Allow empty enum with non-empty mixins

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

@@ -54,6 +54,15 @@ private EnumShape(Builder builder) {
         }
     }
 
+    private EnumShape(Builder builder, Map<String, MemberShape> members) {
+        super(builder);
+        this.members = members;
+        validateMemberShapeIds();
+        if (members.size() < 1) {
+            throw new SourceException("enum shapes must have at least one member", getSourceLocation());
+        }
+    }
+
     /**
      * Gets a map of enum member names to their corresponding values.
      *
@@ -291,8 +300,11 @@ public static final class Builder extends StringShape.Builder {
 
         @Override
         public EnumShape build() {
-            addSyntheticEnumTrait();
-            return new EnumShape(this);
+            // Collect members from enum and mixins
+            Map<String, MemberShape> aggregatedMembers =
+                NamedMemberUtils.computeMixinMembers(getMixins(), members, getId(), getSourceLocation());
+            addSyntheticEnumTrait(aggregatedMembers.values());
+            return new EnumShape(this, aggregatedMembers);
         }
 
         /**
@@ -302,10 +314,10 @@ public EnumShape build() {
          * the enum trait, without having to manually add the trait or risk that it
          * gets serialized.
          */
-        private void addSyntheticEnumTrait() {
+        private void addSyntheticEnumTrait(Collection<MemberShape> memberShapes) {
             SyntheticEnumTrait.Builder builder = SyntheticEnumTrait.builder();
             builder.sourceLocation(getSourceLocation());
-            for (MemberShape member : members.get().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] {}