Bug 1642949 - Replace uses of RemoveElementAt by RemoveLastElement/PopLastElement where possible. r=necko-reviewers,froydnj

Differential Revision: https://phabricator.services.mozilla.com/D78027

dom/base/CustomElementRegistry.cpp

@@ -241,19 +241,23 @@ class MOZ_RAII AutoConstructionStackEntry final {
       : mStack(aStack) {
     MOZ_ASSERT(aElement->IsHTMLElement() || aElement->IsXULElement());
 
+#ifdef DEBUG
     mIndex = mStack.Length();
+#endif
     mStack.AppendElement(aElement);
   }
 
   ~AutoConstructionStackEntry() {
     MOZ_ASSERT(mIndex == mStack.Length() - 1,
                "Removed element should be the last element");
-    mStack.RemoveElementAt(mIndex);
+    mStack.RemoveLastElement();
   }
 
  private:
   nsTArray<RefPtr<Element>>& mStack;
+#ifdef DEBUG
   uint32_t mIndex;
+#endif
 };
 
 }  // namespace
@@ -1295,7 +1299,7 @@ void CustomElementReactionsStack::PopAndInvokeElementQueue() {
       lastIndex == mReactionsStack.Length() - 1,
       "reactions created by InvokeReactions() should be consumed and removed");
 
-  mReactionsStack.RemoveElementAt(lastIndex);
+  mReactionsStack.RemoveLastElement();
   mIsElementQueuePushedForCurrentRecursionDepth = false;
 }
 

dom/base/ResponsiveImageSelector.cpp

@@ -204,7 +204,7 @@ void ResponsiveImageSelector::SetDefaultSource(const nsAString& aURLString,
   int32_t candidates = mCandidates.Length();
   if (candidates && (mCandidates[candidates - 1].Type() ==
                      ResponsiveImageCandidate::eCandidateType_Default)) {
-    mCandidates.RemoveElementAt(candidates - 1);
+    mCandidates.RemoveLastElement();
   }
 
   mDefaultSourceURL = aURLString;

dom/base/nsDOMMutationObserver.cpp

@@ -911,16 +911,15 @@ void nsDOMMutationObserver::EnterMutationHandling() { ++sMutationLevel; }
 void nsDOMMutationObserver::LeaveMutationHandling() {
   if (sCurrentlyHandlingObservers &&
       sCurrentlyHandlingObservers->Length() == sMutationLevel) {
-    nsTArray<RefPtr<nsDOMMutationObserver>>& obs =
-        sCurrentlyHandlingObservers->ElementAt(sMutationLevel - 1);
+    nsTArray<RefPtr<nsDOMMutationObserver>> obs =
+        sCurrentlyHandlingObservers->PopLastElement();
     for (uint32_t i = 0; i < obs.Length(); ++i) {
       nsDOMMutationObserver* o = static_cast<nsDOMMutationObserver*>(obs[i]);
       if (o->mCurrentMutations.Length() == sMutationLevel) {
         // It is already in pending mutations.
-        o->mCurrentMutations.RemoveElementAt(sMutationLevel - 1);
+        o->mCurrentMutations.RemoveLastElement();
       }
     }
-    sCurrentlyHandlingObservers->RemoveElementAt(sMutationLevel - 1);
   }
   --sMutationLevel;
 }

dom/base/nsPlainTextSerializer.cpp

@@ -383,13 +383,7 @@ void nsPlainTextSerializer::PushBool(nsTArray<bool>& aStack, bool aValue) {
 }
 
 bool nsPlainTextSerializer::PopBool(nsTArray<bool>& aStack) {
-  bool returnValue = false;
-  uint32_t size = aStack.Length();
-  if (size > 0) {
-    returnValue = aStack.ElementAt(size - 1);
-    aStack.RemoveElementAt(size - 1);
-  }
-  return returnValue;
+  return aStack.Length() ? aStack.PopLastElement() : false;
 }
 
 bool nsPlainTextSerializer::IsIgnorableRubyAnnotation(

dom/base/nsXMLContentSerializer.cpp

@@ -432,7 +432,7 @@ void nsXMLContentSerializer::PopNameSpaceDeclsFor(nsIContent* aOwner) {
     if (mNameSpaceStack[index].mOwner != aOwner) {
       break;
     }
-    mNameSpaceStack.RemoveElementAt(index);
+    mNameSpaceStack.RemoveLastElement();
   }
 }
 

dom/cache/AutoUtils.cpp

@@ -306,8 +306,7 @@ void AutoChildOpArgs::Add(JSContext* aCx, const InternalRequest& aRequest,
 
