Allow empty enum with non-empty mixins

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

@@ -305,7 +305,9 @@ public EnumShape build() {
         private void addSyntheticEnumTrait() {
             SyntheticEnumTrait.Builder builder = SyntheticEnumTrait.builder();
             builder.sourceLocation(getSourceLocation());
-            for (MemberShape member : members.get().values()) {
+            Collection<MemberShape> memberShapes = NamedMemberUtils.computeMixinMembers(
+                getMixins(), members, getId(), getSourceLocation()).values();
+            for (MemberShape member : memberShapes) {
                 try {
                     builder.addEnum(EnumShape.enumDefinitionFromMember(member));
                 } catch (IllegalStateException e) {

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/mixins/empty-enum-mixin.errors

@@ -0,0 +1,2 @@
+[ERROR] smithy.example#EmptyEnumMixin: enum must have at least one entry | Model
+[ERROR] smithy.example#EmptyEnum: Cannot apply mixin to smithy.example#EmptyEnum: smithy.example#EmptyEnumMixin not found | Model

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/mixins/empty-enum-mixin.smithy

@@ -0,0 +1,8 @@
+$version: "2.0"
+
+namespace smithy.example
+
+@mixin
+enum EmptyEnumMixin {}
+
+enum EmptyEnum with [EmptyEnumMixin] {}

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/mixins/non-empty-enum-mixin.smithy

@@ -0,0 +1,10 @@
+$version: "2.0"
+
+namespace smithy.example
+
+@mixin
+enum NonEmptyEnumMixin {
+    VALUE = "value"
+}
+
+enum NonEmptyEnum with [NonEmptyEnumMixin] {}