Support feature-gated rust attributes (#2813)

Fixes #2801 by adding a `attr.rust.custom_clause`.

### What

### 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/2813) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2813)
- [Docs
preview](https://rerun.io/preview/pr%3Acmc%2Fcodegen_feature_gates/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Acmc%2Fcodegen_feature_gates/examples)

crates/re_types/definitions/rerun/components/instance_key.fbs

@@ -15,6 +15,8 @@ struct InstanceKey (
   "attr.python.array_aliases": "int, npt.NDArray[np.uint64]",
   "attr.rerun.legacy_fqname": "rerun.instance_key",
   "attr.rust.derive": "Copy, Hash, PartialEq, Eq, PartialOrd, Ord",
+  "attr.rust.custom_clause":
+    'cfg_attr(feature = "serde", derive(::serde::Serialize, ::serde::Deserialize))',
   order: 100
 ) {
   value: uint64 (order: 100);

crates/re_types/definitions/rust/attributes.fbs

@@ -11,5 +11,11 @@ attribute "attr.rust.tuple_struct";
 /// E.g. "attr.rust.derive": "Copy"`.
 attribute "attr.rust.derive";
 
+/// Apply to any object to generate an arbitrary clause.
+///
+/// The value of the attribute will be trimmed out but otherwise left as-is.
+/// E.g. "attr.rust.custom_clause": "cfg_attr(feature = "serde", derive(::serde::Serialize))"`.
+attribute "attr.rust.custom_clause";
+
 /// Apply to any object to generate a #repr clause with the specified value.
 attribute "attr.rust.repr";

crates/re_types/src/components/instance_key_ext.rs

@@ -71,21 +71,3 @@ impl std::fmt::Display for InstanceKey {
         }
     }
 }
-
-// TODO(jleibs): allow cfg_attr for codegen
-
-#[cfg(feature = "serde")]
-impl serde::Serialize for InstanceKey {
-    #[inline]
-    fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
-        self.0.serialize(serializer)
-    }
-}
-
-#[cfg(feature = "serde")]
-impl<'de> serde::Deserialize<'de> for InstanceKey {
-    #[inline]
-    fn deserialize<D: serde::Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
-        u64::deserialize(deserializer).map(Self::from)
-    }
-}

crates/re_types_builder/src/codegen/rust.rs

@@ -14,8 +14,8 @@ use crate::{
     codegen::{StringExt as _, AUTOGEN_WARNING},
     ArrowRegistry, CodeGenerator, Docs, ElementType, Object, ObjectField, ObjectKind, Objects,
     Type, ATTR_RERUN_COMPONENT_OPTIONAL, ATTR_RERUN_COMPONENT_RECOMMENDED,
-    ATTR_RERUN_COMPONENT_REQUIRED, ATTR_RERUN_LEGACY_FQNAME, ATTR_RUST_DERIVE, ATTR_RUST_REPR,
-    ATTR_RUST_TUPLE_STRUCT,
+    ATTR_RERUN_COMPONENT_REQUIRED, ATTR_RERUN_LEGACY_FQNAME, ATTR_RUST_CUSTOM_CLAUSE,
+    ATTR_RUST_DERIVE, ATTR_RUST_REPR, ATTR_RUST_TUPLE_STRUCT,
 };
 
 // TODO(cmc): it'd be nice to be able to generate vanilla comments (as opposed to doc-comments)
@@ -382,6 +382,7 @@ impl QuotedObject {
         let quoted_derive_clone_debug = quote_derive_clone_debug();
         let quoted_derive_clause = quote_meta_clause_from_obj(obj, ATTR_RUST_DERIVE, "derive");
         let quoted_repr_clause = quote_meta_clause_from_obj(obj, ATTR_RUST_REPR, "repr");
+        let quoted_custom_clause = quote_meta_clause_from_obj(obj, ATTR_RUST_CUSTOM_CLAUSE, "");
 
         let quoted_fields = fields
             .iter()
@@ -405,6 +406,7 @@ impl QuotedObject {
             #quoted_derive_clone_debug
             #quoted_derive_clause
             #quoted_repr_clause
+            #quoted_custom_clause
             #quoted_struct
 
             #quoted_from_impl
@@ -450,6 +452,7 @@ impl QuotedObject {
         let quoted_derive_clone_debug = quote_derive_clone_debug();
         let quoted_derive_clause = quote_meta_clause_from_obj(obj, ATTR_RUST_DERIVE, "derive");
         let quoted_repr_clause = quote_meta_clause_from_obj(obj, ATTR_RUST_REPR, "repr");
+        let quoted_custom_clause = quote_meta_clause_from_obj(obj, ATTR_RUST_CUSTOM_CLAUSE, "");
 
         let quoted_fields = fields.iter().map(|obj_field| {
             let ObjectField {
@@ -490,6 +493,7 @@ impl QuotedObject {
             #quoted_derive_clone_debug
             #quoted_derive_clause
             #quoted_repr_clause
+            #quoted_custom_clause
             pub enum #name {
                 #(#quoted_fields,)*
             }
@@ -658,10 +662,16 @@ fn quote_derive_clone_debug() -> TokenStream {
 fn quote_meta_clause_from_obj(obj: &Object, attr: &str, clause: &str) -> TokenStream {
     let quoted = obj
         .try_get_attr::<String>(attr)
-        .map(|what| {
-            syn::parse_str::<syn::MetaList>(&format!("{clause}({what})"))
-                .with_context(|| format!("illegal meta clause: {what:?}"))
-                .unwrap()
+        .map(|contents| {
+            if clause.is_empty() {
+                syn::parse_str::<syn::Meta>(contents.as_str())
+                    .with_context(|| format!("illegal meta clause: {clause:?}"))
+                    .unwrap()
+            } else {
+                syn::parse_str::<syn::Meta>(&format!("{clause}({contents})"))
+                    .with_context(|| format!("illegal meta clause: {clause}({contents})"))
+                    .unwrap()
+            }
         })
         .map(|clause| quote!(#[#clause]));
     quote!(#quoted)

crates/re_types_builder/src/lib.rs

@@ -153,6 +153,7 @@ pub const ATTR_RERUN_LEGACY_FQNAME: &str = "attr.rerun.legacy_fqname";
 pub const ATTR_PYTHON_ALIASES: &str = "attr.python.aliases";
 pub const ATTR_PYTHON_ARRAY_ALIASES: &str = "attr.python.array_aliases";
 
+pub const ATTR_RUST_CUSTOM_CLAUSE: &str = "attr.rust.custom_clause";
 pub const ATTR_RUST_DERIVE: &str = "attr.rust.derive";
 pub const ATTR_RUST_REPR: &str = "attr.rust.repr";
 pub const ATTR_RUST_TUPLE_STRUCT: &str = "attr.rust.tuple_struct";