Add flattenAndRemoveMixins build transform

docs/source-2.0/guides/smithy-build-json.rst

@@ -1518,6 +1518,28 @@ key is not in the provided ``keys`` list.
         }
     }
 
+.. _flattenAndRemoveMixins:
+
+flattenAndRemoveMixins
+----------------------
+
+Flattens :ref:`mixins <Mixins>` out of the model and into their local shapes.
+
+.. code-block:: json
+
+    {
+        "version": "1.0",
+        "projections": {
+            "exampleProjection": {
+                "transforms": [
+                    {
+                        "name": "flattenAndRemoveMixins"
+                    }
+                ]
+            }
+        }
+    }
+
 .. _flattenNamespaces:
 
 flattenNamespaces

smithy-build/src/main/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixins.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.build.transforms;
+
+import software.amazon.smithy.build.ProjectionTransformer;
+import software.amazon.smithy.build.TransformContext;
+import software.amazon.smithy.model.Model;
+
+/**
+ * {@code flattenAndRemoveMixins} flattens mixins out of the model.
+ */
+public final class FlattenAndRemoveMixins implements ProjectionTransformer {
+
+    @Override
+    public String getName() {
+        return "flattenAndRemoveMixins";
+    }
+
+    @Override
+    public Model transform(TransformContext context) {
+        Model model = context.getModel();
+        return context.getTransformer().flattenAndRemoveMixins(model);
+    }
+}

smithy-build/src/test/java/software/amazon/smithy/build/transforms/FlattenAndRemoveMixinsTest.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.build.transforms;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.contains;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+import software.amazon.smithy.model.Model;
+import software.amazon.smithy.model.shapes.ShapeId;
+import software.amazon.smithy.model.shapes.StructureShape;
+import software.amazon.smithy.model.traits.MixinTrait;
+import software.amazon.smithy.model.transform.ModelTransformer;
+
+public class FlattenAndRemoveMixinsTest {
+
+    @Test
+    void compareTransform() {
+        Model before = Model.assembler()
+                .addImport(FlattenAndRemoveMixinsTest.class.getResource("flatten-and-remove-mixins.smithy"))
+                .assemble()
+                .unwrap();
+        Model result = ModelTransformer.create().flattenAndRemoveMixins(before);
+
+        assertTrue(result.getShapesWithTrait(MixinTrait.class).isEmpty());
+        assertThat(result.expectShape(ShapeId.from("smithy.example#Foo"), StructureShape.class).getMemberNames(),
+                contains("bar", "foo"));
+    }
+}

smithy-build/src/test/resources/software/amazon/smithy/build/transforms/flatten-and-remove-mixins.smithy

@@ -0,0 +1,12 @@
+$version: "2.0"
+
+namespace smithy.example
+
+@mixin
+structure FooMixin {
+    bar: String
+}
+
+structure Foo with [FooMixin] {
+    foo: String
+}