       if (aRv.Failed()) {
         CleanupChild(pair.request().body(), Delete);
-        args.requestResponseList().RemoveElementAt(
-            args.requestResponseList().Length() - 1);
+        args.requestResponseList().RemoveLastElement();
       }
 
       break;

dom/console/Console.cpp

@@ -1935,7 +1935,7 @@ static bool ProcessArguments(JSContext* aCx, const Sequence<JS::Value>& aData,
         // If there isn't any output but there's already a style, then
         // discard the previous style and use the next one instead.
         if (output.IsEmpty() && !aStyles.IsEmpty()) {
-          aStyles.TruncateLength(aStyles.Length() - 1);
+          aStyles.RemoveLastElement();
         }
 
         if (NS_WARN_IF(!FlushOutput(aCx, aSequence, output))) {
@@ -2084,9 +2084,7 @@ static bool UnstoreGroupName(nsAString& aName,
     return false;
   }
 
-  uint32_t pos = aGroupStack->Length() - 1;
-  aName = (*aGroupStack)[pos];
-  aGroupStack->RemoveElementAt(pos);
+  aName = aGroupStack->PopLastElement();
   return true;
 }
 

dom/events/EventDispatcher.cpp

@@ -149,9 +149,8 @@ class EventTargetChainItem {
 
   static void DestroyLast(nsTArray<EventTargetChainItem>& aChain,
                           EventTargetChainItem* aItem) {
-    uint32_t lastIndex = aChain.Length() - 1;
-    MOZ_ASSERT(&aChain[lastIndex] == aItem);
-    aChain.RemoveElementAt(lastIndex);
+    MOZ_ASSERT(&aChain.LastElement() == aItem);
+    aChain.RemoveLastElement();
   }
 
   static EventTargetChainItem* GetFirstCanHandleEventTarget(

dom/html/TextControlState.cpp

@@ -1389,8 +1389,7 @@ TextControlState::TextControlState(TextControlElement* aOwningElement)
 TextControlState* TextControlState::Construct(
     TextControlElement* aOwningElement) {
   if (sReleasedInstances && !sReleasedInstances->IsEmpty()) {
-    TextControlState* state = sReleasedInstances->LastElement();
-    sReleasedInstances->RemoveLastElement();
+    TextControlState* state = sReleasedInstances->PopLastElement();
     state->mTextCtrlElement = aOwningElement;
     state->mBoundFrame = nullptr;
     state->mSelectionProperties = SelectionProperties();

dom/html/nsHTMLContentSink.cpp

@@ -551,15 +551,13 @@ HTMLContentSink::~HTMLContentSink() {
     mNotificationTimer->Cancel();
   }
 
-  int32_t numContexts = mContextStack.Length();
-
-  if (mCurrentContext == mHeadContext && numContexts > 0) {
+  if (mCurrentContext == mHeadContext && !mContextStack.IsEmpty()) {
     // Pop off the second html context if it's not done earlier
-    mContextStack.RemoveElementAt(--numContexts);
+    mContextStack.RemoveLastElement();
   }
 
-  int32_t i;
-  for (i = 0; i < numContexts; i++) {
+  for (int32_t i = 0, numContexts = mContextStack.Length(); i < numContexts;
+       i++) {
     SinkContext* sc = mContextStack.ElementAt(i);
     if (sc) {
       sc->End();
@@ -716,11 +714,8 @@ HTMLContentSink::SetParser(nsParserBase* aParser) {
 nsresult HTMLContentSink::CloseHTML() {
   if (mHeadContext) {
     if (mCurrentContext == mHeadContext) {
-      uint32_t numContexts = mContextStack.Length();
-
       // Pop off the second html context if it's not done earlier
-      mCurrentContext = mContextStack.ElementAt(--numContexts);
-      mContextStack.RemoveElementAt(numContexts);
+      mCurrentContext = mContextStack.PopLastElement();
     }
 
     mHeadContext->End();
@@ -842,9 +837,7 @@ void HTMLContentSink::CloseHeadContext() {
   }
 
   if (!mContextStack.IsEmpty()) {
-    uint32_t n = mContextStack.Length() - 1;
-    mCurrentContext = mContextStack.ElementAt(n);
-    mContextStack.RemoveElementAt(n);
+    mCurrentContext = mContextStack.PopLastElement();
   }
 }
 

dom/indexedDB/ActorsParent.cpp

@@ -12496,11 +12496,8 @@ bool ConnectionPool::ScheduleTransaction(TransactionInfo* aTransactionInfo,
         return false;
       }
     } else {
-      const uint32_t lastIndex = mIdleThreads.Length() - 1;
-
-      dbInfo->mThreadInfo = std::move(mIdleThreads[lastIndex].mThreadInfo);
-
-      mIdleThreads.RemoveElementAt(lastIndex);
+      dbInfo->mThreadInfo =
+          std::move(mIdleThreads.PopLastElement().mThreadInfo);
 
