Fix IDL serialization of enums with mixins

smithy-model/src/main/java/software/amazon/smithy/model/shapes/SmithyIdlModelSerializer.java

@@ -477,12 +477,17 @@ private void writeEnumMembers(Collection<MemberShape> members) {
 
         private void applyIntroducedTraits(Collection<MemberShape> members) {
             for (MemberShape member : members) {
+                Map<ShapeId, Trait> introducedTraits = new LinkedHashMap<>(member.getIntroducedTraits());
+
+                // The @enumValue trait is serialized using the `=` IDL syntax, so remove it here.
+                introducedTraits.remove(EnumValueTrait.ID);
+
                 // Use short form for a single trait, and block form for multiple traits.
-                if (member.getIntroducedTraits().size() == 1) {
+                if (introducedTraits.size() == 1) {
                     codeWriter.writeInline("apply $I ", member.getId());
                     serializeTraits(member.getIntroducedTraits(), TraitFeature.NO_SPECIAL_DOCS_SYNTAX);
                     codeWriter.write("");
-                } else if (!member.getIntroducedTraits().isEmpty()) {
+                } else if (!introducedTraits.isEmpty()) {
                     codeWriter.openBlock("apply $I {", "}", member.getId(), () -> {
                         // Only serialize local traits, and don't use special documentation syntax here.
                         serializeTraits(member.getIntroducedTraits(), TraitFeature.NO_SPECIAL_DOCS_SYNTAX);

smithy-model/src/test/java/software/amazon/smithy/model/shapes/SmithyIdlModelSerializerTest.java

@@ -243,4 +243,20 @@ public void sortsAlphabetically() {
 
         assertThat(modelResult, equalTo(IoUtils.readUtf8Url(resource).replace("\r\n", "\n")));
     }
+
+    @Test
+    public void handlesEnumMixins() {
+        URL resource = getClass().getResource("idl-serialization/enum-mixin-input.smithy");
+        Model model = Model.assembler().addImport(resource).assemble().unwrap();
+        Map<Path, String> serialized = SmithyIdlModelSerializer.builder().build().serialize(model);
+
+        if (serialized.size() != 1) {
+            throw new RuntimeException("Exactly one smithy file should be output for generated tests.");
+        }
+
+        String expectedOutput = IoUtils.readUtf8Resource(getClass(), "idl-serialization/enum-mixin-output.smithy")
+                .replaceAll("\\R", "\n");
+        String serializedString = serialized.entrySet().iterator().next().getValue();
+        Assertions.assertEquals(expectedOutput, serializedString);
+    }
 }

smithy-model/src/test/resources/software/amazon/smithy/model/shapes/idl-serialization/enum-mixin-input.smithy

@@ -0,0 +1,23 @@
+$version: "2.0"
+
+namespace ns.foo
+
+@mixin
+intEnum IntMixin {
+    FOO = 1
+    BAR = 2
+}
+
+intEnum IntEnum with [IntMixin] {
+    FOO = 1
+    BAR = 2
+}
+
+@mixin
+enum StringMixin {
+    BAZ = "baz"
+}
+
+enum StringEnum with [StringMixin] {
+    BAZ = "baz"
+}

smithy-model/src/test/resources/software/amazon/smithy/model/shapes/idl-serialization/enum-mixin-output.smithy

@@ -0,0 +1,18 @@
+$version: "2.0"
+
+namespace ns.foo
+
+intEnum IntEnum with [IntMixin] {}
+
+@mixin
+intEnum IntMixin {
+    FOO = 1
+    BAR = 2
+}
+
+enum StringEnum with [StringMixin] {}
+
+@mixin
+enum StringMixin {
+    BAZ = "baz"
+}