Add datatype to component reflection and use it (preferred) for place…

…holders (#7944)

### What

* Fixes #7940

Tested by removing the fallback provider for `ShowLabels` from points
which prior to this fix exhibits the same issue as above

### 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 the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7944?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/7944?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/7944)
- [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/rust/reflection.rs

@@ -142,8 +142,8 @@ fn generate_component_reflection(
         let quoted_reflection = quote! {
             ComponentReflection {
                 docstring_md: #docstring_md,
-
                 custom_placeholder: #custom_placeholder,
+                datatype: #type_name::arrow_datatype(),
             }
         };
         quoted_pairs.push(quote! { (#quoted_name, #quoted_reflection) });

crates/store/re_types_core/src/loggable.rs

@@ -91,14 +91,6 @@ pub trait Loggable: 'static + Send + Sync + Clone + Sized + SizeBytes {
 ///
 /// Any [`Loggable`] with a [`Loggable::Name`] set to [`ComponentName`] automatically implements
 /// [`Component`].
-///
-/// All built-in components should implement the [`Default`] trait, so that the Viewer has a placeholder
-/// value that it can display for ui editors in absence of a value.
-/// In absence of an obvious default value, they should be implemented such that they are versatile
-/// and informative for the user when using the viewer.
-///
-/// This is not enforced via a trait bound, since it's only necessary for components known to the Rerun Viewer.
-/// It is not a hard requirement for custom components that are only used on the SDK side.
 pub trait Component: Loggable<Name = ComponentName> {}
 
 impl<L: Loggable<Name = ComponentName>> Component for L {}

crates/store/re_types_core/src/reflection.rs

@@ -219,6 +219,9 @@ pub struct ComponentReflection {
     /// Typically, this is only used when `FallbackProvider`s are not available.
     /// If there's no custom placeholder, a placeholder can be derived from the arrow datatype.
     pub custom_placeholder: Option<Box<dyn arrow2::array::Array>>,
+
+    /// Datatype of the component.
+    pub datatype: arrow2::datatypes::DataType,
 }
 
 /// Runtime reflection about archetypes.