Bug 1527519 Part 1 - Make do_QueryFrame more const-friendly, and mark nsIFrame::IsBlockFrameOrSubclass() as a const method. r=mats

This patch makes do_QueryFrame() accept const frame pointer e.g.
"const nsIFrame*", and also helps eliminate a few const_cast in Part 3.

Note that the fast path of do_QueryFrame is const-correct, but the slow
path is not (due to nsIFrame::QueryFrame() returns void*).

For example:

```
const nsIFrame* f;
nsBlockFrame* a = do_QueryFrame(f); // fast path, compile error.
nsIAnonymousContentCreator* b = do_QueryFrame(f); // slow path, still compiles.
```

Differential Revision: https://phabricator.services.mozilla.com/D19860

--HG--
extra : moz-landing-system : lando

layout/generic/nsFrame.cpp

@@ -7241,8 +7241,8 @@ bool nsIFrame::IsBlockWrapper() const {
           pseudoType == nsCSSAnonBoxes::cellContent());
 }
 
-bool nsIFrame::IsBlockFrameOrSubclass() {
-  nsBlockFrame* thisAsBlock = do_QueryFrame(this);
+bool nsIFrame::IsBlockFrameOrSubclass() const {
+  const nsBlockFrame* thisAsBlock = do_QueryFrame(this);
   return !!thisAsBlock;
 }
 

layout/generic/nsFrame.h

@@ -100,7 +100,7 @@ enum class TableSelection : uint32_t;
   NS_DECL_QUERYFRAME_TARGET(class)                                             \
   static constexpr nsIFrame::ClassID kClassID = nsIFrame::ClassID::class##_id; \
   void* operator new(size_t, nsIPresShell*) MOZ_MUST_OVERRIDE;                 \
-  nsQueryFrame::FrameIID GetFrameId() override MOZ_MUST_OVERRIDE {             \
+  nsQueryFrame::FrameIID GetFrameId() const override MOZ_MUST_OVERRIDE {       \
     return nsQueryFrame::class##_id;                                           \
   }
 
@@ -111,7 +111,7 @@ enum class TableSelection : uint32_t;
 
 #define NS_DECL_ABSTRACT_FRAME(class)                                   \
   void* operator new(size_t, nsIPresShell*) MOZ_MUST_OVERRIDE = delete; \
-  virtual nsQueryFrame::FrameIID GetFrameId() override MOZ_MUST_OVERRIDE = 0;
+  nsQueryFrame::FrameIID GetFrameId() const override MOZ_MUST_OVERRIDE = 0;
 
 //----------------------------------------------------------------------
 
@@ -155,7 +155,7 @@ class nsFrame : public nsBox {
   // nsQueryFrame
   NS_DECL_QUERYFRAME
   NS_DECL_QUERYFRAME_TARGET(nsFrame)
-  virtual nsQueryFrame::FrameIID GetFrameId() MOZ_MUST_OVERRIDE {
+  virtual nsQueryFrame::FrameIID GetFrameId() const MOZ_MUST_OVERRIDE {
     return kFrameIID;
   }
   void* operator new(size_t, nsIPresShell*) MOZ_MUST_OVERRIDE;

layout/generic/nsIFrame.h

@@ -2883,14 +2883,8 @@ class nsIFrame : public nsQueryFrame {
   /**
    * Returns true if the frame is an instance of nsBlockFrame or one of its
    * subclasses.
-   *
-   * XXXdholbert this is non-const because it uses nsIFrame::QueryFrame which
-   * is non-const.  If we need this accessor to be 'const' down the road, the
-   * right way to do it would be to make the QueryFrame machinery
-   * const-friendly. But it may not be worth the trouble, because we rarely
-   * handle const frame pointers anyway.
    */
-  bool IsBlockFrameOrSubclass();
+  bool IsBlockFrameOrSubclass() const;
 
   /**
    * Get this frame's CSS containing block.

layout/generic/nsQueryFrame.h

@@ -19,17 +19,17 @@
       nsQueryFrame::classname##_id;               \
   typedef classname Has_NS_DECL_QUERYFRAME_TARGET;
 
-#define NS_DECL_QUERYFRAME void* QueryFrame(FrameIID id) override;
+#define NS_DECL_QUERYFRAME void* QueryFrame(FrameIID id) const override;
 
 #define NS_QUERYFRAME_HEAD(class)               \
-  void* class ::QueryFrame(FrameIID id) { \
+  void* class ::QueryFrame(FrameIID id) const { \
     switch (id) {
 #define NS_QUERYFRAME_ENTRY(class)                                            \
   case class ::kFrameIID: {                                                   \
     static_assert(                                                            \
         mozilla::IsSame<class, class ::Has_NS_DECL_QUERYFRAME_TARGET>::value, \
         #class " must declare itself as a queryframe target");                \
-    return static_cast<class*>(this);                                         \
+    return const_cast<class*>(static_cast<const class*>(this));               \
   }
 
 #define NS_QUERYFRAME_ENTRY_CONDITIONAL(class, condition)                \
@@ -39,7 +39,7 @@
           mozilla::IsSame<class,                                         \
                           class ::Has_NS_DECL_QUERYFRAME_TARGET>::value, \
           #class " must declare itself as a queryframe target");         \
-      return static_cast<class*>(this);                                  \
+      return const_cast<class*>(static_cast<const class*>(this));        \
     }                                                                    \
     break;
 
@@ -85,7 +85,7 @@ class nsQueryFrame {
 #undef ABSTRACT_FRAME_ID
   };
 
-  virtual void* QueryFrame(FrameIID id) = 0;
+  virtual void* QueryFrame(FrameIID id) const = 0;
 };
 
 class nsIFrame;
@@ -104,7 +104,7 @@ class do_QueryFrameHelper {
   template <class Dest>
   operator Dest*() {
     static_assert(
-        mozilla::IsSame<Dest,
+        mozilla::IsSame<typename mozilla::RemoveConst<Dest>::Type,
                         typename Dest::Has_NS_DECL_QUERYFRAME_TARGET>::value,
         "Dest must declare itself as a queryframe target");
     if (!mRawPtr) {

layout/mathml/nsMathMLSelectedFrame.h

@@ -11,6 +11,8 @@
 
 class nsMathMLSelectedFrame : public nsMathMLContainerFrame {
  public:
+  NS_DECL_ABSTRACT_FRAME(nsMathMLSelectedFrame)
+
   NS_IMETHOD
   TransmitAutomaticData() override;
 
@@ -35,8 +37,6 @@ class nsMathMLSelectedFrame : public nsMathMLContainerFrame {
                       const ReflowInput& aReflowInput,
                       nsReflowStatus& aStatus) override;
 
-  virtual nsQueryFrame::FrameIID GetFrameId() override = 0;
-
  protected:
   nsMathMLSelectedFrame(ComputedStyle* aStyle, nsPresContext* aPresContext,
                         ClassID aID)
@@ -49,9 +49,6 @@ class nsMathMLSelectedFrame : public nsMathMLContainerFrame {
   nsIFrame* mSelectedFrame;
 
   bool mInvalidMarkup;
-
- private:
-  void* operator new(size_t, nsIPresShell*) MOZ_MUST_OVERRIDE = delete;
 };
 
 #endif /* nsMathMLSelectedFrame_h___ */