       AdjustIdleTimer();
     }

dom/media/webaudio/AudioNode.cpp

@@ -161,16 +161,12 @@ void AudioNode::DisconnectFromGraph() {
 
   // Disconnect inputs. We don't need them anymore.
   while (!mInputNodes.IsEmpty()) {
-    size_t i = mInputNodes.Length() - 1;
-    RefPtr<AudioNode> input = mInputNodes[i].mInputNode;
-    mInputNodes.RemoveElementAt(i);
-    input->mOutputNodes.RemoveElement(this);
+    InputNode inputNode = mInputNodes.PopLastElement();
+    inputNode.mInputNode->mOutputNodes.RemoveElement(this);
   }
 
   while (!mOutputNodes.IsEmpty()) {
-    size_t i = mOutputNodes.Length() - 1;
-    RefPtr<AudioNode> output = std::move(mOutputNodes[i]);
-    mOutputNodes.RemoveElementAt(i);
+    RefPtr<AudioNode> output = mOutputNodes.PopLastElement();
     size_t inputIndex = FindIndexOfNode(output->mInputNodes, this);
     // It doesn't matter which one we remove, since we're going to remove all
     // entries for this node anyway.
@@ -180,9 +176,7 @@ void AudioNode::DisconnectFromGraph() {
   }
 
   while (!mOutputParams.IsEmpty()) {
-    size_t i = mOutputParams.Length() - 1;
-    RefPtr<AudioParam> output = std::move(mOutputParams[i]);
-    mOutputParams.RemoveElementAt(i);
+    RefPtr<AudioParam> output = mOutputParams.PopLastElement();
     size_t inputIndex = FindIndexOfNode(output->InputNodes(), this);
     // It doesn't matter which one we remove, since we're going to remove all
     // entries for this node anyway.

dom/media/webaudio/AudioNode.h

@@ -143,6 +143,10 @@ class AudioNode : public DOMEventTargetHelper, public nsSupportsWeakReference {
   }
 
   struct InputNode final {
+    InputNode() = default;
+    InputNode(const InputNode&) = delete;
+    InputNode(InputNode&&) = default;
+
     ~InputNode() {
       if (mTrackPort) {
         mTrackPort->Destroy();

dom/media/webaudio/AudioParam.cpp

@@ -55,9 +55,7 @@ void AudioParam::DisconnectFromGraphAndDestroyTrack() {
              "mRefCnt.stabilizeForDeletion()");
 
   while (!mInputNodes.IsEmpty()) {
-    uint32_t i = mInputNodes.Length() - 1;
-    RefPtr<AudioNode> input = mInputNodes[i].mInputNode;
-    mInputNodes.RemoveElementAt(i);
+    RefPtr<AudioNode> input = mInputNodes.PopLastElement().mInputNode;
     input->RemoveOutputParam(this);
   }
 

dom/plugins/base/nsNPAPIPluginInstance.cpp

@@ -781,9 +781,7 @@ nsresult nsNPAPIPluginInstance::PushPopupsEnabledState(bool aEnabled) {
 }
 
 nsresult nsNPAPIPluginInstance::PopPopupsEnabledState() {
-  int32_t last = mPopupStates.Length() - 1;
-
-  if (last < 0) {
+  if (mPopupStates.IsEmpty()) {
     // Nothing to pop.
     return NS_OK;
   }
@@ -791,11 +789,7 @@ nsresult nsNPAPIPluginInstance::PopPopupsEnabledState() {
   nsCOMPtr<nsPIDOMWindowOuter> window = GetDOMWindow();
   if (!window) return NS_ERROR_FAILURE;
 
-  PopupBlocker::PopupControlState& oldState = mPopupStates[last];
-
-  PopupBlocker::PopPopupControlState(oldState);
-
-  mPopupStates.RemoveElementAt(last);
+  PopupBlocker::PopPopupControlState(mPopupStates.PopLastElement());
 
   return NS_OK;
 }

dom/plugins/ipc/PluginModuleChild.cpp

@@ -1818,13 +1818,14 @@ void PluginModuleChild::EnteredCall() { mIncallPumpingStack.AppendElement(); }
 
 void PluginModuleChild::ExitedCall() {
   NS_ASSERTION(mIncallPumpingStack.Length(), "mismatched entered/exited");
-  uint32_t len = mIncallPumpingStack.Length();
-  const IncallFrame& f = mIncallPumpingStack[len - 1];
+  const IncallFrame& f = mIncallPumpingStack.LastElement();
   if (f._spinning)
     MessageLoop::current()->SetNestableTasksAllowed(
         f._savedNestableTasksAllowed);
 
