C++ annotation context (#2948)

### What


* Fixes #2794
* Part of #2919 

Small codegen fix for nested array serialization was required.
Adds example & test & roundtrip test for annotation context to C++

Sneaked in: Fixes a few annoyances with VSCode Cpp/CMake config.

<img width="1016" alt="image"
src="https://github.com/rerun-io/rerun/assets/1220815/31a764bc-cb51-4420-a75f-0da0eb6bfd39">


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x ] I have tested [demo.rerun.io](https://demo.rerun.io/pr/2948) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2948)
- [Docs
preview](https://rerun.io/preview/pr%3Aandreas%2Fcpp%2Fannotation-context/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Aandreas%2Fcpp%2Fannotation-context/examples)

.vscode/settings.json

@@ -79,4 +79,6 @@
         "--config",
         "rerun_py/pyproject.toml"
     ],
+    "cmake.buildDirectory": "${workspaceRoot}/build/",
+    "C_Cpp.autoAddFileAssociations": false
 }

crates/re_types/definitions/rerun/archetypes/annotation_context.fbs

@@ -26,6 +26,13 @@ namespace rerun.archetypes;
 /// \rs ```ignore
 /// \rs \include:../../../../../docs/code-examples/annotation_context_rects.rs
 /// \rs ```
+///
+/// \cpp ## Example
+/// \cpp
+/// \cpp ```
+//  \cpp TODO(#2786): implement Rect2D cpp example and link it here instead.
+/// \cpp \include:../../../../../docs/code-examples/annotation_context_arrows_v2.cpp
+/// \cpp ```
 table AnnotationContext (
   "attr.rust.derive": "Eq, PartialEq",
   order: 100

crates/re_types_builder/src/codegen/cpp/mod.rs

@@ -1304,9 +1304,9 @@ fn quote_append_field_to_builder(
             quote! {
                 auto #value_builder = static_cast<arrow::#value_builder_type*>(#builder->value_builder());
                 #NEWLINE_TOKEN #NEWLINE_TOKEN
+                ARROW_RETURN_NOT_OK(#builder->AppendValues(num_elements));
                 static_assert(sizeof(elements[0].#field_name) == sizeof(elements[0]));
                 ARROW_RETURN_NOT_OK(#value_builder->AppendValues(#field_ptr_accessor, num_elements * #num_items_per_value, nullptr));
-                ARROW_RETURN_NOT_OK(#builder->AppendValues(num_elements));
             }
         } else {
             let value_reserve_factor = match &field.typ {
@@ -1348,8 +1348,8 @@ fn quote_append_field_to_builder(
                     for (auto elem_idx = 0; elem_idx < num_elements; elem_idx += 1) {
                         const auto& element = elements[elem_idx];
                         if (element.#field_name.has_value()) {
-                            #append_value
                             ARROW_RETURN_NOT_OK(#builder->Append());
+                            #append_value
                         } else {
                             ARROW_RETURN_NOT_OK(#builder->AppendNull());
                         }
@@ -1360,8 +1360,8 @@ fn quote_append_field_to_builder(
                     #setup
                     for (auto elem_idx = 0; elem_idx < num_elements; elem_idx += 1) {
                         const auto& element = elements[elem_idx];
-                        #append_value
                         ARROW_RETURN_NOT_OK(#builder->Append());
+                        #append_value
                     }
                 }
             }
@@ -1477,7 +1477,9 @@ fn quote_append_single_value_to_builder(
                     let fqname = quote_fqname_as_type_path(includes, fqname);
                     let field_ptr_accessor = quote_field_ptr_access(typ, value_access);
                     quote! {
-                        ARROW_RETURN_NOT_OK(#fqname::fill_arrow_array_builder(#value_builder, #field_ptr_accessor, #num_items_per_element));
+                        if (#field_ptr_accessor) {
+                            ARROW_RETURN_NOT_OK(#fqname::fill_arrow_array_builder(#value_builder, #field_ptr_accessor, #num_items_per_element));
+                        }
                     }
                 }
             }

docs/code-examples/annotation_context_arrows_v2.cpp

@@ -0,0 +1,25 @@
+// Log an annotation context to assign a label and color to each class
+
+#include <rerun.hpp>
+
+namespace rr = rerun;
+
+int main() {
+    auto rr_stream = rr::RecordingStream("annotation_context_rects");
+    rr_stream.connect("127.0.0.1:9876");
+
+    // Log an annotation context to assign a label and color to each class
+    rr_stream.log(
+        "/",
+        rr::archetypes::AnnotationContext({
+            rr::datatypes::AnnotationInfo(1, "red", rr::components::Color(255, 0, 0)),
+            rr::datatypes::AnnotationInfo(2, "green", rr::components::Color(0, 255, 0)),
+        })
+    );
+
+    // Log a batch of 2 arrows with different `class_ids`
+    rr_stream.log(
+        "arrows",
+        rr::archetypes::Arrows3D({{1.0f, 0.0f, 0.0f}, {0.0f, 1.0f, 0.0f}}).with_class_ids({1, 2})
+    );
+}

rerun_cpp/src/rerun/components/annotation_context_ext.cpp

@@ -0,0 +1,31 @@
+#include <utility>
+#include "annotation_context.hpp"
+
+// Uncomment for better auto-complete while editing the extension.
+// #define EDIT_EXTENSION
+
+namespace rerun {
+    namespace components {
+#ifdef EDIT_EXTENSION
+        struct AnnotationContextExt {
+            std::vector<rerun::datatypes::ClassDescriptionMapElem> class_map;
+
+#define AnnotationContext AnnotationContextExt
+
+            // [CODEGEN COPY TO HEADER START]
+
+            AnnotationContext(
+                std::initializer_list<rerun::datatypes::ClassDescription> class_descriptions
+            ) {
+                class_map.reserve(class_descriptions.size());
+                for (const auto& class_description : class_descriptions) {
+                    class_map.emplace_back(std::move(class_description));
+                }
+            }
+
+            // [CODEGEN COPY TO HEADER END]
+        };
+#endif
+
+    } // namespace components
+} // namespace rerun