Limit streaming traits to top level members

A streaming blob on a nested structure doesn't make much sense, so
this updates the streaming trait to only apply to members of
operation input/output and errors.

Fundamentally this doesn't change the way that serde code would be
generated, since at the end of the day simple types like blobs will
never get generated types.

Author: JordonPhillips

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

@@ -240,7 +240,7 @@ string since
 /// Indicates that the the data stored in the shape is very large and should not
 /// be stored in memory, or that the size of the data stored in the shape is
 /// unknown at the start of a request
-@trait(selector: "blob")
+@trait(selector: ":each(operation -[input, output]-> structure > :test(member > blob), :test(member:of(structure[trait|error]) > blob))")
 @tags(["diff.error.const"])
 structure streaming {
     /// Indicates that the stream must have a known size.

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/streaming-trait.errors

@@ -0,0 +1 @@
+[ERROR] ns.foo#InvalidStreamingStructure$Body: Trait `streaming` cannot be applied to `ns.foo#InvalidStreamingStructure$Body`. This trait may only be applied to shapes that match the following selector: :each(operation -[input, output]-> structure > :test(member > blob), :test(member:of(structure[trait|error]) > blob)) | TraitTarget

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/streaming-trait.json

@@ -0,0 +1,52 @@
+{
+    "smithy": "0.4.0",
+    "ns.foo": {
+        "shapes": {
+            "Blob": {
+                "type": "blob"
+            },
+            "StreamingOperation": {
+                "type": "operation",
+                "input": "StreamingInput",
+                "output": "StreamingOutput"
+            },
+            "StreamingInput": {
+                "type": "structure",
+                "members": {
+                    "Body": {
+                        "target": "Blob",
+                        "smithy.api#streaming": true
+                    }
+                }
+            },
+            "StreamingOutput": {
+                "type": "structure",
+                "members": {
+                    "Body": {
+                        "target": "Blob",
+                        "smithy.api#streaming": true
+                    }
+                }
+            },
+            "StreamingError": {
+                "type": "structure",
+                "members": {
+                    "Body": {
+                        "target": "Blob",
+                        "smithy.api#streaming": true
+                    }
+                },
+                "smithy.api#error": "client"
+            },
+            "InvalidStreamingStructure": {
+                "type": "structure",
+                "members": {
+                    "Body": {
+                        "target": "Blob",
+                        "smithy.api#streaming": true
+                    }
+                }
+            }
+        }
+    }
+}

smithy-openapi/src/test/resources/software/amazon/smithy/openapi/fromsmithy/mappers/streaming-service.smithy

@@ -12,8 +12,8 @@ operation StreamingOperation() -> Output
 
 structure Output {
   @httpPayload
+  @streaming
   body: StreamingPayload,
 }
 
-@streaming
 blob StreamingPayload