-  mIncallPumpingStack.TruncateLength(len - 1);
+  // XXX Is RemoveLastElement intentionally called only after calling
+  // SetNestableTasksAllowed? Otherwise, PopLastElement could be used above.
+  mIncallPumpingStack.RemoveLastElement();
 }
 
 LRESULT CALLBACK PluginModuleChild::CallWindowProcHook(int nCode, WPARAM wParam,

dom/webbrowserpersist/nsWebBrowserPersist.cpp

@@ -1696,9 +1696,7 @@ void nsWebBrowserPersist::FinishSaveDocumentInternal(nsIURI* aFile,
   }
 
   if (mWalkStack.Length() > 0) {
-    mozilla::UniquePtr<WalkData> toWalk;
-    mWalkStack.LastElement().swap(toWalk);
-    mWalkStack.TruncateLength(mWalkStack.Length() - 1);
+    mozilla::UniquePtr<WalkData> toWalk = mWalkStack.PopLastElement();
     // Bounce this off the event loop to avoid stack overflow.
     typedef StoreCopyPassByRRef<decltype(toWalk)> WalkStorage;
     auto saveMethod = &nsWebBrowserPersist::SaveDocumentDeferred;

dom/workers/Queue.h

@@ -49,9 +49,7 @@ struct StorageWithTArray {
       return false;
     }
 
-    uint32_t index = aStorage.Length() - 1;
-    aEntry = aStorage.ElementAt(index);
-    aStorage.RemoveElementAt(index);
+    aEntry = aStorage.PopLastElement();
     return true;
   }
 

dom/workers/RuntimeService.cpp

@@ -1358,7 +1358,7 @@ bool RuntimeService::ScheduleWorker(WorkerPrivate& aWorkerPrivate) {
   {
     MutexAutoLock lock(mMutex);
     if (!mIdleThreadArray.IsEmpty()) {
-      thread = mIdleThreadArray.PopLastElement().mThread;
+      thread = std::move(mIdleThreadArray.PopLastElement().mThread);
     }
   }
 

dom/xml/nsXMLContentSink.cpp

@@ -798,14 +798,12 @@ nsresult nsXMLContentSink::PushContent(nsIContent* aContent) {
 }
 
 void nsXMLContentSink::PopContent() {
-  int32_t count = mContentStack.Length();
-
-  if (count == 0) {
+  if (mContentStack.IsEmpty()) {
     NS_WARNING("Popping empty stack");
     return;
   }
 
-  mContentStack.RemoveElementAt(count - 1);
+  mContentStack.RemoveLastElement();
 }
 
 bool nsXMLContentSink::HaveNotifiedForCurrentContent() const {

dom/xslt/base/txStack.h

@@ -44,9 +44,7 @@ class txStack : private nsTArray<void*> {
     void* object = nullptr;
     NS_ASSERTION(!isEmpty(), "popping from empty stack");
     if (!isEmpty()) {
-      const uint32_t count = Length() - 1;
-      object = ElementAt(count);
-      RemoveElementAt(count);
+      object = PopLastElement();
     }
     return object;
   }

dom/xslt/xslt/txExecutionState.cpp

@@ -331,13 +331,8 @@ nsresult txExecutionState::pushBool(bool aBool) {
 
 bool txExecutionState::popBool() {
   NS_ASSERTION(mBoolStack.Length(), "popping from empty stack");
-  uint32_t last = mBoolStack.Length() - 1;
-  NS_ENSURE_TRUE(last != (uint32_t)-1, false);
 
-  bool res = mBoolStack.ElementAt(last);
-  mBoolStack.RemoveElementAt(last);
-
-  return res;
+  return mBoolStack.IsEmpty() ? false : mBoolStack.PopLastElement();
 }
 
 nsresult txExecutionState::pushResultHandler(txAXMLEventHandler* aHandler) {

dom/xslt/xslt/txStylesheetCompiler.cpp

@@ -589,13 +589,11 @@ nsresult txStylesheetCompilerState::pushPtr(void* aPtr, enumStackType aType) {
 }
 
 void* txStylesheetCompilerState::popPtr(enumStackType aType) {
-  uint32_t stacklen = mTypeStack.Length();
-  if (stacklen == 0) {
+  if (mTypeStack.IsEmpty()) {
     MOZ_CRASH("Attempt to pop when type stack is empty");
   }
 
