Add validation and codegen for custom validation exception traits (#4317)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

Adds ability to define custom validation exceptions per RFC:

-
https://github.com/smithy-lang/smithy-rs/blob/custom-validation-rfc/design/src/rfcs/rfc0047_custom_validation.md

## Description
<!--- Describe your changes in detail -->

Adds ability to use the following traits:
- @validationException
- @validationMessage
- @validationFieldList
- @validationFieldName
- @validationFieldMessage

to define a custom validation exception for a service or operation.

Updated Smithy validation ensures:
- The custom validation exception shape also has @error trait
- The custom validation exception shape has exactly one member with the
@validationMessage trait
- Default constructibility if it contains constrained shapes
- At most one custom validation exception is defined
- `smithy.framework#ValidationException` is not used in an operation or
service is a custom validation exception is defined
- Operations with constrained input have exactly one of the default
validation exception or a custom validation exception attached to their
errors.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

- For unit testing, the following were added/updated:
-
`codegen-server-traits/src/test/kotlin/software/amazon/smithy/rust/codegen/server/traits/ValidationExceptionTraitTest.kt`
-
`codegen-server-traits/src/test/kotlin/software/amazon/smithy/rust/codegen/server/traits/ValidationMessageTraitTest.kt`
-
`codegen-server-traits/src/test/kotlin/software/amazon/smithy/rust/codegen/server/traits/ValidationFieldListTraitTest.kt`
-
`codegen-server-traits/src/test/kotlin/software/amazon/smithy/rust/codegen/server/traits/ValidationFieldNameTraitTest.kt`
-
`codegen-server-traits/src/test/kotlin/software/amazon/smithy/rust/codegen/server/traits/ValidationFieldMessageTraitTest.kt`
-
`codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraintsAreNotUsedTest.kt`
-
`codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/customizations/CustomValidationExceptionDecoratorTest.kt`
-
`codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/validators/CustomValidationExceptionValidatorTest.kt`
- For integration testing
- serverIntegrationTests in
`codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/customizations/CustomValidationExceptionDecoratorTest.kt`
- For e2e integration testing
-
`codegen-server-test/custom-test-models/custom-validation-exception.smithy`
    - `codegen-server-test/build.gradle.kts`
- For regression testing
- The existing e2e integration tests in `codegen-server-tests` use
common models in `codegen-core/common-test-models`, which use the
default `smithy.framework#ValidationException`

Ran with `export RUSTFLAGS="-D warnings -A clippy::redundant_closure -A
non_local_definitions"` until fixed in server runtime crates. See
#4122.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Author: jasgin

.changelog/1759254918.md

@@ -0,0 +1,11 @@
+---
+applies_to: ["server"]
+authors: ["jasgin"]
+references: ["smithy-rs#4317"]
+breaking: false
+new_feature: true
+bug_fix: false
+---
+Adds validators and codegen support for the custom traits custom traits `@validationException`, `@validationMessage`,
+`@validationFieldList`, `@validationFieldName`, and `@validationFieldMessage` for defining a custom validation exception
+to use instead of `smithy.framework#ValidationException`.

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/util/Smithy.kt

@@ -132,6 +132,8 @@ fun Shape.targetOrSelf(model: Model): Shape =
         else -> this
     }
 
+fun MemberShape.targetShape(model: Model): Shape = model.expectShape(this.target)
+
 /** Kotlin sugar for hasTrait() check. e.g. shape.hasTrait<EnumTrait>() instead of shape.hasTrait(EnumTrait::class.java) */
 inline fun <reified T : Trait> Shape.hasTrait(): Boolean = hasTrait(T::class.java)
 

codegen-server-test/build.gradle.kts

@@ -33,7 +33,7 @@ smithy {
     format.set(false)
 }
 
-val allCodegenTests = "../codegen-core/common-test-models".let { commonModels ->
+val commonCodegenTests = "../codegen-core/common-test-models".let { commonModels ->
     listOf(
         CodegenTest(
             "crate#Config",
@@ -118,6 +118,18 @@ val allCodegenTests = "../codegen-core/common-test-models".let { commonModels ->
 // When iterating on protocol tests use this to speed up codegen:
 // .filter { it.module == "rpcv2Cbor_extras" || it.module == "rpcv2Cbor_extras_no_initial_response" }
 
