Bug 1641085 Part 1 - Add move semantic to nsFrameList, and use it on SetOverflowFrames(). r=mats

It's useful to use `std::move()` to indicate the frames' ownership in one list
is transferred to the another list.

For a frame list managed by AutoFrameListPtr, after moving its frames to
another list, it can be automatically deleted when it is going out of
scope.

Differential Revision: https://phabricator.services.mozilla.com/D88455

layout/forms/nsFieldSetFrame.cpp

@@ -925,7 +925,7 @@ void nsFieldSetFrame::EnsureChildContinuation(nsIFrame* aChild,
       if (nsFrameList* oc = GetOverflowFrames()) {
         oc->AppendFrames(nullptr, nifs);
       } else {
-        SetOverflowFrames(nifs);
+        SetOverflowFrames(std::move(nifs));
       }
     }
   }

layout/generic/nsColumnSetFrame.cpp

@@ -847,9 +847,9 @@ nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowChildren(
       kidNextInFlow->MarkSubtreeDirty();
       // Move any of our leftover columns to our overflow list. Our
       // next-in-flow will eventually pick them up.
-      const nsFrameList& continuationColumns = mFrames.RemoveFramesAfter(child);
+      nsFrameList continuationColumns = mFrames.RemoveFramesAfter(child);
       if (continuationColumns.NotEmpty()) {
-        SetOverflowFrames(continuationColumns);
+        SetOverflowFrames(std::move(continuationColumns));
       }
       child = nullptr;
 

layout/generic/nsContainerFrame.cpp

@@ -1636,7 +1636,7 @@ void nsContainerFrame::PushChildren(nsIFrame* aFromChild,
     nextInFlow->mFrames.InsertFrames(nextInFlow, nullptr, tail);
   } else {
     // Add the frames to our overflow list
-    SetOverflowFrames(tail);
+    SetOverflowFrames(std::move(tail));
   }
 }
 
@@ -2043,7 +2043,7 @@ void nsContainerFrame::MergeSortedOverflow(nsFrameList& aList) {
   if (overflow) {
     MergeSortedFrameLists(*overflow, aList, GetContent());
   } else {
-    SetOverflowFrames(aList);
+    SetOverflowFrames(std::move(aList));
   }
 }
 

layout/generic/nsContainerFrame.h

@@ -564,10 +564,10 @@ class nsContainerFrame : public nsSplittableFrame {
   /**
    * Set the overflow list.  aOverflowFrames must not be an empty list.
    */
-  void SetOverflowFrames(const nsFrameList& aOverflowFrames) {
+  void SetOverflowFrames(nsFrameList&& aOverflowFrames) {
     MOZ_ASSERT(aOverflowFrames.NotEmpty(), "Shouldn't be called");
     SetProperty(OverflowProperty(),
-                new (PresShell()) nsFrameList(aOverflowFrames));
+                new (PresShell()) nsFrameList(std::move(aOverflowFrames)));
   }
 
   /**

layout/generic/nsFirstLetterFrame.cpp

@@ -259,9 +259,9 @@ void nsFirstLetterFrame::Reflow(nsPresContext* aPresContext,
       if (!IsFloating()) {
         CreateNextInFlow(kid);
         // And then push it to our overflow list
-        const nsFrameList& overflow = mFrames.RemoveFramesAfter(kid);
+        nsFrameList overflow = mFrames.RemoveFramesAfter(kid);
         if (overflow.NotEmpty()) {
-          SetOverflowFrames(overflow);
+          SetOverflowFrames(std::move(overflow));
         }
       } else if (!kid->GetNextInFlow()) {
         // For floating first letter frames (if a continuation wasn't already

layout/generic/nsFrameList.h

@@ -80,8 +80,28 @@ class nsFrameList {
     VerifyList();
   }
 
+  // XXX: Ideally, copy constructor should be removed because a frame should be
+  // owned by one list.
   nsFrameList(const nsFrameList& aOther) = default;
 
+  // XXX: ideally, copy assignment should be removed because we should use move
+  // assignment to transfer the ownership.
+  nsFrameList& operator=(const nsFrameList& aOther) = default;
+
+  /**
+   * Move the frames in aOther to this list. aOther becomes empty after this
+   * operation.
+   */
+  nsFrameList(nsFrameList&& aOther)
+      : mFirstChild(aOther.mFirstChild), mLastChild(aOther.mLastChild) {
+    aOther.Clear();
+    VerifyList();
+  }
+  nsFrameList& operator=(nsFrameList&& aOther) {
+    SetFrames(aOther);
+    return *this;
+  }
+
   /**
    * Infallibly allocate a nsFrameList from the shell arena.
    */

layout/generic/nsIFrame.h

@@ -5734,6 +5734,7 @@ template <bool IsLessThanOrEqual(nsIFrame*, nsIFrame*)>
 template <bool IsLessThanOrEqual(nsIFrame*, nsIFrame*)>
 /* static */ void nsIFrame::SortFrameList(nsFrameList& aFrameList) {
   nsIFrame* head = MergeSort<IsLessThanOrEqual>(aFrameList.FirstChild());
+  aFrameList.Clear();
   aFrameList = nsFrameList(head, nsLayoutUtils::GetLastSibling(head));
   MOZ_ASSERT(IsFrameListSorted<IsLessThanOrEqual>(aFrameList),
              "After we sort a frame list, it should be in sorted order...");

layout/tables/nsTableFrame.cpp

@@ -2107,7 +2107,7 @@ void nsTableFrame::PushChildren(const RowGroupArray& aRowGroups,
     nextInFlow->mFrames.InsertFrames(nextInFlow, prevSibling, frames);
   } else {
     // Add the frames to our overflow list.
-    SetOverflowFrames(frames);
+    SetOverflowFrames(std::move(frames));
   }
 }