-  enumStackType type = mTypeStack.ElementAt(stacklen - 1);
-  mTypeStack.RemoveElementAt(stacklen - 1);
+  enumStackType type = mTypeStack.PopLastElement();
   void* value = mOtherStack.pop();
 
 #ifdef TX_DEBUG_STACK

editor/libeditor/TypeInState.cpp

@@ -207,15 +207,9 @@ void TypeInState::ClearProp(nsAtom* aProp, nsAtom* aAttr) {
  * Caller assumes ownership of PropItem and must delete it.
  */
 UniquePtr<PropItem> TypeInState::TakeClearProperty() {
-  size_t count = mClearedArray.Length();
-  if (!count) {
-    return nullptr;
-  }
-
-  --count;  // indices are zero based
-  PropItem* propItem = mClearedArray[count];
-  mClearedArray.RemoveElementAt(count);
-  return UniquePtr<PropItem>(propItem);
+  return mClearedArray.Length()
+             ? UniquePtr<PropItem>{mClearedArray.PopLastElement()}
+             : nullptr;
 }
 
 /**
@@ -223,14 +217,8 @@ UniquePtr<PropItem> TypeInState::TakeClearProperty() {
  * Caller assumes ownership of PropItem and must delete it.
  */
 UniquePtr<PropItem> TypeInState::TakeSetProperty() {
-  size_t count = mSetArray.Length();
-  if (!count) {
-    return nullptr;
-  }
-  count--;  // indices are zero based
-  PropItem* propItem = mSetArray[count];
-  mSetArray.RemoveElementAt(count);
-  return UniquePtr<PropItem>(propItem);
+  return mSetArray.Length() ? UniquePtr<PropItem>{mSetArray.PopLastElement()}
+                            : nullptr;
 }
 
 /**

gfx/layers/ImageContainer.cpp

@@ -84,10 +84,7 @@ UniquePtr<uint8_t[]> BufferRecycleBin::GetBuffer(uint32_t aSize) {
     return UniquePtr<uint8_t[]>(new (fallible) uint8_t[aSize]);
   }
 
-  uint32_t last = mRecycledBuffers.Length() - 1;
-  UniquePtr<uint8_t[]> result = std::move(mRecycledBuffers[last]);
-  mRecycledBuffers.RemoveElementAt(last);
-  return result;
+  return mRecycledBuffers.PopLastElement();
 }
 
 void BufferRecycleBin::ClearRecycledBuffers() {

gfx/layers/LayerSorter.cpp

@@ -301,12 +301,9 @@ void SortLayersBy3DZOrder(nsTArray<Layer*>& aLayers) {
   // and remove edges from it.
   do {
     if (!noIncoming.IsEmpty()) {
-      uint32_t last = noIncoming.Length() - 1;
-
-      Layer* layer = noIncoming.ElementAt(last);
+      Layer* layer = noIncoming.PopLastElement();
       MOZ_ASSERT(layer);  // don't let null layer pointers sneak into sortedList
 
-      noIncoming.RemoveElementAt(last);
       sortedList.AppendElement(layer);
 
       nsTArray<DirectedGraph<Layer*>::Edge> outgoing;

gfx/layers/wr/IpcResourceUpdateQueue.cpp

@@ -72,11 +72,9 @@ layers::OffsetRange ShmSegmentsWriter::Write(Range<uint8_t> aBytes) {
       if (!AllocChunk()) {
         // Allocation failed, so roll back to the state at the start of this
         // Write() call and abort.
-        for (size_t i = mSmallAllocs.Length(); currAllocLen < i; i--) {
-          MOZ_ASSERT(i > 0);
-          RefCountedShmem& shm = mSmallAllocs.ElementAt(i - 1);
+        while (mSmallAllocs.Length() > currAllocLen) {
+          RefCountedShmem shm = mSmallAllocs.PopLastElement();
           RefCountedShm::Dealloc(mShmAllocator, shm);
-          mSmallAllocs.RemoveElementAt(i - 1);
         }
         MOZ_ASSERT(mSmallAllocs.Length() == currAllocLen);
         return layers::OffsetRange(0, start, 0);

