Ignore types changes to same type

Author: mtdowling

smithy-model/src/main/java/software/amazon/smithy/model/transform/ChangeShapeType.java

@@ -85,7 +85,10 @@ static ChangeShapeType downgradeEnums(Model model) {
     Model transform(ModelTransformer transformer, Model model) {
         return transformer.mapShapes(model, shape -> {
             if (shapeToType.containsKey(shape.getId())) {
-                return shape.accept(new Retype(shapeToType.get(shape.getId()), synthesizeEnumNames));
+                ShapeType targetType = shapeToType.get(shape.getId());
+                return targetType == shape.getType()
+                       ? shape
+                       : shape.accept(new Retype(targetType, synthesizeEnumNames));
             } else {
                 return shape;
             }

smithy-model/src/test/java/software/amazon/smithy/model/transform/ChangeShapeTypeTest.java

@@ -242,6 +242,14 @@ public void cannotConvertStructureToAnythingButUnion() {
         });
     }
 
+    @Test
+    public void ignoresConversionToSameType() {
+        Shape startShape = StructureShape.builder().id(ShapeId.from("smithy.example#Test")).build();
+        Model model = Model.assembler().addShape(startShape).assemble().unwrap();
+
+        ModelTransformer.create().changeShapeType(model, MapUtils.of(startShape.getId(), ShapeType.STRUCTURE));
+    }
+
     @Test
     public void cannotConvertUnionToAnythingButStructure() {
         Shape startShape = UnionShape.builder()