fix(internal/librarian/rust): map recursive fields to Box

internal/sidekick/rust/annotate.go

@@ -505,6 +505,8 @@ type fieldAnnotations struct {
 	IsBoxed bool
 	// If true, it requires a serde_with::serde_as() transformation.
 	SerdeAs string
+	// If true, the field is boxed in the prost generated type.
+	MapToBoxed bool
 	// If true, use `wkt::internal::is_default()` to skip the field
 	SkipIfIsDefault bool
 	// If true, this is a `wkt::Value` field, and requires super-extra custom
@@ -1479,6 +1481,7 @@ func (c *codec) annotateField(field *api.Field, message *api.Message, model *api
 	if field.Recursive || (field.Typez == api.MESSAGE_TYPE && field.IsOneOf) {
 		ann.IsBoxed = true
 	}
+	ann.MapToBoxed = mapToBoxed(field, message, model)
 	field.Codec = ann
 	if field.Typez == api.MESSAGE_TYPE {
 		if msg, ok := model.State.MessageByID[field.TypezID]; ok && msg.IsMap {
@@ -1651,3 +1654,39 @@ func isIdempotent(p *api.PathInfo) string {
 	}
 	return "true"
 }
+
+// mapToBoxed returns true if the prost generated type for this field is boxed.
+// Prost boxes fields that would cause an infinitely sized struct, which happens
+// on recursive cycles that are not broken by a repeated or map field.
+func mapToBoxed(field *api.Field, message *api.Message, model *api.API) bool {
+	if field.Typez != api.MESSAGE_TYPE || field.Repeated || field.Map {
+		return false
+	}
+
+	var check func(typezID string, targetID string, visited map[string]bool) bool
+	check = func(typezID string, targetID string, visited map[string]bool) bool {
+		if typezID == targetID {
+			return true
+		}
+		if visited[typezID] {
+			return false
+		}
+		visited[typezID] = true
+		msg, ok := model.State.MessageByID[typezID]
+		if !ok {
+			return false
+		}
+		for _, f := range msg.Fields {
+			if f.Typez != api.MESSAGE_TYPE || f.Repeated || f.Map {
+				continue
+			}
+			if check(f.TypezID, targetID, visited) {
+				return true
+			}
+		}
+		return false
+	}
+
+	visited := make(map[string]bool)
+	return check(field.TypezID, message.ID, visited)
+}

internal/sidekick/rust/annotate_field_test.go

@@ -183,6 +183,7 @@ func TestFieldAnnotations(t *testing.T) {
 		PrimitiveFieldType:    "crate::model::TestMessage",
 		AddQueryParameter:     `let builder = req.boxed_field.as_ref().map(|p| serde_json::to_value(p).map_err(Error::ser) ).transpose()?.into_iter().fold(builder, |builder, v| { use gaxi::query_parameter::QueryParameter; v.add(builder, "boxedField") });`,
 		IsBoxed:               true,
+		MapToBoxed:            true,
 		SkipIfIsDefault:       true,
 		FieldTypeIsParentType: true,
 	}
@@ -295,6 +296,7 @@ func TestRecursiveFieldAnnotations(t *testing.T) {
 		ValueField:            value_field,
 		SerdeAs:               "std::collections::HashMap<wkt::internal::I32, serde_with::Same>",
 		IsBoxed:               true,
+		MapToBoxed:            true,
 		SkipIfIsDefault:       true,
 		FieldTypeIsParentType: true,
 	}
@@ -318,6 +320,7 @@ func TestRecursiveFieldAnnotations(t *testing.T) {
 		PrimitiveFieldType:    "crate::model::TestMessage",
 		AddQueryParameter:     `let builder = req.oneof_field().map(|p| serde_json::to_value(p).map_err(Error::ser) ).transpose()?.into_iter().fold(builder, |builder, p| { use gaxi::query_parameter::QueryParameter; p.add(builder, "oneofField") });`,
 		IsBoxed:               true,
+		MapToBoxed:            true,
 		SkipIfIsDefault:       true,
 		OtherFieldsInGroup:    []*api.Field{},
 		FieldTypeIsParentType: true,
@@ -342,6 +345,7 @@ func TestRecursiveFieldAnnotations(t *testing.T) {
 		PrimitiveFieldType:    "crate::model::TestMessage",
 		AddQueryParameter:     `let builder = req.repeated_field.as_ref().map(|p| serde_json::to_value(p).map_err(Error::ser) ).transpose()?.into_iter().fold(builder, |builder, v| { use gaxi::query_parameter::QueryParameter; v.add(builder, "repeatedField") });`,
 		IsBoxed:               true,
+		MapToBoxed:            false,
 		SkipIfIsDefault:       true,
 		FieldTypeIsParentType: true,
 	}
@@ -365,6 +369,7 @@ func TestRecursiveFieldAnnotations(t *testing.T) {
 		PrimitiveFieldType:    "crate::model::TestMessage",
 		AddQueryParameter:     `let builder = { use gaxi::query_parameter::QueryParameter; serde_json::to_value(&req.message_field).map_err(Error::ser)?.add(builder, "messageField") };`,
 		IsBoxed:               true,
+		MapToBoxed:            true,
 		SkipIfIsDefault:       true,
 		FieldTypeIsParentType: true,
 	}
@@ -508,6 +513,7 @@ func TestSameTypeNameFieldAnnotations(t *testing.T) {
 		PrimitiveFieldType: "rusty_test_inner_v1::model::TestMessage",
 		AddQueryParameter:  `let builder = req.oneof_field().map(|p| serde_json::to_value(p).map_err(Error::ser) ).transpose()?.into_iter().fold(builder, |builder, p| { use gaxi::query_parameter::QueryParameter; p.add(builder, "oneofField") });`,
 		IsBoxed:            true,
+		MapToBoxed:         false,
 		SkipIfIsDefault:    true,
 		OtherFieldsInGroup: []*api.Field{},
 		AliasInExamples:    "OneofField",

internal/sidekick/rust/templates/convert-prost/message.mustache

@@ -35,7 +35,12 @@ impl gaxi::prost::ToProto<{{Codec.RelativeName}}> for {{Codec.QualifiedName}} {
             {{Codec.FieldName}}: self.{{Codec.FieldName}}.to_proto()?,
             {{/Optional}}
             {{#Optional}}
+            {{^Codec.MapToBoxed}}
             {{Codec.FieldName}}: self.{{Codec.FieldName}}.map(|v| v.to_proto()).transpose()?,
+            {{/Codec.MapToBoxed}}
+            {{#Codec.MapToBoxed}}
+            {{Codec.FieldName}}: self.{{Codec.FieldName}}.map(|v| v.to_proto().map(std::boxed::Box::new)).transpose()?,
+            {{/Codec.MapToBoxed}}
             {{/Optional}}
             {{/Singular}}
             {{#Repeated}}