layout/base/PresShell.cpp

@@ -607,11 +607,10 @@ void PresShell::DirtyRootsList::Remove(nsIFrame* aFrame) {
 nsIFrame* PresShell::DirtyRootsList::PopShallowestRoot() {
   // List is sorted in order of decreasing depth, so there are no deeper
   // frames than the last one.
-  const FrameAndDepth& lastFAD = mList.LastElement();
+  const FrameAndDepth& lastFAD = mList.PopLastElement();
   nsIFrame* frame = lastFAD.mFrame;
   // We don't expect frame to change depths.
   MOZ_ASSERT(frame->GetDepthInFrameTree() == lastFAD.mDepth);
-  mList.RemoveLastElement();
   return frame;
 }
 

layout/base/nsBidiPresUtils.cpp

@@ -453,7 +453,7 @@ struct MOZ_STACK_CLASS BidiParagraphData {
     MOZ_ASSERT(mEmbeddingStack.Length(), "embedding/override underflow");
     MOZ_ASSERT(aCh == mEmbeddingStack.LastElement());
     AppendPopChar(aCh);
-    mEmbeddingStack.TruncateLength(mEmbeddingStack.Length() - 1);
+    mEmbeddingStack.RemoveLastElement();
   }
 
   void ClearBidiControls() {

layout/base/nsPresArena.cpp

@@ -27,12 +27,10 @@ template <size_t ArenaSize, typename ObjectId, size_t ObjectIdCount>
 nsPresArena<ArenaSize, ObjectId, ObjectIdCount>::~nsPresArena() {
 #if defined(MOZ_HAVE_MEM_CHECKS)
   for (FreeList* entry = mFreeLists; entry != ArrayEnd(mFreeLists); ++entry) {
-    nsTArray<void*>::index_type len;
-    while ((len = entry->mEntries.Length())) {
-      void* result = entry->mEntries.ElementAt(len - 1);
-      entry->mEntries.RemoveElementAt(len - 1);
+    for (void* result : entry->mEntries) {
       MOZ_MAKE_MEM_UNDEFINED(result, entry->mEntrySize);
     }
+    entry->mEntries.Clear();
   }
 #endif
 }

layout/style/ServoStyleSet.cpp

@@ -981,10 +981,7 @@ bool ServoStyleSet::EnsureUniqueInnerOnCSSSheets() {
   });
 
   while (!queue.IsEmpty()) {
-    uint32_t idx = queue.Length() - 1;
-    auto* sheet = queue[idx].first;
-    SheetOwner owner = queue[idx].second;
-    queue.RemoveElementAt(idx);
+    auto [sheet, owner] = queue.PopLastElement();
 
     // Only call EnsureUniqueInner for complete sheets. If we do call it on
     // incomplete sheets, we'll cause problems when the sheet is actually

layout/svg/SVGTextFrame.cpp

@@ -1627,7 +1627,7 @@ nsTextFrame* TextFrameIterator::Next() {
           mCurrentPosition -= mCurrentFrame->GetPosition();
           if (mCurrentFrame->GetContent()->IsSVGElement(nsGkAtoms::textPath)) {
             // Pop off the <textPath> frame if this is a <textPath>.
-            mTextPathFrames.TruncateLength(mTextPathFrames.Length() - 1);
+            mTextPathFrames.RemoveLastElement();
           }
           // Pop off the current baseline.
           PopBaseline();
@@ -1674,7 +1674,7 @@ void TextFrameIterator::PushBaseline(nsIFrame* aNextFrame) {
 
 void TextFrameIterator::PopBaseline() {
   NS_ASSERTION(!mBaselines.IsEmpty(), "popped too many baselines");
-  mBaselines.TruncateLength(mBaselines.Length() - 1);
+  mBaselines.RemoveLastElement();
 }
 
 // -----------------------------------------------------------------------------

layout/tables/nsTableFrame.cpp

@@ -570,7 +570,7 @@ void nsTableFrame::InsertCol(nsTableColFrame& aColFrame, int32_t aColIndex) {
           nsTableColType lastColType = lastCol->GetColType();
           if (eColAnonymousCell == lastColType) {
             // remove the col from the cache
-            mColFrames.RemoveElementAt(numCacheCols - 1);
+            mColFrames.RemoveLastElement();
             // remove the col from the synthetic col group
             nsTableColGroupFrame* lastColGroup =
                 (nsTableColGroupFrame*)mColGroups.LastChild();

netwerk/cache2/CacheIndexIterator.cpp

@@ -36,8 +36,7 @@ nsresult CacheIndexIterator::GetNextHash(SHA1Sum::Hash* aHash) {
     return mStatus;
   }
 
