Operations with an event stream member shape must have `ValidationExc…

…eption` (#3814)

This PR addresses an issue where, if an operation's input includes an
event streaming member, the builder for the operation input or output
may raise a `ConstraintViolation` when `build_enforcing_all_constraints`
is called and the event streaming member field is not set. This occurs
because the member shape is required.

The standard error message that is shown when `ValidationException` is
not attached to an operation is also displayed in this case:

*Operation test#TestOperation takes in input that is constrained
(https://awslabs.github.io/smithy/2.0/spec/constraint-traits.html), and
as such can fail with a validation exception. You must model this
behavior in the operation shape in your model file.*

```smithy
use smithy.framework#ValidationException

operation TestOperation {
    ...
    errors: [..., ValidationException] // <-- Add this.
}
```
Closes: [3813](#3813)

---------

Co-authored-by: Fahad Zubair <[email protected]>

.changelog/7869753.md

@@ -0,0 +1,9 @@
+---
+applies_to: ["server"]
+authors: ["drganjoo"]
+references: ["smithy-rs#3813"]
+breaking: true
+new_feature: false
+bug_fix: true
+---
+Operations with event stream member shapes must include `ValidationException` in the errors list. This is necessary because the member shape is a required field, and the builder for the operation input or output returns a `std::result::Result` with the error set to `crate::model::ValidationExceptionField`.

codegen-core/common-test-models/constraints.smithy

@@ -190,6 +190,7 @@ operation StreamingBlobOperation {
 operation EventStreamsOperation {
     input: EventStreamsOperationInputOutput,
     output: EventStreamsOperationInputOutput,
+    errors: [ValidationException]
 }
 
 structure ConstrainedShapesOperationInputOutput {

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraints.kt

@@ -28,6 +28,7 @@ import software.amazon.smithy.model.traits.UniqueItemsTrait
 import software.amazon.smithy.rust.codegen.core.smithy.DirectedWalker
 import software.amazon.smithy.rust.codegen.core.smithy.traits.SyntheticEventStreamUnionTrait
 import software.amazon.smithy.rust.codegen.core.util.expectTrait
+import software.amazon.smithy.rust.codegen.core.util.hasEventStreamMember
 import software.amazon.smithy.rust.codegen.core.util.hasTrait
 import software.amazon.smithy.rust.codegen.core.util.inputShape
 import software.amazon.smithy.rust.codegen.core.util.orNull
@@ -190,7 +191,7 @@ fun operationShapesThatMustHaveValidationException(
         .filter { operationShape ->
             // Walk the shapes reachable via this operation input.
             walker.walkShapes(operationShape.inputShape(model))
-                .any { it is SetShape || it is EnumShape || it.hasConstraintTrait() }
+                .any { it is SetShape || it is EnumShape || it.hasConstraintTrait() || it.hasEventStreamMember(model) }
         }
         .toSet()
 }
@@ -207,7 +208,6 @@ fun validateOperationsWithConstrainedInputHaveValidationExceptionAttached(
     // `ValidationException` attached in `errors`. https://github.com/smithy-lang/smithy-rs/pull/1199#discussion_r809424783
     // TODO(https://github.com/smithy-lang/smithy-rs/issues/1401): This check will go away once we add support for
     //  `disableDefaultValidation` set to `true`, allowing service owners to map from constraint violations to operation errors.
-    val walker = DirectedWalker(model)
     val operationsWithConstrainedInputWithoutValidationExceptionSet =
         operationShapesThatMustHaveValidationException(model, service)
             .filter { !it.errors.contains(validationExceptionShapeId) }

codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraintsAreNotUsedTest.kt

@@ -81,6 +81,51 @@ internal class ValidateUnsupportedConstraintsAreNotUsedTest {
             """.trimIndent()
     }
 
+    @Test
+    fun `should detect when event streams are used, even without constraints, as the event member is required`() {
+        val model =
+            """
+            $baseModel
+            structure TestInputOutput {
+                eventStream: EventStream
+            }
+            @streaming
+            union EventStream {
+                message: Message,
+                error: Error
+            }
+            structure Message {
+                lengthString: String
+            }
+            structure Error {
+                message: String
+            }
+            """.asSmithyModel()
+        val service = model.lookup<ServiceShape>("test#TestService")
+        val validationResult =
+            validateOperationsWithConstrainedInputHaveValidationExceptionAttached(
+                model,
+                service,
+                SmithyValidationExceptionConversionGenerator.SHAPE_ID,
+            )
+
+        validationResult.messages shouldHaveSize 1
+
+        // Asserts the exact message, to ensure the formatting is appropriate.
+        validationResult.messages[0].message shouldBe
+            """
+            Operation test#TestOperation takes in input that is constrained (https://awslabs.github.io/smithy/2.0/spec/constraint-traits.html), and as such can fail with a validation exception. You must model this behavior in the operation shape in your model file.
+            ```smithy
+            use smithy.framework#ValidationException
+
+            operation TestOperation {
+                ...
+                errors: [..., ValidationException] // <-- Add this.
+            }
+            ```
+            """.trimIndent()
+    }
+
     private val constraintTraitOnStreamingBlobShapeModel =
         """
         $baseModel