Bug 1559546 - Introduce nsAtom::IsEmpty. r=njn

Both for symmetry with other string APIs, and also to prevent footguns (since I
debugged for a while a typo where I wrote nsGkAtoms::empty rather than
nsGkAtoms::_empty).

We could use null here, but that will not be possible in the future when I use
the rust representation of more grid data structures (at least without
increasing memory usage).

So I think I'll keep using ::_empty as a signaling value for "no grid
identifier".

Differential Revision: https://phabricator.services.mozilla.com/D35120

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

layout/generic/nsGridContainerFrame.cpp

@@ -944,7 +944,7 @@ class MOZ_STACK_CLASS nsGridContainerFrame::LineNameMap {
   uint32_t FindNamedLine(nsAtom* aName, int32_t* aNth, uint32_t aFromIndex,
                          const nsTArray<uint32_t>& aImplicitLines) const {
     MOZ_ASSERT(aName);
-    MOZ_ASSERT(aName != nsGkAtoms::_empty);
+    MOZ_ASSERT(!aName->IsEmpty());
     MOZ_ASSERT(aNth && *aNth != 0);
     if (*aNth > 0) {
       return FindLine(aName, aNth, aFromIndex, aImplicitLines);
@@ -3385,7 +3385,7 @@ int32_t nsGridContainerFrame::Grid::ResolveLine(
     const nsStylePosition* aStyle) {
   MOZ_ASSERT(!aLine.IsAuto());
   int32_t line = 0;
-  if (aLine.mLineName == nsGkAtoms::_empty) {
+  if (aLine.mLineName->IsEmpty()) {
     MOZ_ASSERT(aNth != 0, "css-grid 9.2: <integer> must not be zero.");
     line = int32_t(aFromIndex) + aNth;
   } else {
@@ -3460,7 +3460,7 @@ nsGridContainerFrame::Grid::ResolveLineRangeHelper(
   if (aStart.mHasSpan) {
     if (aEnd.mHasSpan || aEnd.IsAuto()) {
       // http://dev.w3.org/csswg/css-grid/#grid-placement-errors
-      if (aStart.mLineName == nsGkAtoms::_empty) {
+      if (aStart.mLineName->IsEmpty()) {
         // span <integer> / span *
         // span <integer> / auto
         return LinePair(kAutoLine, aStart.mInteger);
@@ -3494,7 +3494,7 @@ nsGridContainerFrame::Grid::ResolveLineRangeHelper(
       return LinePair(start, 1);  // XXX subgrid explicit size instead of 1?
     }
     if (aEnd.mHasSpan) {
-      if (aEnd.mLineName == nsGkAtoms::_empty) {
+      if (aEnd.mLineName->IsEmpty()) {
         // auto / span <integer>
         MOZ_ASSERT(aEnd.mInteger != 0);
         return LinePair(start, aEnd.mInteger);
@@ -3520,7 +3520,7 @@ nsGridContainerFrame::Grid::ResolveLineRangeHelper(
   int32_t nth = aEnd.mInteger == 0 ? 1 : aEnd.mInteger;
   if (aEnd.mHasSpan) {
     if (MOZ_UNLIKELY(start < 0)) {
-      if (aEnd.mLineName == nsGkAtoms::_empty) {
+      if (aEnd.mLineName->IsEmpty()) {
         return LinePair(start, start + nth);
       }
       from = 0;

xpcom/ds/nsAtom.h

@@ -75,6 +75,13 @@ class nsAtom {
   // unchanged.
   bool IsAsciiLowercase() const { return mIsAsciiLowercase; }
 
+  // This function returns true if this is the empty atom. This is exactly
+  // equivalent to `this == nsGkAtoms::_empty`, but it's a bit less foot-gunny,
+  // since we also have `nsGkAtoms::empty`.
+  //
+  // Defined in nsGkAtoms.h
+  inline bool IsEmpty() const;
+
   // We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
   // of this type is special.
   inline MozExternalRefCountType AddRef();

xpcom/ds/nsGkAtoms.h

@@ -180,4 +180,8 @@ class nsGkAtoms {
 #undef GK_ATOM
 };
 
+inline bool nsAtom::IsEmpty() const {
+  return this == nsGkAtoms::_empty;
+}
+
 #endif /* nsGkAtoms_h___ */