[Impeller] Dont synthesize fake index buffer if none was provided to VBB (#41969)

Followup from flutter/flutter#126572

Author: Jonah Williams

impeller/renderer/renderer_unittests.cc

@@ -1007,9 +1007,18 @@ TEST_P(RendererTest, DefaultIndexSize) {
   // Default to 16bit index buffer size, as this is a reasonable default and
   // supported on all backends without extensions.
   VertexBufferBuilder<VS::PerVertexData> vertex_builder;
+  vertex_builder.AppendIndex(0u);
   ASSERT_EQ(vertex_builder.GetIndexType(), IndexType::k16bit);
 }
 
+TEST_P(RendererTest, DefaultIndexBehavior) {
+  using VS = BoxFadeVertexShader;
+
+  // Do not create any index buffer if no indices were provided.
+  VertexBufferBuilder<VS::PerVertexData> vertex_builder;
+  ASSERT_EQ(vertex_builder.GetIndexType(), IndexType::kNone);
+}
+
 TEST_P(RendererTest, VertexBufferBuilder) {
   // Does not create index buffer if one is provided.
   using VS = BoxFadeVertexShader;

impeller/renderer/vertex_buffer_builder.h

@@ -30,13 +30,16 @@ class VertexBufferBuilder {
   ~VertexBufferBuilder() = default;
 
   constexpr impeller::IndexType GetIndexType() const {
+    if (indices_.size() == 0) {
+      return impeller::IndexType::kNone;
+    }
     if constexpr (sizeof(IndexType) == 2) {
       return impeller::IndexType::k16bit;
-    } else if (sizeof(IndexType) == 4) {
+    }
+    if (sizeof(IndexType) == 4) {
       return impeller::IndexType::k32bit;
-    } else {
-      return impeller::IndexType::kUnknown;
     }
+    return impeller::IndexType::kUnknown;
   }
 
   void SetLabel(std::string label) { label_ = std::move(label); }
@@ -121,29 +124,23 @@ class VertexBufferBuilder {
     return buffer->AsBufferView();
   }
 
-  std::vector<IndexType> CreateIndexBuffer() const {
-    if (indices_.size() > 0) {
-      return indices_;
-    }
-
-    // So dumb! We don't actually need an index buffer right now. But we will
-    // once de-duplication is done. So assume this is always done.
-    std::vector<IndexType> index_buffer;
-    for (size_t i = 0; i < vertices_.size(); i++) {
-      index_buffer.push_back(i);
-    }
-    return index_buffer;
-  }
+  std::vector<IndexType> CreateIndexBuffer() const { return indices_; }
 
   BufferView CreateIndexBufferView(HostBuffer& buffer) const {
     const auto index_buffer = CreateIndexBuffer();
+    if (index_buffer.size() == 0) {
+      return {};
+    }
     return buffer.Emplace(index_buffer.data(),
                           index_buffer.size() * sizeof(IndexType),
                           alignof(IndexType));
   }
 
   BufferView CreateIndexBufferView(Allocator& allocator) const {
     const auto index_buffer = CreateIndexBuffer();
+    if (index_buffer.size() == 0) {
+      return {};
+    }
     auto buffer = allocator.CreateBufferWithCopy(
         reinterpret_cast<const uint8_t*>(index_buffer.data()),
         index_buffer.size() * sizeof(IndexType));