diff -r b029d63565a2 -r c1ba166f3fc4 src/Structures.cpp
--- a/src/Structures.cpp	Mon Sep 17 00:10:01 2012 -0700
+++ b/src/Structures.cpp	Sat Nov 17 18:41:01 2012 -0800
@@ -51,7 +51,7 @@
 }
 
 Descriptor *
-Descriptor::New(Zone *zone, Handle<String> name, Handle<Type> type, unsigned offset)
+Descriptor::New(Zone *zone, Handle<String> name, Handle<Type> type)
 {
     Local<Descriptor> desc(zone,
         Descriptor::cast(zone->allocate(MapKind_Descriptor, sizeof(Descriptor), Heap::Tenure_Old)));
@@ -60,13 +60,31 @@
 
     desc->name_ = name;
     desc->type_ = type;
-    desc->offset_ = offset;
+    desc->offset_ = unsigned(-1);
     return desc;
 }
 
 StructMap *
-StructMap::New(Zone *zone, const StructureBufferBuilder &builder, Handle<FixedArray> composed)
+StructMap::New(Zone *zone, const StructureBufferBuilder &builder, Handle<StructType> type)
 {
+    Vector<unsigned> composedIndexes;
+    for (size_t i = 0; i < type->fields()->length(); i++) {
+        Descriptor *desc = Descriptor::cast(type->fields()->at(i));
+        if (desc->type()->isArray() || desc->type()->isStruct()) {
+            if (!composedIndexes.append(i))
+                return NULL;
+        }
+    }
+
+    Local<FixedArray> composed(zone, FixedArray::New(zone, composedIndexes.length(), Heap::Tenure_Old));
+    if (!composed)
+        return NULL;
+
+    for (size_t i = 0; i < composedIndexes.length(); i++) {
+        Descriptor *desc = Descriptor::cast(type->fields()->at(composedIndexes[i]));
+        composed->set(zone, i, desc);
+    }
+
     size_t bytesNeeded = sizeof(StructMap) +
                          (builder.traceableOffsetCount() * sizeof(unsigned));
 
@@ -77,7 +95,7 @@
 
     map->structureSize_ = builder.structureSize();
     map->traceOffsetCount_ = builder.traceableOffsetCount();
-    map->type_ = NULL;
+    map->type_ = type;
     map->composed_ = composed;
     memcpy(const_cast<unsigned *>(map->traceOffsets()), builder.offsets(),
            sizeof(unsigned) * builder.traceableOffsetCount());
@@ -107,14 +125,17 @@
             desc = Descriptor::cast(map->composed()->at(i));
 
             if (desc->type()->isStruct()) {
-                childStructMap = StructMap::cast(desc->type()->newMap());
+                childStructMap = StructMap::cast(StructType::cast(desc->type())->newMap());
                 child = Struct::New(zone, childStructMap, tenure);
             } else if (desc->type()->isArray()) {
+                assert(false);
+#if 0
                 childArrayMap = ArrayMap::cast(desc->type()->newMap());
                 if (desc->type()->isFixedLength())
                     child = Array::NewFixed(zone, childArrayMap, tenure);
                 else
                     child = Array::NewEmpty(zone, childArrayMap, tenure);
+#endif
             }
             if (!child)
                 return NULL;