Bug 1292618: Add Gecko_ClearPODTArray to clear arrays of types without destructors. r=heycam

MozReview-Commit-ID: 3RRvrXn6ZTD

layout/style/ServoBindings.cpp

@@ -709,10 +709,24 @@ Gecko_CreateGradient(uint8_t aShape,
 void
 Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity, size_t aElemSize)
 {
-  auto base = reinterpret_cast<nsTArray_base<nsTArrayInfallibleAllocator, nsTArray_CopyWithMemutils> *>(aArray);
+  auto base =
+    reinterpret_cast<nsTArray_base<nsTArrayInfallibleAllocator,
+                                   nsTArray_CopyWithMemutils> *>(aArray);
+
   base->EnsureCapacity<nsTArrayInfallibleAllocator>(aCapacity, aElemSize);
 }
 
+void
+Gecko_ClearPODTArray(void* aArray, size_t aElementSize, size_t aElementAlign)
+{
+  auto base =
+    reinterpret_cast<nsTArray_base<nsTArrayInfallibleAllocator,
+                                   nsTArray_CopyWithMemutils> *>(aArray);
+
+  base->template ShiftData<nsTArrayInfallibleAllocator>(0, base->Length(), 0,
+                                                        aElementSize, aElementAlign);
+}
+
 void
 Gecko_EnsureImageLayersLength(nsStyleImageLayers* aLayers, size_t aLen)
 {

layout/style/ServoBindings.h

@@ -225,6 +225,11 @@ void Gecko_StoreStyleDifference(RawGeckoNode* node, nsChangeHint change);
 // friend function declaration in nsTArray.h
 void Gecko_EnsureTArrayCapacity(void* array, size_t capacity, size_t elem_size);
 
+// Same here, `array` must be an nsTArray<T>, for some T.
+//
+// Important note: Only valid for POD types, since destructors won't be run
+// otherwise. This is ensured with rust traits for the relevant structs.
+void Gecko_ClearPODTArray(void* array, size_t elem_size, size_t elem_align);
 
 void Gecko_EnsureImageLayersLength(nsStyleImageLayers* layers, size_t len);
 

xpcom/glue/nsTArray.h

@@ -328,7 +328,13 @@ struct nsTArray_SafeElementAtHelper<mozilla::OwningNonNull<E>, Derived>
   }
 };
 
-extern "C" void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity, size_t aElemSize);
+// Servo bindings.
+extern "C" void Gecko_EnsureTArrayCapacity(void* aArray,
+                                           size_t aCapacity,
+                                           size_t aElementSize);
+extern "C" void Gecko_ClearPODTArray(void* aArray,
+                                     size_t aElementSize,
+                                     size_t aElementAlign);
 
 MOZ_NORETURN MOZ_COLD void
 InvalidArrayIndex_CRASH(size_t aIndex, size_t aLength);
@@ -346,7 +352,10 @@ class nsTArray_base
   // the same free().
   template<class Allocator, class Copier>
   friend class nsTArray_base;
-  friend void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity, size_t aElemSize);
+  friend void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity,
+                                         size_t aElemSize);
+  friend void Gecko_ClearPODTArray(void* aTArray, size_t aElementSize,
+                                   size_t aElementAlign);
 
 protected:
   typedef nsTArrayHeader Header;