Added new code sections to support deser overrides

Fixes #1053

Author: timocov

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/DeserializerGenerator.java

@@ -8,6 +8,7 @@
 import software.amazon.smithy.codegen.core.Symbol;
 import software.amazon.smithy.codegen.core.SymbolProvider;
 import software.amazon.smithy.java.codegen.CodegenUtils;
+import software.amazon.smithy.java.codegen.sections.MemberDeserializerSection;
 import software.amazon.smithy.java.codegen.writer.JavaWriter;
 import software.amazon.smithy.java.core.serde.event.EventStream;
 import software.amazon.smithy.model.Model;
@@ -66,7 +67,7 @@ final class DeserializerGenerator extends ShapeVisitor.DataShapeVisitor<Void> im
 
     @Override
     public void run() {
-        writer.pushState();
+        writer.pushState(new MemberDeserializerSection(shape, schemaName, deserializer));
         writer.putContext("schemaName", schemaName);
         writer.putContext("deserializer", deserializer);
         shape.accept(this);

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/SerializerMemberGenerator.java

@@ -9,6 +9,7 @@
 import software.amazon.smithy.codegen.core.directed.ContextualDirective;
 import software.amazon.smithy.java.codegen.CodeGenerationContext;
 import software.amazon.smithy.java.codegen.CodegenUtils;
+import software.amazon.smithy.java.codegen.sections.MemberSerializerSection;
 import software.amazon.smithy.java.codegen.writer.JavaWriter;
 import software.amazon.smithy.java.core.schema.Unit;
 import software.amazon.smithy.model.Model;
@@ -66,9 +67,10 @@ final class SerializerMemberGenerator extends ShapeVisitor.DataShapeVisitor<Void
 
     @Override
     public void run() {
-        writer.pushState();
+        var shapeSchemaVariable = "$SCHEMA";
+        writer.pushState(new MemberSerializerSection(shape, state, shapeSchemaVariable));
         writer.putContext("state", state);
-        writer.putContext("schema", "$SCHEMA");
+        writer.putContext("schema", shapeSchemaVariable);
         shape.accept(this);
         writer.popState();
     }

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/sections/MemberDeserializerSection.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.codegen.sections;
+
+import software.amazon.smithy.model.shapes.Shape;
+import software.amazon.smithy.utils.CodeSection;
+
+/**
+ * Adds a section for member deserializer code.
+ *
+ * @param targetedShape A shape that the deserializer code section is created for
+ * @param shapeSchemaVariable Name of a local variable containing the shape's schema
+ * @param deserializerVariable Name of a local variable containing the deserializer instance
+ */
+public record MemberDeserializerSection(Shape targetedShape, String shapeSchemaVariable, String deserializerVariable)
+        implements CodeSection {}

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/sections/MemberSerializerSection.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.codegen.sections;
+
+import software.amazon.smithy.model.shapes.Shape;
+import software.amazon.smithy.utils.CodeSection;
+
+/**
+ * Adds a section for member serializer code.
+ *
+ * @param targetedShape A shape that the serializer code section is created for
+ * @param stateVariable Name of a local variable containing the state to serialize
+ * @param shapeSchemaVariable Name of a local variable containing the schema for a given {@code targetedShape}
+ */
+public record MemberSerializerSection(Shape targetedShape, String stateVariable, String shapeSchemaVariable)
+        implements CodeSection {}