Angle datatype stores now only radians (#6916)

### What

* Part of #6831

Previously, `Angle` was an arrow union between radian & degree. As part
of the effort of simplifying our data representations, this is now
always just radians.
Ran into a nameclash for C++'s `Angle` class and had to resort to a new
attribute that allows renaming the field for C++ only.

Simplifies the `Angle` datatype to only store radians.
As far as I can tell I managed to do this in a way that should break
almost no existing usage (ignoring deprecation warnings)

### Checklist
* [x] "passes" main test
* [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 the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6916?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6916?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/6916)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

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

@@ -16,7 +16,7 @@ use crate::{
     format_path,
     objects::ObjectClass,
     ArrowRegistry, Docs, ElementType, GeneratedFiles, Object, ObjectField, ObjectKind, Objects,
-    Reporter, Type, ATTR_CPP_NO_FIELD_CTORS,
+    Reporter, Type, ATTR_CPP_NO_FIELD_CTORS, ATTR_CPP_RENAME_FIELD,
 };
 
 use self::array_builder::{arrow_array_builder_type, arrow_array_builder_type_object};
@@ -445,7 +445,7 @@ impl QuotedObject {
             .iter()
             .map(|obj_field| {
                 let docstring = quote_field_docs(objects, obj_field);
-                let field_name = format_ident!("{}", obj_field.name);
+                let field_name = field_name_identifier(obj_field);
                 let field_type = quote_archetype_field_type(&mut hpp_includes, obj_field);
                 let field_type = if obj_field.is_nullable {
                     hpp_includes.insert_system("optional");
@@ -476,7 +476,7 @@ impl QuotedObject {
                 .iter()
                 .map(|obj_field| {
                     let field_type = quote_archetype_field_type(&mut hpp_includes, obj_field);
-                    let field_ident = format_ident!("{}", obj_field.name);
+                    let field_ident = field_name_identifier(obj_field);
                     // C++ compilers give warnings for re-using the same name as the member variable.
                     let parameter_ident = format_ident!("_{}", obj_field.name);
                     (
@@ -498,7 +498,7 @@ impl QuotedObject {
 
         // Builder methods for all optional components.
         for obj_field in obj.fields.iter().filter(|field| field.is_nullable) {
-            let field_ident = format_ident!("{}", obj_field.name);
+            let field_ident = field_name_identifier(obj_field);
             // C++ compilers give warnings for re-using the same name as the member variable.
             let parameter_ident = format_ident!("_{}", obj_field.name);
             let method_ident = format_ident!("with_{}", obj_field.name);
@@ -663,7 +663,7 @@ impl QuotedObject {
                     objects,
                     &mut hpp_includes,
                     obj_field,
-                    &format_ident!("{}", obj_field.name),
+                    &field_name_identifier(obj_field),
                 );
                 quote! {
                     #NEWLINE_TOKEN
@@ -819,7 +819,7 @@ impl QuotedObject {
             }
         })
         .chain(obj.fields.iter().map(|obj_field| {
-            let ident = format_ident!("{}", obj_field.name);
+            let ident = field_name_identifier(obj_field);
             quote! {
                 #ident,
             }
@@ -896,7 +896,7 @@ impl QuotedObject {
         for obj_field in &obj.fields {
             let snake_case_name = obj_field.snake_case_name();
             let field_name = format_ident!("{}", snake_case_name);
-            let tag_name = format_ident!("{}", obj_field.name);
+            let tag_name = field_name_identifier(obj_field);
 
             let method = if obj_field.typ == Type::Unit {
                 let method_name = format_ident!("is_{}", snake_case_name);
@@ -950,7 +950,7 @@ impl QuotedObject {
                 }
             })
             .chain(obj.fields.iter().map(|obj_field| {
-                let tag_ident = format_ident!("{}", obj_field.name);
+                let tag_ident = field_name_identifier(obj_field);
                 let field_ident = format_ident!("{}", obj_field.snake_case_name());
 
                 if obj_field.typ.has_default_destructor(objects) {
@@ -988,7 +988,7 @@ impl QuotedObject {
             let mut placement_new_arms = Vec::new();
             let mut trivial_memcpy_cases = Vec::new();
             for obj_field in &obj.fields {
-                let tag_ident = format_ident!("{}", obj_field.name);
+                let tag_ident = field_name_identifier(obj_field);
                 let case = quote!(case detail::#tag_typename::#tag_ident:);
 
                 // Inferring from trivial destructability that we don't need to call the copy constructor is a little bit wonky,
@@ -1215,7 +1215,7 @@ impl QuotedObject {
             .enumerate()
             .map(|(i, obj_field)| {
                 let docstring = quote_field_docs(objects, obj_field);
-                let field_name = format_ident!("{}", obj_field.name);
+                let field_name = field_name_identifier(obj_field);
 
                 // We assign the arrow type index to the enum fields to make encoding simpler and faster:
                 let arrow_type_index = proc_macro2::Literal::usize_unsuffixed(1 + i); // 0 is reserved for `_null_markers`
@@ -1263,6 +1263,14 @@ impl QuotedObject {
     }
 }
 
