,7 +25,7 @@ ZeroCopyNSIOutputStream::ZeroCopyNSIOutputStream(nsCOMPtr<nsIOutputStream>& out)
 ZeroCopyNSIOutputStream::~ZeroCopyNSIOutputStream()
 {
   if (!failed())
-    NS_WARN_IF(NS_FAILED(writeBuffer()));
+    Unused << NS_WARN_IF(NS_FAILED(writeBuffer()));
 }
 
 nsresult

dom/audiochannel/AudioChannelService.cpp

@@ -1349,8 +1349,8 @@ AudioChannelService::AudioChannelWindow::NotifyAudioAudibleChanged(nsPIDOMWindow
 {
   RefPtr<AudioPlaybackRunnable> runnable =
     new AudioPlaybackRunnable(aWindow, aAudible, aReason);
-  nsresult rv = NS_DispatchToCurrentThread(runnable);
-  NS_WARN_IF(NS_FAILED(rv));
+  DebugOnly<nsresult> rv = NS_DispatchToCurrentThread(runnable);
+  NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "NS_DispatchToCurrentThread failed");
 }
 
 void
@@ -1360,6 +1360,6 @@ AudioChannelService::AudioChannelWindow::NotifyChannelActive(uint64_t aWindowID,
 {
   RefPtr<NotifyChannelActiveRunnable> runnable =
     new NotifyChannelActiveRunnable(aWindowID, aChannel, aActive);
-  nsresult rv = NS_DispatchToCurrentThread(runnable);
-  NS_WARN_IF(NS_FAILED(rv));
+  DebugOnly<nsresult> rv = NS_DispatchToCurrentThread(runnable);
+  NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "NS_DispatchToCurrentThread failed");
 }

dom/base/MultipartBlobImpl.cpp

@@ -167,7 +167,7 @@ void
 MultipartBlobImpl::InitializeBlob(ErrorResult& aRv)
 {
   SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  NS_WARNING_ASSERTION(!aRv.Failed(), "SetLengthAndModifiedDate failed");
 }
 
 void
@@ -221,7 +221,7 @@ MultipartBlobImpl::InitializeBlob(JSContext* aCx,
 
   mBlobImpls = blobSet.GetBlobImpls();
   SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  NS_WARNING_ASSERTION(!aRv.Failed(), "SetLengthAndModifiedDate failed");
 }
 
 void
@@ -352,7 +352,7 @@ MultipartBlobImpl::InitializeChromeFile(Blob& aBlob,
   mBlobImpls = blobSet.GetBlobImpls();
 
   SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  NS_WARNING_ASSERTION(!aRv.Failed(), "SetLengthAndModifiedDate failed");
 }
 
 void
@@ -424,7 +424,7 @@ MultipartBlobImpl::InitializeChromeFile(nsPIDOMWindowInner* aWindow,
   mBlobImpls = blobSet.GetBlobImpls();
 
   SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  NS_WARNING_ASSERTION(!aRv.Failed(), "SetLengthAndModifiedDate failed");
 }
 
 void

dom/base/ScreenOrientation.cpp

@@ -399,9 +399,11 @@ ScreenOrientation::UnlockDeviceOrientation()
   // Remove event listener in case of fullscreen lock.
   nsCOMPtr<EventTarget> target = do_QueryInterface(GetOwner()->GetDoc());
   if (target) {
-    nsresult rv = target->RemoveSystemEventListener(NS_LITERAL_STRING("fullscreenchange"),
-                                                    mFullScreenListener, /* useCapture */ true);
-    NS_WARN_IF(NS_FAILED(rv));
+    DebugOnly<nsresult> rv =
+      target->RemoveSystemEventListener(NS_LITERAL_STRING("fullscreenchange"),
+                                        mFullScreenListener,
+                                        /* useCapture */ true);
+    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "RemoveSystemEventListener failed");
   }
 
   mFullScreenListener = nullptr;
@@ -530,18 +532,18 @@ ScreenOrientation::Notify(const hal::ScreenConfiguration& aConfiguration)
   mAngle = aConfiguration.angle();
   mType = InternalOrientationToType(orientation);
 
-  nsresult rv;
+  DebugOnly<nsresult> rv;
   if (mScreen && mType != previousOrientation) {
     // Use of mozorientationchange is deprecated.
     rv = mScreen->DispatchTrustedEvent(NS_LITERAL_STRING("mozorientationchange"));
-    NS_WARN_IF(NS_FAILED(rv));
+    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "DispatchTrustedEvent failed");
   }
 
   if (doc->Hidden() && !mVisibleListener) {
     mVisibleListener = new VisibleEventListener();
     rv = doc->AddSystemEventListener(NS_LITERAL_STRING("visibilitychange"),
                                      mVisibleListener, /* useCapture = */ true);
-    NS_WARN_IF(NS_FAILED(rv));
+    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "AddSystemEventListener failed");
     return;
   }
 
@@ -557,7 +559,7 @@ ScreenOrientation::Notify(const hal::ScreenConfiguration& aConfiguration)
     nsCOMPtr<nsIRunnable> runnable = NewRunnableMethod(this,
       &ScreenOrientation::DispatchChangeEvent);
     rv = NS_DispatchToMainThread(runnable);
-    NS_WARN_IF(NS_FAILED(rv));
+    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "NS_DispatchToMainThread failed");
   }
 }
 
@@ -567,16 +569,16 @@ ScreenOrientation::UpdateActiveOrientationLock(ScreenOrientationInternal aOrient
   if (aOrientation == eScreenOrientation_None) {
     hal::UnlockScreenOrientation();
   } else {
-    bool rv = hal::LockScreenOrientation(aOrientation);
-    NS_WARN_IF(!rv);
+    DebugOnly<bool> ok = hal::LockScreenOrientation(aOrientation);
+    NS_WARNING_ASSERTION(ok, "hal::LockScreenOrientation failed");
   }
 }
 
 void
 ScreenOrientation::DispatchChangeEvent()
 {
-  nsresult rv = DispatchTrustedEvent(NS_LITERAL_STRING("change"));
-  NS_WARN_IF(NS_FAILED(rv));
+  DebugOnly<nsresult> rv = DispatchTrustedEvent(NS_LITERAL_STRING("change"));
+  NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "DispatchTrustedEvent failed");
 }
 
 JSObject*