-  memcpy(aHash, mRecords[mRecords.Length() - 1]->mHash, sizeof(SHA1Sum::Hash));
-  mRecords.RemoveLastElement();
+  memcpy(aHash, mRecords.PopLastElement()->mHash, sizeof(SHA1Sum::Hash));
 
   return NS_OK;
 }

netwerk/protocol/http/nsHttpChannel.cpp

@@ -9035,10 +9035,10 @@ void nsHttpChannel::PushRedirectAsyncFunc(nsContinueRedirectionFunc func) {
 }
 
 void nsHttpChannel::PopRedirectAsyncFunc(nsContinueRedirectionFunc func) {
-  MOZ_ASSERT(func == mRedirectFuncStack[mRedirectFuncStack.Length() - 1],
+  MOZ_ASSERT(func == mRedirectFuncStack.LastElement(),
              "Trying to pop wrong method from redirect async stack!");
 
-  mRedirectFuncStack.TruncateLength(mRedirectFuncStack.Length() - 1);
+  mRedirectFuncStack.RemoveLastElement();
 }
 
 //-----------------------------------------------------------------------------

netwerk/protocol/http/nsHttpConnectionMgr.cpp

@@ -728,7 +728,7 @@ HttpConnectionBase* nsHttpConnectionMgr::FindCoalescableConnectionByHashKey(
         listOfWeakConns->Elements()[j] =
             listOfWeakConns->Elements()[listLen - 1];
       }
-      listOfWeakConns->RemoveElementAt(listLen - 1);
+      listOfWeakConns->RemoveLastElement();
       MOZ_ASSERT(listOfWeakConns->Length() == listLen - 1);
       listLen--;
       continue;  // without adjusting iterator

toolkit/components/commandlines/nsCommandLine.cpp

@@ -96,9 +96,7 @@ nsCommandLine::RemoveArguments(int32_t aStart, int32_t aEnd) {
   NS_ENSURE_ARG_MIN(aStart, 0);
   NS_ENSURE_ARG_MAX(uint32_t(aEnd) + 1, mArgs.Length());
 
-  for (int32_t i = aEnd; i >= aStart; --i) {
-    mArgs.RemoveElementAt(i);
-  }
+  mArgs.RemoveElementsAt(mArgs.begin() + aStart, mArgs.begin() + aEnd + 1);
 
   return NS_OK;
 }

toolkit/components/reputationservice/ApplicationReputation.cpp

@@ -990,12 +990,10 @@ nsresult PendingLookup::LookupNext() {
   // If a url is in blocklist we should call PendingLookup::OnComplete directly.
   MOZ_ASSERT(mBlocklistCount == 0);
 
-  int index = mAnylistSpecs.Length() - 1;
   nsCString spec;
-  if (index >= 0) {
+  if (!mAnylistSpecs.IsEmpty()) {
     // Check the source URI only.
-    spec = mAnylistSpecs[index];
-    mAnylistSpecs.RemoveElementAt(index);
+    spec = mAnylistSpecs.PopLastElement();
     RefPtr<PendingDBLookup> lookup(new PendingDBLookup(this));
 
     // We don't need to check whitelist if the file is not a binary file.
@@ -1004,11 +1002,9 @@ nsresult PendingLookup::LookupNext() {
     return lookup->LookupSpec(spec, type);
   }
 
-  index = mBlocklistSpecs.Length() - 1;
-  if (index >= 0) {
+  if (!mBlocklistSpecs.IsEmpty()) {
     // Check the referrer and redirect chain.
-    spec = mBlocklistSpecs[index];
-    mBlocklistSpecs.RemoveElementAt(index);
+    spec = mBlocklistSpecs.PopLastElement();
     RefPtr<PendingDBLookup> lookup(new PendingDBLookup(this));
     return lookup->LookupSpec(spec, LookupType::BlocklistOnly);
   }
@@ -1024,11 +1020,9 @@ nsresult PendingLookup::LookupNext() {
   MOZ_ASSERT_IF(!mIsBinaryFile, mAllowlistSpecs.Length() == 0);
 
   // Only binary signatures remain.
-  index = mAllowlistSpecs.Length() - 1;
-  if (index >= 0) {
-    spec = mAllowlistSpecs[index];
+  if (!mAllowlistSpecs.IsEmpty()) {
+    spec = mAllowlistSpecs.PopLastElement();
     LOG(("PendingLookup::LookupNext: checking %s on allowlist", spec.get()));
-    mAllowlistSpecs.RemoveElementAt(index);
     RefPtr<PendingDBLookup> lookup(new PendingDBLookup(this));
     return lookup->LookupSpec(spec, LookupType::AllowlistOnly);
   }

toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp

@@ -540,7 +540,7 @@ nsresult nsUrlClassifierPrefixSet::WritePrefixes(
       totalDeltas += deltaLength;
       indexStarts.AppendElement(totalDeltas);
     }
-    indexStarts.RemoveElementAt(indexSize);  // we don't use the last element
+    indexStarts.RemoveLastElement();  // we don't use the last element
     MOZ_ASSERT(indexStarts.Length() == indexSize);
   }
 

widget/cocoa/nsCocoaUtils.mm

@@ -1421,8 +1421,7 @@ void nsCocoaUtils::ResolveMediaCapturePromises(bool aGranted, PromiseArray& aPro
 
