Fixed errors and added complete union serialization implementation

- Added core serialization fixes in JsonSerializerGenerator and QuerySerializerGenerator
- Fixed variable reference consistency (_inner vs inner) to prevent unused warnings
- Corrected test structure with shared model and proper test locations
- Ensured protocol_serde module generation works correctly
- All tests now pass without compilation errors or unused variable warnings

Author: vcjana

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/JsonSerializerGenerator.kt

@@ -555,14 +555,28 @@ class JsonSerializerGenerator(
                     }
                     rustBlock("match input") {
                         for (member in context.shape.members()) {
+                            val memberShape = model.expectShape(member.target)
                             val variantName =
                                 if (member.isTargetUnit()) {
                                     "${symbolProvider.toMemberName(member)}"
+                                } else if (memberShape.isStructureShape &&
+                                    memberShape.asStructureShape().get().allMembers.isEmpty()
+                                ) {
+                                    // Unit structs don't serialize inner, so it is never accessed
+                                    "${symbolProvider.toMemberName(member)}(_inner)"
                                 } else {
                                     "${symbolProvider.toMemberName(member)}(inner)"
                                 }
                             withBlock("#T::$variantName => {", "},", unionSymbol) {
-                                serializeMember(MemberContext.unionMember(context, "inner", member, jsonName))
+                                val innerRef =
+                                    if (memberShape.isStructureShape &&
+                                        memberShape.asStructureShape().get().allMembers.isEmpty()
+                                    ) {
+                                        "_inner"
+                                    } else {
+                                        "inner"
+                                    }
+                                serializeMember(MemberContext.unionMember(context, innerRef, member, jsonName))
                             }
                         }
                         if (codegenTarget.renderUnknownVariant()) {

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/QuerySerializerGenerator.kt

@@ -371,10 +371,18 @@ abstract class QuerySerializerGenerator(private val codegenContext: CodegenConte
                                     "$memberName(inner)"
                                 }
                             withBlock("#T::$variantName => {", "},", unionSymbol) {
+                                val innerRef =
+                                    if (memberShape.isStructureShape &&
+                                        memberShape.asStructureShape().get().allMembers.isEmpty()
+                                    ) {
+                                        "_inner"
+                                    } else {
+                                        "inner"
+                                    }
                                 serializeMember(
                                     MemberContext.unionMember(
                                         context.copy(writerExpression = "writer"),
-                                        "inner",
+                                        innerRef,
                                         member,
                                     ),
                                 )

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/JsonSerializerGeneratorTest.kt

@@ -33,28 +33,6 @@ import software.amazon.smithy.rust.codegen.core.util.inputShape
 import software.amazon.smithy.rust.codegen.core.util.lookup
 
 class JsonSerializerGeneratorTest {
-    private val unionWithUnitStructModel =
-        """
-        namespace test
-        use aws.protocols#restJson1
-
-        union TestUnion {
-            unitMember: Unit
-        }
-
-        // Empty unit struct that doesn't need serialization parameters
-        structure Unit {}
-
-        @http(uri: "/test", method: "POST")
-        operation TestOp {
-            input: TestInput
-        }
-
-        structure TestInput {
-            union: TestUnion
-        }
-        """.asSmithyModel()
-
     private val baseModel =
         """
         namespace test
@@ -371,7 +349,7 @@ class JsonSerializerGeneratorTest {
     fun `union with unit struct doesn't cause unused variable warning`() {
         // Regression test for https://github.com/smithy-lang/smithy-rs/issues/4308
         // This test ensures that union serialization with unit structs compiles without unused variable warnings.
-        val model = RecursiveShapeBoxer().transform(OperationNormalizer.transform(unionWithUnitStructModel))
+        val model = RecursiveShapeBoxer().transform(OperationNormalizer.transform(QuerySerializerGeneratorTest.unionWithUnitStructModel))
 
         val codegenContext = testCodegenContext(model)
         val symbolProvider = codegenContext.symbolProvider

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/QuerySerializerGeneratorTest.kt

@@ -22,44 +22,40 @@ import software.amazon.smithy.rust.codegen.core.util.inputShape
 import software.amazon.smithy.rust.codegen.core.util.lookup
 
 class QuerySerializerGeneratorTest {
+    companion object {
+        val unionWithUnitStructModel =
+            """
+            namespace test
+
+            union TestUnion {
+                unitMember: Unit
+            }
+
+            structure Unit {}
+
+            @http(uri: "/test", method: "POST")
+            operation TestOp {
+                input: TestInput
+            }
+
+            structure TestInput {
+                union: TestUnion
+            }
+            """.asSmithyModel()
+    }
+
     @Test
     fun `union with unit struct doesn't cause unused variable warning`() {
         // Regression test for https://github.com/smithy-lang/smithy-rs/issues/4308
-        // This test ensures that Query serialization with unit structs compiles without unused variable warnings.
-        // Query serialization for unit structs generates empty blocks where the variable would be unused,
-        // so the fix uses '_inner' (with underscore prefix) to suppress the unused variable warning.
-        val model =
-            RecursiveShapeBoxer().transform(
-                OperationNormalizer.transform(
-                    """
-                    namespace test
-
-                    union TestUnion {
-                        unitMember: Unit
-                    }
-
-                    structure Unit {}
-
-                    @http(uri: "/test", method: "POST")
-                    operation TestOp {
-                        input: TestInput
-                    }
-
-                    structure TestInput {
-                        union: TestUnion
-                    }
-                    """.asSmithyModel(),
-                ),
-            )
+        val model = RecursiveShapeBoxer().transform(OperationNormalizer.transform(unionWithUnitStructModel))
 
         val codegenContext = testCodegenContext(model)
         val symbolProvider = codegenContext.symbolProvider
-
-        val parserSerializer = AwsQuerySerializerGenerator(codegenContext)
-        val operationGenerator = parserSerializer.operationInputSerializer(model.lookup("test#TestOp"))
-
         val project = TestWorkspace.testProject(symbolProvider)
 
+        val querySerializer = Ec2QuerySerializerGenerator(codegenContext)
+        val operationGenerator = querySerializer.operationInputSerializer(model.lookup("test#TestOp"))
+
         // Render all necessary structures and unions
         model.lookup<StructureShape>("test#Unit").renderWithModelBuilder(model, symbolProvider, project)
         model.lookup<OperationShape>("test#TestOp").inputShape(model).renderWithModelBuilder(model, symbolProvider, project)
@@ -70,7 +66,7 @@ class QuerySerializerGeneratorTest {
 
         project.lib {
             unitTest(
-                "query_union_serialization_fix_works",
+                "test_query_union_serialization",
                 """
                 use test_model::{TestUnion, Unit};
 
@@ -88,7 +84,6 @@ class QuerySerializerGeneratorTest {
             )
         }
 
-        // The test passes if the generated code compiles without unused variable warnings
         project.compileAndTest()
     }
 }