+fn field_name_identifier(obj_field: &ObjectField) -> Ident {
+    if let Some(name) = obj_field.try_get_attr::<String>(ATTR_CPP_RENAME_FIELD) {
+        format_ident!("{}", name)
+    } else {
+        format_ident!("{}", obj_field.name)
+    }
+}
+
 fn single_field_constructor_methods(
     obj: &Object,
     hpp_includes: &mut Includes,
@@ -1303,7 +1311,7 @@ fn add_copy_assignment_and_constructor(
     objects: &Objects,
 ) -> Vec<Method> {
     let mut methods = Vec::new();
-    let field_ident = format_ident!("{}", target_field.name);
+    let field_ident = field_name_identifier(target_field);
     let param_ident = format_ident!("{}_", obj_field.name);
 
     // We keep parameter passing for assignment & ctors simple by _always_ passing by value.
@@ -1551,7 +1559,7 @@ fn archetype_serialize(type_ident: &Ident, obj: &Object, hpp_includes: &mut Incl
 
     let num_fields = quote_integer(obj.fields.len() + 1); // Plus one for the indicator.
     let push_batches = obj.fields.iter().map(|field| {
-        let field_name = format_ident!("{}", field.name);
+        let field_name = field_name_identifier(field);
         let field_accessor = quote!(archetype.#field_name);
 
         let push_back = quote! {
@@ -1829,7 +1837,7 @@ fn quote_append_field_to_builder(
     includes: &mut Includes,
     objects: &Objects,
 ) -> TokenStream {
-    let field_name = format_ident!("{}", field.name);
+    let field_name = field_name_identifier(field);
 
     if let Some(elem_type) = field.typ.plural_inner() {
         let value_builder = format_ident!("value_builder");

crates/build/re_types_builder/src/codegen/rust/serializer.rs

@@ -447,12 +447,13 @@ fn quote_arrow_field_serializer(
             // hence `unwrap_or_default` (unless elements_are_nullable is false)
             let quoted_transparent_mapping = if inner_is_arrow_transparent {
                 let inner_obj = inner_obj.as_ref().unwrap();
-                let quoted_inner_obj_type = quote_fqname_as_type_path(&inner_obj.fqname);
                 let is_tuple_struct = is_tuple_struct_from_obj(inner_obj);
                 let quoted_member_accessor = if is_tuple_struct {
                     quote!(0)
                 } else {
-                    quote!(#quoted_inner_obj_type)
+                    let inner_field_name =
+                        format_ident!("{}", inner_obj.fields[0].snake_case_name());
+                    quote!(#inner_field_name)
                 };
 
                 if elements_are_nullable {

crates/build/re_types_builder/src/lib.rs

@@ -198,6 +198,7 @@ pub const ATTR_RUST_TUPLE_STRUCT: &str = "attr.rust.tuple_struct";
 pub const ATTR_RUST_GENERATE_FIELD_INFO: &str = "attr.rust.generate_field_info";
 
 pub const ATTR_CPP_NO_FIELD_CTORS: &str = "attr.cpp.no_field_ctors";
+pub const ATTR_CPP_RENAME_FIELD: &str = "attr.cpp.rename_field";
 
 pub const ATTR_DOCS_UNRELEASED: &str = "attr.docs.unreleased";
 pub const ATTR_DOCS_CATEGORY: &str = "attr.docs.category";

crates/store/re_types/definitions/cpp/attributes.fbs

@@ -2,3 +2,7 @@ namespace cpp.attributes;
 
 /// Don't emit parameterized constructors for fields, only copy, move and default constructors.
 attribute "attr.cpp.no_field_ctors";
+
+/// Changes the name of a field in the generated C++ code.
+/// This can be used to avoid name clashes with C++ keywords or methods.
+attribute "attr.cpp.rename_field";

crates/store/re_types/definitions/rerun/datatypes/angle.fbs

@@ -1,22 +1,23 @@
 include "arrow/attributes.fbs";
 include "rust/attributes.fbs";
 include "fbs/attributes.fbs";
+include "cpp/attributes.fbs";
 
 include "./float32.fbs";
 
 namespace rerun.datatypes;
 
 // ---
 
-/// Angle in either radians or degrees.
-union Angle (
-  "attr.rust.derive": "Copy, PartialEq"
+/// Angle in radians.
+struct Angle (
+  "attr.arrow.transparent",
+  "attr.python.aliases": "float, int",
+  "attr.python.array_aliases": "npt.ArrayLike, Sequence[float], Sequence[int]",
+  "attr.rust.derive": "Copy, Default, PartialEq, PartialOrd, bytemuck::Pod, bytemuck::Zeroable",
+  "attr.rust.repr": "transparent",
+  "attr.cpp.no_field_ctors"
 ) {
   /// Angle in radians. One turn is equal to 2π (or τ) radians.
-  /// \py Only one of `degrees` or `radians` should be set.
-  Radians: rerun.datatypes.Float32 (transparent),
-
-  /// Angle in degrees. One turn is equal to 360 degrees.
-  /// \py Only one of `degrees` or `radians` should be set.
-  Degrees: rerun.datatypes.Float32 (transparent),
+  radians: float (order: 100, "attr.cpp.rename_field": "angle_radians"); // Rename field to avoid nameclash with `radians` function.
 }