   // Remove each promise from the list and resolve it.
   while (aPromiseList->Length() > 0) {
-    RefPtr<Promise> promise = aPromiseList->LastElement();
-    aPromiseList->RemoveLastElement();
+    RefPtr<Promise> promise = aPromiseList->PopLastElement();
 
     // Resolve on main thread
     nsCOMPtr<nsIRunnable> runnable(NS_NewRunnableFunction(

widget/gtk/WindowSurfaceWayland.cpp

@@ -969,19 +969,16 @@ void WindowSurfaceWayland::CacheImageSurface(
   WindowImageSurface surf = WindowImageSurface(mImageSurface, aRegion);
 
   if (mDelayedImageCommits.Length()) {
-    int lastSurf = mDelayedImageCommits.Length() - 1;
-    if (surf.OverlapsSurface(mDelayedImageCommits[lastSurf])) {
+    auto lastSurf = mDelayedImageCommits.PopLastElement();
+    if (surf.OverlapsSurface(lastSurf)) {
 #ifdef MOZ_LOGGING
       {
-        gfx::IntRect size = mDelayedImageCommits[lastSurf]
-                                .GetUpdateRegion()
-                                ->GetBounds()
-                                .ToUnknownRect();
+        gfx::IntRect size =
+            lastSurf.GetUpdateRegion()->GetBounds().ToUnknownRect();
         LOGWAYLAND(("    removing [ %d, %d] -> [%d x %d]\n", size.x, size.y,
                     size.width, size.height));
       }
 #endif
-      mDelayedImageCommits.RemoveElementAt(lastSurf);
     }
   }
 

xpcom/base/nsMemoryReporterManager.cpp

@@ -2046,9 +2046,8 @@ void nsMemoryReporterManager::EndProcessReport(uint32_t aGeneration,
   while (s->mNumProcessesRunning < s->mConcurrencyLimit &&
          !s->mChildrenPending.IsEmpty()) {
     // Pop last element from s->mChildrenPending
-    RefPtr<MemoryReportingProcess> nextChild;
-    nextChild.swap(s->mChildrenPending.LastElement());
-    s->mChildrenPending.TruncateLength(s->mChildrenPending.Length() - 1);
+    const RefPtr<MemoryReportingProcess> nextChild =
+        s->mChildrenPending.PopLastElement();
     // Start report (if the child is still alive).
     if (StartChildReport(nextChild, s)) {
       ++s->mNumProcessesRunning;

xpcom/ds/nsExpirationTracker.h

@@ -189,12 +189,13 @@ class ExpirationTrackerImpl {
     MOZ_ASSERT(generation.Length() > index && generation[index] == aObj,
                "Object is lying about its index");
     // Move the last object to fill the hole created by removing aObj
-    uint32_t last = generation.Length() - 1;
-    T* lastObj = generation[last];
-    generation[index] = lastObj;
+    T* lastObj = generation.PopLastElement();
+    // XXX It looks weird that index might point to the element that was just
+    // removed. Is that really correct?
+    if (index < generation.Length()) {
+      generation[index] = lastObj;
+    }
     lastObj->GetExpirationState()->mIndexInGeneration = index;
-    generation.RemoveElementAt(last);
-    MOZ_ASSERT(generation.Length() == last);
     state->mGeneration = nsExpirationState::NOT_TRACKED;
     // We do not check whether we need to stop the timer here. The timer
     // will check that itself next time it fires. Checking here would not

xpcom/ds/nsTPriorityQueue.h

@@ -93,11 +93,15 @@ class nsTPriorityQueue {
    */
   T Pop() {
     MOZ_ASSERT(!mElements.IsEmpty(), "Empty queue");
-    T pop = mElements[0];
+    T pop = std::move(mElements[0]);
+
+    if (mElements.Length() == 1) {
+      mElements.Clear();
+      return pop;
+    }
 
     // Move last to front
-    mElements[0] = mElements[mElements.Length() - 1];
-    mElements.TruncateLength(mElements.Length() - 1);
+    mElements[0] = mElements.PopLastElement();
 
     // Sift down
     size_type i = 0;