From 0d84c5d71ef17f6c761b2150b446c5b09810339c Mon Sep 17 00:00:00 2001
From: Ian Caffey <mail@iancaffey.com>
Date: Fri, 25 Aug 2023 13:30:42 -0500
Subject: [PATCH] Support removing unused mixins

---
 .../transform/FlattenAndRemoveMixins.java     |  2 +-
 .../model/transform/ModelTransformerTest.java | 30 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/smithy-model/src/main/java/software/amazon/smithy/model/transform/FlattenAndRemoveMixins.java b/smithy-model/src/main/java/software/amazon/smithy/model/transform/FlattenAndRemoveMixins.java
index 5aa3cfa5da4..ad790d78c3b 100644
--- a/smithy-model/src/main/java/software/amazon/smithy/model/transform/FlattenAndRemoveMixins.java
+++ b/smithy-model/src/main/java/software/amazon/smithy/model/transform/FlattenAndRemoveMixins.java
@@ -38,7 +38,7 @@ Model transform(ModelTransformer transformer, Model model) {
             }
         }
 
-        if (!updatedShapes.isEmpty()) {
+        if (!updatedShapes.isEmpty() || !toRemove.isEmpty()) {
             Model.Builder builder = model.toBuilder();
             updatedShapes.forEach(builder::addShape);
             // Don't use the removeShapes transform because that would further mutate shapes and remove the things
diff --git a/smithy-model/src/test/java/software/amazon/smithy/model/transform/ModelTransformerTest.java b/smithy-model/src/test/java/software/amazon/smithy/model/transform/ModelTransformerTest.java
index 3e54ff288ce..ee267e7b431 100644
--- a/smithy-model/src/test/java/software/amazon/smithy/model/transform/ModelTransformerTest.java
+++ b/smithy-model/src/test/java/software/amazon/smithy/model/transform/ModelTransformerTest.java
@@ -145,6 +145,36 @@ public void canFilterAndRemoveMixinsWhenMixinsArePresent() {
                    Matchers.equalTo(concrete.toBuilder().flattenMixins().build()));
     }
 
+    @Test
+    public void canFilterAndRemoveMixinsWhenUnusedMixinsArePresent() {
+        ModelTransformer transformer = ModelTransformer.create();
+        Model.Builder builder = Model.builder();
+        StringShape string = StringShape.builder().id("smithy.example#String").build();
+        StructureShape mixin1 = StructureShape.builder()
+                .id("smithy.example#Mixin1")
+                .addTrait(MixinTrait.builder().build())
+                .addMember("a", string.getId())
+                .build();
+        StructureShape mixin2 = StructureShape.builder()
+                .id("smithy.example#Mixin2")
+                .addMember("b", string.getId())
+                .addTrait(MixinTrait.builder().build())
+                .build();
+        StructureShape mixin3 = StructureShape.builder()
+                .id("smithy.example#Mixin3")
+                .addMember("c", string.getId())
+                .addTrait(MixinTrait.builder().build())
+                .addMixin(mixin2)
+                .build();
+        builder.addShapes(mixin1, mixin2, mixin3);
+        Model model = builder.build();
+        Model result = transformer.flattenAndRemoveMixins(model);
+
+        assertThat(result.toSet(), Matchers.not(Matchers.hasItem(mixin1)));
+        assertThat(result.toSet(), Matchers.not(Matchers.hasItem(mixin2)));
+        assertThat(result.toSet(), Matchers.not(Matchers.hasItem(mixin3)));
+    }
+
     @ParameterizedTest
     @MethodSource("flattenShapesData")
     public void flattenShapes(String name) {