Allow the xmlFlattened trait to be on members

docs/source/spec/xml.rst

@@ -98,9 +98,9 @@ Summary
     Moves serialized collection members from their collection element to that
     of the collection's container.
 Trait selector
-    ``collection``
+    ``:test(collection, member:of(structure) > collection)``
 
-    *Any list or set*
+    *Any list or set or any structure member that targets a list or set*
 Value type
     Annotation trait
 

smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude-traits.smithy

@@ -148,7 +148,7 @@ structure xmlAttribute {}
 
 /// Moves serialized collection members from their collection element to that of
 /// the collection's container.
-@trait(selector: "collection")
+@trait(selector: ":test(collection, member:of(structure) > collection)")
 @tags(["diff.error.const"])
 structure xmlFlattened {}
 

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/xml-flattened.json

@@ -0,0 +1,37 @@
+{
+    "smithy": "0.4.0",
+    "ns.foo": {
+        "shapes": {
+            "FlatCollection": {
+                "type": "list",
+                "member": {
+                    "target": "smithy.api#String"
+                },
+                "smithy.api#xmlFlattened": true
+            },
+            "NotFlatCollection": {
+                "type": "list",
+                "member": {
+                    "target": "smithy.api#String"
+                }
+            },
+            "ValidStructure1": {
+                "type": "structure",
+                "members": {
+                    "flatList": {
+                        "target": "FlatCollection"
+                    }
+                }
+            },
+            "ValidStructure2": {
+                "type": "structure",
+                "members": {
+                    "flatList": {
+                        "target": "NotFlatCollection",
+                        "smithy.api#xmlFlattened": true
+                    }
+                }
+            }
+        }
+    }
+}