+val customCodegenTests = "custom-test-models".let { customModels ->
+    listOf(
+        CodegenTest(
+            "com.aws.example#CustomValidationExample",
+            "custom-validation-exception-example",
+            imports = listOf("$customModels/custom-validation-exception.smithy"),
+        ),
+    )
+}
+
+val allCodegenTests = commonCodegenTests + customCodegenTests
+
 project.registerGenerateSmithyBuildTask(rootProject, pluginName, allCodegenTests)
 project.registerGenerateCargoWorkspaceTask(rootProject, pluginName, allCodegenTests, workingDirUnderBuildDir)
 project.registerGenerateCargoConfigTomlTask(layout.buildDirectory.dir(workingDirUnderBuildDir).get().asFile)

codegen-server-test/custom-test-models/custom-validation-exception.smithy

@@ -0,0 +1,70 @@
+$version: "2.0"
+
+namespace com.aws.example
+
+use aws.protocols#restJson1
+use smithy.framework.rust#validationException
+use smithy.framework.rust#validationFieldList
+use smithy.framework.rust#validationFieldMessage
+use smithy.framework.rust#validationFieldName
+use smithy.framework.rust#validationMessage
+
+@restJson1
+service CustomValidationExample {
+    version: "1.0.0"
+    operations: [
+        TestOperation
+    ]
+    errors: [
+        MyCustomValidationException
+    ]
+}
+
+@http(method: "POST", uri: "/test")
+operation TestOperation {
+    input: TestInput
+}
+
+structure TestInput {
+    @required
+    @length(min: 1, max: 10)
+    name: String
+
+    @range(min: 1, max: 100)
+    age: Integer
+}
+
+@error("client")
+@httpError(400)
+@validationException
+structure MyCustomValidationException {
+    @required
+    @validationMessage
+    customMessage: String
+
+    @required
+    @default("testReason1")
+    reason: ValidationExceptionReason
+
+    @validationFieldList
+    customFieldList: CustomValidationFieldList
+}
+
+enum ValidationExceptionReason {
+    TEST_REASON_0 = "testReason0"
+    TEST_REASON_1 = "testReason1"
+}
+
+structure CustomValidationField {
+    @required
+    @validationFieldName
+    customFieldName: String
+
+    @required
+    @validationFieldMessage
+    customFieldMessage: String
+}
+
+list CustomValidationFieldList {
+    member: CustomValidationField
+}

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

@@ -10,6 +10,8 @@ import software.amazon.smithy.model.Model
 import software.amazon.smithy.model.shapes.BlobShape
 import software.amazon.smithy.model.shapes.ByteShape
 import software.amazon.smithy.model.shapes.CollectionShape
+import software.amazon.smithy.model.shapes.EnumShape
+import software.amazon.smithy.model.shapes.IntEnumShape
 import software.amazon.smithy.model.shapes.IntegerShape
 import software.amazon.smithy.model.shapes.LongShape
 import software.amazon.smithy.model.shapes.MapShape
@@ -97,6 +99,39 @@ fun Shape.isDirectlyConstrained(symbolProvider: SymbolProvider): Boolean =
             this.members().any { !symbolProvider.toSymbol(it).isOptional() && !it.hasNonNullDefault() }
         }
 
+        else -> this.isDirectlyConstrainedHelper()
+    }
+
+/**
+ * Finds shapes that are directly constrained in validation phase, which means the shape is a:
+ * - [StructureShape] with a required member that does not have a non-null default
+ * - [EnumShape]
+ * - [IntEnumShape]
+ * - [MemberShape] that is required and does not have a non-null default
+ *
+ * We use this rather than [Shape.isDirectlyConstrained] to check for constrained shapes in validation phase because
+ * the [SymbolProvider] has not yet been created
+ */
+fun Shape.isDirectlyConstrainedForValidation(): Boolean =
+    when (this) {
+        is StructureShape -> {
+            // we use `member.isOptional` here because the issue outlined in (https://github.com/smithy-lang/smithy-rs/issues/1302)
+            // should not be relevant in validation phase
+            this.members().any { !it.isOptional && !it.hasNonNullDefault() }
+        }
+
+        // For alignment with
+        // (https://github.com/smithy-lang/smithy-rs/blob/custom-validation-rfc/design/src/rfcs/rfc0047_custom_validation.md#terminology)
+        // TODO(move to [isDirectlyConstrainerHelper] if they can be safely applied to [isDirectlyConstrained] without breaking implications)
+        is EnumShape -> true
+        is IntEnumShape -> true
+        is MemberShape -> !this.isOptional && !this.hasNonNullDefault()
+
+        else -> this.isDirectlyConstrainedHelper()
+    }
+
+private fun Shape.isDirectlyConstrainedHelper(): Boolean =
+    when (this) {
         is MapShape -> this.hasTrait<LengthTrait>()
         is StringShape -> this.hasTrait<EnumTrait>() || supportedStringConstraintTraits.any { this.hasTrait(it) }
         is CollectionShape -> supportedCollectionConstraintTraits.any { this.hasTrait(it) }
@@ -129,11 +164,27 @@ fun Shape.canReachConstrainedShape(
         DirectedWalker(model).walkShapes(this).toSet().any { it.isDirectlyConstrained(symbolProvider) }
     }
 
