Add a attr.rust.no_placeholder attribute to codegen to force a comp…

…onent's reflection to not have a placeholder value specified.

This enables the component not to `impl Default`

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

@@ -6,7 +6,7 @@ use quote::{format_ident, quote};
 
 use crate::{
     codegen::{autogen_warning, Target},
-    ObjectKind, Objects, Reporter, ATTR_RERUN_COMPONENT_REQUIRED,
+    ObjectKind, Objects, Reporter, ATTR_RERUN_COMPONENT_REQUIRED, ATTR_RUST_NO_PLACEHOLDER,
 };
 
 use super::util::{append_tokens, doc_as_lines};
@@ -111,11 +111,22 @@ fn generate_component_reflection(
             obj.is_experimental(),
         )
         .join("\n");
+
+        let placeholder = if obj.attrs.has(ATTR_RUST_NO_PLACEHOLDER) {
+            quote! {
+                None
+            }
+        } else {
+            quote! {
+                Some(#type_name::default().to_arrow()?)
+            }
+        };
+
         let quoted_reflection = quote! {
             ComponentReflection {
                 docstring_md: #docstring_md,
 
-                placeholder: Some(#type_name::default().to_arrow()?),
+                placeholder: #placeholder,
             }
         };
         quoted_pairs.push(quote! { (#quoted_name, #quoted_reflection) });

crates/build/re_types_builder/src/lib.rs

@@ -199,6 +199,7 @@ pub const ATTR_RUST_NEW_PUB_CRATE: &str = "attr.rust.new_pub_crate";
 pub const ATTR_RUST_OVERRIDE_CRATE: &str = "attr.rust.override_crate";
 pub const ATTR_RUST_REPR: &str = "attr.rust.repr";
 pub const ATTR_RUST_TUPLE_STRUCT: &str = "attr.rust.tuple_struct";
+pub const ATTR_RUST_NO_PLACEHOLDER: &str = "attr.rust.no_placeholder";
 pub const ATTR_CPP_NO_FIELD_CTORS: &str = "attr.cpp.no_field_ctors";
 pub const ATTR_CPP_RENAME_FIELD: &str = "attr.cpp.rename_field";
 

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

@@ -41,3 +41,8 @@ attribute "attr.rust.new_pub_crate";
 /// an object of kind `Blueprint` with `attr.rust.override_crate=re_viewport`, the final
 /// object will be generated in `re_viewport/src/blueprint`.
 attribute "attr.rust.override_crate";
+
+/// No reflection placeholder value will be generated for this component.
+///
+/// This enables the component to not `impl Default`. The drawback is that this may result in a `  
+attribute "attr.rust.no_placeholder";