From 84e751b80f722a864b9b3714cd0fd1dee4368760 Mon Sep 17 00:00:00 2001
From: Thomas BESSOU <thomas.bessou@hotmail.fr>
Date: Sun, 27 Oct 2024 15:59:27 +0100
Subject: [PATCH] Fix direct lookup newtype schema generation (fixes #25)

---
 .../field_types_and_instantiations.rs         | 11 ++++-
 .../src/build_schema/mod.rs                   | 40 ++++++++++++-------
 .../src/build_schema/type_lookup.rs           |  1 +
 3 files changed, 36 insertions(+), 16 deletions(-)

diff --git a/serde_avro_derive_macros/src/build_schema/field_types_and_instantiations.rs b/serde_avro_derive_macros/src/build_schema/field_types_and_instantiations.rs
index 9d2ccf0..455a381 100644
--- a/serde_avro_derive_macros/src/build_schema/field_types_and_instantiations.rs
+++ b/serde_avro_derive_macros/src/build_schema/field_types_and_instantiations.rs
@@ -7,6 +7,11 @@ pub(super) struct FieldTypeAndInstantiationsBuilder<'t, 'm> {
 	pub(super) errors: &'m mut TokenStream,
 	pub(super) namespace: &'m Option<String>,
 	pub(super) expand_namespace_var: bool,
+	/// Whether at least one of the fields id directly looked up in the schema,
+	/// without necessarily inserting a new node. This is useful for newtype
+	/// struct serialization without logical type, whose type (and lookup)
+	/// should be directly the inner one
+	pub(super) has_direct_lookup: bool,
 }
 
 pub(super) enum FieldKind<'a> {
@@ -203,8 +208,10 @@ impl<'t> FieldTypeAndInstantiationsBuilder<'t, '_> {
 						}
 					}
 				}
-				override_field_instantiation
-					.unwrap_or_else(|| quote! { builder.find_or_build::<#ty>() })
+				override_field_instantiation.unwrap_or_else(|| {
+					self.has_direct_lookup = true;
+					quote! { builder.find_or_build::<#ty>() }
+				})
 			}
 			Some(logical_type_litstr) => {
 				let logical_type_str_raw = logical_type_litstr.value();
diff --git a/serde_avro_derive_macros/src/build_schema/mod.rs b/serde_avro_derive_macros/src/build_schema/mod.rs
index d2b84bc..f390ee1 100644
--- a/serde_avro_derive_macros/src/build_schema/mod.rs
+++ b/serde_avro_derive_macros/src/build_schema/mod.rs
@@ -107,6 +107,7 @@ pub(crate) fn schema_impl(input: SchemaDeriveInput) -> Result<TokenStream, Error
 		errors: &mut errors,
 		namespace: &input.namespace,
 		expand_namespace_var: false,
+		has_direct_lookup: false,
 	};
 
 	let type_lookup: syn::Type;
@@ -125,20 +126,31 @@ pub(crate) fn schema_impl(input: SchemaDeriveInput) -> Result<TokenStream, Error
 							struct_name: name_ident,
 						},
 					);
-				let namespace_var = field_types_and_instantiations
-					.expand_namespace_var
-					.then(lazy_compute_namespace);
-				(type_lookup, type_lookup_decl, _) = type_lookup::build_type_lookup(
-					type_ident,
-					&generics,
-					None,
-					std::slice::from_ref(&field_type),
-				);
-				quote! {
-					let n_nodes = builder.nodes.len();
-					#namespace_var
-					let new_node_key = #field_instantiation;
-					assert_eq!(n_nodes, new_node_key.idx());
+				if field_types_and_instantiations.has_direct_lookup {
+					// This single field is a direct lookup (no logical type,
+					// etc), so this is the special case we need to just
+					// expand into forwarding.
+					type_lookup = parse_quote! { <#field_type as serde_avro_derive::BuildSchema>::TypeLookup };
+					type_lookup_decl = None;
+					quote! {
+						<#field_type as serde_avro_derive::BuildSchema>::append_schema(builder);
+					}
+				} else {
+					let namespace_var = field_types_and_instantiations
+						.expand_namespace_var
+						.then(lazy_compute_namespace);
+					(type_lookup, type_lookup_decl, _) = type_lookup::build_type_lookup(
+						type_ident,
+						&generics,
+						None,
+						std::slice::from_ref(&field_type),
+					);
+					quote! {
+						let n_nodes = builder.nodes.len();
+						#namespace_var
+						let new_node_key = #field_instantiation;
+						assert_eq!(n_nodes, new_node_key.idx());
+					}
 				}
 			} else {
 				// named struct
diff --git a/serde_avro_derive_macros/src/build_schema/type_lookup.rs b/serde_avro_derive_macros/src/build_schema/type_lookup.rs
index ff2d819..93b84d3 100644
--- a/serde_avro_derive_macros/src/build_schema/type_lookup.rs
+++ b/serde_avro_derive_macros/src/build_schema/type_lookup.rs
@@ -1,5 +1,6 @@
 use super::*;
 
+/// Returned boolean is `has_non_lifetime_generics`
 pub(super) fn build_type_lookup(
 	type_ident: &syn::Ident,
 	generics: &syn::Generics,