+/**
+ * Whether this shape (or the shape's target for [MemberShape]s) can reach constrained shapes for validations.
+ *
+ * We use this rather than [Shape.canReachConstrainedShape] to check for constrained shapes in validation phase because
+ * the [SymbolProvider] has not yet been created
+ */
+fun Shape.canReachConstrainedShapeForValidation(model: Model): Boolean =
+    if (this is MemberShape) {
+        this.targetCanReachConstrainedShapeForValidation(model)
+    } else {
+        DirectedWalker(model).walkShapes(this).toSet().any { it.isDirectlyConstrainedForValidation() }
+    }
+
 fun MemberShape.targetCanReachConstrainedShape(
     model: Model,
     symbolProvider: SymbolProvider,
 ): Boolean = model.expectShape(this.target).canReachConstrainedShape(model, symbolProvider)
 
+fun MemberShape.targetCanReachConstrainedShapeForValidation(model: Model): Boolean =
+    model.expectShape(this.target).canReachConstrainedShapeForValidation(model)
+
 fun Shape.hasPublicConstrainedWrapperTupleType(
     model: Model,
     publicConstrainedTypes: Boolean,

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

@@ -20,6 +20,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.SymbolVisitor
 import software.amazon.smithy.rust.codegen.server.smithy.customizations.CustomValidationExceptionWithReasonDecorator
 import software.amazon.smithy.rust.codegen.server.smithy.customizations.ServerRequiredCustomizations
 import software.amazon.smithy.rust.codegen.server.smithy.customizations.SmithyValidationExceptionDecorator
+import software.amazon.smithy.rust.codegen.server.smithy.customizations.UserProvidedValidationExceptionDecorator
 import software.amazon.smithy.rust.codegen.server.smithy.customize.CombinedServerCodegenDecorator
 import software.amazon.smithy.rust.codegen.server.smithy.customize.ServerCodegenDecorator
 import software.amazon.smithy.rust.codegen.server.smithy.testutil.ServerDecoratableBuildPlugin
@@ -50,6 +51,7 @@ class RustServerCodegenPlugin : ServerDecoratableBuildPlugin() {
             CombinedServerCodegenDecorator.fromClasspath(
                 context,
                 ServerRequiredCustomizations(),
+                UserProvidedValidationExceptionDecorator(),
                 SmithyValidationExceptionDecorator(),
                 CustomValidationExceptionWithReasonDecorator(),
                 *decorator,

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

@@ -238,6 +238,7 @@ open class ServerCodegenVisitor(
 
         val validationExceptionShapeId = validationExceptionConversionGenerator.shapeId
         for (validationResult in listOf(
+            validateModelHasAtMostOneValidationException(model, service),
             codegenDecorator.postprocessValidationExceptionNotAttachedErrorMessage(
                 validateOperationsWithConstrainedInputHaveValidationExceptionAttached(
                     model,
@@ -246,6 +247,13 @@ open class ServerCodegenVisitor(
                 ),
             ),
             validateUnsupportedConstraints(model, service, codegenContext.settings.codegenConfig),
+            codegenDecorator.postprocessMultipleValidationExceptionsErrorMessage(
+                validateOperationsWithConstrainedInputHaveOneValidationExceptionAttached(
+                    model,
+                    service,
+                    validationExceptionShapeId,
+                ),
+            ),
         )) {
             for (logMessage in validationResult.messages) {
                 // TODO(https://github.com/smithy-lang/smithy-rs/issues/1756): These are getting duplicated.