--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -13,16 +13,17 @@
 #include "nsDOMTokenList.h"
 #include "nsIDOMNode.h"
 #include "nsIDocument.h"
 #include "nsINode.h"
 #include "nsIPrincipal.h"
 #include "nsNameSpaceManager.h"
 #include "nsString.h"
 #include "nsStyleStruct.h"
+#include "nsTArray.h"
 #include "nsStyleUtil.h"
 #include "StyleStructContext.h"
 
 #include "mozilla/EventStates.h"
 #include "mozilla/dom/Element.h"
 
 uint32_t
 Gecko_ChildrenCount(RawGeckoNode* aNode)
@@ -555,16 +556,22 @@ Gecko_CreateGradient(uint8_t aShape,
 
   for (uint32_t i = 0; i < aStopCount; i++) {
     result->mStops.AppendElement(dummyStop);
   }
 
   return result;
 }
 
+void
+Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity, size_t aElemSize) {
+  auto base = reinterpret_cast<nsTArray_base<nsTArrayInfallibleAllocator, nsTArray_CopyWithMemutils> *>(aArray);
+  base->EnsureCapacity<nsTArrayInfallibleAllocator>(aCapacity, aElemSize);
+}
+
 #define STYLE_STRUCT(name, checkdata_cb)                                      \
                                                                               \
 void                                                                          \
 Gecko_Construct_nsStyle##name(nsStyle##name* ptr)                             \
 {                                                                             \
   new (ptr) nsStyle##name(StyleStructContext::ServoContext());                \
 }                                                                             \
                                                                               \

--- a/layout/style/ServoBindings.h
+++ b/layout/style/ServoBindings.h
@@ -176,16 +176,21 @@ void Gecko_SetMozBinding(nsStyleDisplay*
                          ThreadSafePrincipalHolder* principal);
 void Gecko_CopyMozBindingFrom(nsStyleDisplay* des, const nsStyleDisplay* src);
 
 // Dirtiness tracking.
 uint32_t Gecko_GetNodeFlags(RawGeckoNode* node);
 void Gecko_SetNodeFlags(RawGeckoNode* node, uint32_t flags);
 void Gecko_UnsetNodeFlags(RawGeckoNode* node, uint32_t flags);
 
+// `array` must be an nsTArray
+// If changing this signature, please update the
+// friend function declaration in nsTArray.h
+void Gecko_EnsureTArrayCapacity(void* array, size_t capacity, size_t elem_size);
+
 // Styleset and Stylesheet management.
 //
 // TODO: Make these return already_AddRefed and UniquePtr when the binding
 // generator is smart enough to handle them.
 RawServoStyleSheet* Servo_StylesheetFromUTF8Bytes(
     const uint8_t* bytes, uint32_t length,
     mozilla::css::SheetParsingMode parsing_mode,
     ThreadSafeURIHolder* base,

--- a/xpcom/glue/nsTArray.h
+++ b/xpcom/glue/nsTArray.h
@@ -331,29 +331,32 @@ struct nsTArray_SafeElementAtHelper<mozi
   {
     if (aIndex < static_cast<const Derived*>(this)->Length()) {
       return static_cast<const Derived*>(this)->ElementAt(aIndex);
     }
     return nullptr;
   }
 };
 
+extern "C" void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity, size_t aElemSize);
+
 //
 // This class serves as a base class for nsTArray.  It shouldn't be used
 // directly.  It holds common implementation code that does not depend on the
 // element type of the nsTArray.
 //
 template<class Alloc, class Copy>
 class nsTArray_base
 {
   // Allow swapping elements with |nsTArray_base|s created using a
   // different allocator.  This is kosher because all allocators use
   // the same free().
   template<class Allocator, class Copier>
   friend class nsTArray_base;
+  friend void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity, size_t aElemSize);
 
 protected:
   typedef nsTArrayHeader Header;
 
 public:
   typedef size_t size_type;
   typedef size_t index_type;