diff --git a/js/public/HeapAPI.h b/js/public/HeapAPI.h
index 473ef8414bbe..71da9e7a4d38 100644
--- a/js/public/HeapAPI.h
+++ b/js/public/HeapAPI.h
@@ -42,6 +42,14 @@ const size_t CellAlignMask = CellAlignBytes - 1;
 
 const size_t CellBytesPerMarkBit = CellAlignBytes;
 
+/*
+ * We sometimes use an index to refer to a cell in an arena. The index for a
+ * cell is found by dividing by the cell alignment so not all indicies refer to
+ * valid cells.
+ */
+const size_t ArenaCellIndexBytes = CellAlignBytes;
+const size_t MaxArenaCellIndex = ArenaSize / CellAlignBytes;
+
 /* These are magic constants derived from actual offsets in gc/Heap.h. */
 #ifdef JS_GC_SMALL_CHUNK_SIZE
 const size_t ChunkMarkBitmapOffset = 258104;
diff --git a/js/src/gc/Heap.h b/js/src/gc/Heap.h
index 8272e4364dff..85cb733102f7 100644
--- a/js/src/gc/Heap.h
+++ b/js/src/gc/Heap.h
@@ -357,14 +357,6 @@ DivideAndRoundUp(size_t numerator, size_t divisor) {
 static_assert(ArenaSize % CellAlignBytes == 0,
               "Arena size must be a multiple of cell alignment");
 
-/*
- * We sometimes use an index to refer to a cell in an arena. The index for a
- * cell is found by dividing by the cell alignment so not all indicies refer to
- * valid cells.
- */
-const size_t ArenaCellIndexBytes = CellAlignBytes;
-const size_t MaxArenaCellIndex = ArenaSize / CellAlignBytes;
-
 /*
  * The mark bitmap has one bit per each possible cell start position. This
  * wastes some space for larger GC things but allows us to avoid division by the