Make a toVariant() function on ArrayInit @override-unit-failures

Suggested by @smith.

hphp/runtime/base/array/array_init.h

@@ -276,12 +276,26 @@ public:
     return *this;
   }
 
+  // Prefer toArray() in new code---it can save a null check when the
+  // compiler can't prove m_data hasn't changed.
   ArrayData *create() {
     ArrayData *ret = m_data;
     m_data = nullptr;
     return ret;
   }
 
+  Array toArray() {
+    auto ptr = m_data;
+    m_data = nullptr;
+    return Array(ptr, Array::ArrayInitCtor::Tag);
+  }
+
+  Variant toVariant() {
+    auto ptr = m_data;
+    m_data = nullptr;
+    return Variant(ptr, Variant::ArrayInitCtor::Tag);
+  }
+
   static ArrayData *CreateParams(int count, ...);
 
 private:

hphp/runtime/base/type_array.h

@@ -82,6 +82,15 @@ class Array : protected SmartPtr<ArrayData> {
   /* implicit */ Array(ArrayData *data) : ArrayBase(data) { }
   /* implicit */ Array(CArrRef arr) : ArrayBase(arr.m_px) { }
 
+  /*
+   * Special constructor for use from ArrayInit that creates an Array
+   * without a null check.
+   */
+  enum class ArrayInitCtor { Tag };
+  explicit Array(ArrayData* ad, ArrayInitCtor)
+    : ArrayBase(ad, ArrayBase::NonNull::Tag)
+  {}
+
   // Move ctor
   Array(Array&& src) : ArrayBase(std::move(src)) {
     static_assert(sizeof(Array) == sizeof(ArrayBase), "Fix this.");

hphp/runtime/base/type_variant.h

@@ -188,6 +188,16 @@ class Variant : private TypedValue {
   Variant(const Variant *v) = delete;
   Variant(Variant *v) = delete;
 
+  /*
+   * Creation constructor from ArrayInit that avoids a null check.
+   */
+  enum class ArrayInitCtor { Tag };
+  explicit Variant(ArrayData* ad, ArrayInitCtor) {
+    m_type = KindOfArray;
+    m_data.parr = ad;
+    ad->incRefCount();
+  }
+
 #ifdef INLINE_VARIANT_HELPER
   inline ALWAYS_INLINE Variant(CVarRef v) { constructValHelper(v); }
   inline ALWAYS_INLINE

hphp/runtime/base/util/smart_ptr.h

@@ -53,6 +53,12 @@ public:
     if (m_px) m_px->incRefCount();
   }
 
+  enum class NonNull { Tag };
+  explicit SmartPtr(T* px, NonNull) : m_px(px) {
+    assert(px);
+    m_px->incRefCount();
+  }
+
   // Move ctor
   SmartPtr(SmartPtr&& src) : m_px(src.get()) {
     src.m_px = nullptr;

hphp/runtime/vm/bytecode.cpp

@@ -2309,12 +2309,10 @@ Array VMExecutionContext::debugBacktrace(bool skip /* = false */,
   // the backtrace
   if (parserFrame) {
     bt.append(
-      Array(
-        ArrayInit(2)
-          .set(s_file, parserFrame->filename, true)
-          .set(s_line, parserFrame->lineNumber, true)
-          .create()
-      )
+      ArrayInit(2)
+        .set(s_file, parserFrame->filename, true)
+        .set(s_line, parserFrame->lineNumber, true)
+        .toVariant()
     );
   }
 
@@ -2360,7 +2358,7 @@ Array VMExecutionContext::debugBacktrace(bool skip /* = false */,
           frame.set(s_function, s_include, true);
           frame.set(s_args, Array::Create(parserFrame->filename), true);
         }
-        bt.append(Array(frame.create()));
+        bt.append(frame.toVariant());
         depth++;
       }
     }
@@ -2479,7 +2477,7 @@ Array VMExecutionContext::debugBacktrace(bool skip /* = false */,
       frame.set(s_args, args, true);
     }
 
-    bt.append(Array(frame.create()));
+    bt.append(frame.toVariant());
     depth++;
   }
   return bt;