Forbid fully recursive unions

docs/source/1.0/spec/core/model.rst

@@ -969,6 +969,8 @@ Smithy allows recursive shape definitions with the following limitations:
    a recursive list in Java ``List<List<List....``).
 2. A structure cannot contain a cyclical set of members marked with the
    :ref:`required-trait` that refers back to itself.
+3. A union MUST contain at least one member that does not refer back to
+   itself.
 
 The following recursive shape definition is **valid**:
 

smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/ShapeRecursionValidator.java

@@ -23,9 +23,11 @@
 import software.amazon.smithy.model.selector.PathFinder;
 import software.amazon.smithy.model.shapes.ListShape;
 import software.amazon.smithy.model.shapes.MapShape;
+import software.amazon.smithy.model.shapes.MemberShape;
 import software.amazon.smithy.model.shapes.SetShape;
 import software.amazon.smithy.model.shapes.Shape;
 import software.amazon.smithy.model.shapes.StructureShape;
+import software.amazon.smithy.model.shapes.UnionShape;
 import software.amazon.smithy.model.traits.RequiredTrait;
 import software.amazon.smithy.model.validation.AbstractValidator;
 import software.amazon.smithy.model.validation.ValidationEvent;
@@ -50,6 +52,7 @@ public List<ValidationEvent> validate(Model model) {
         List<ValidationEvent> events = new ArrayList<>();
         validateListMapSetShapes(finder, model, events);
         validateStructurePaths(finder, model, events);
+        validateUnions(model, events);
         return events;
     }
 
@@ -108,4 +111,18 @@ private String formatPath(PathFinder.Path path) {
         }
         return joiner.toString();
     }
+
+    private void validateUnions(Model model, List<ValidationEvent> events) {
+        nextUnion: for (UnionShape union : model.getUnionShapes()) {
+            // Don't claim the union is recursive if it's empty (which is also invalid).
+            if (!union.getAllMembers().isEmpty()) {
+                for (MemberShape member : union.getAllMembers().values()) {
+                    if (!member.getTarget().equals(member.getContainer())) {
+                        continue nextUnion;
+                    }
+                }
+                events.add(error(union, "Unions must contain at least one member that is not directly recursive"));
+            }
+        }
+    }
 }

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/union-cant-be-fully-recursive.errors

@@ -0,0 +1 @@
+[ERROR] smithy.example#RecursiveUnion: Unions must contain at least one member that is not directly recursive | ShapeRecursion

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/union-cant-be-fully-recursive.smithy

@@ -0,0 +1,15 @@
+$version: "1.0"
+
+namespace smithy.example
+
+// It's impossible to provide a value for this union.
+union RecursiveUnion {
+    a: RecursiveUnion,
+    b: RecursiveUnion,
+}
+
+union NotFullyRecursiveUnion {
+    a: RecursiveUnion,
+    b: RecursiveUnion,
+    c: String
+}