Error out if ignoreUnsupportedConstraintTraits has no effect

Now that constraint traits are supported in server SDKs (with some
corner case caveats, see #1401), this flag will almost always be useless
for those early adopters of constraint traits. It is thus convenient to
inform the user that they should remove it.

CHANGELOG.next.toml

@@ -80,3 +80,9 @@ message = "Add `into_segments` method to `AggregatedBytes`, for zero-copy conver
 references = ["smithy-rs#2525"]
 meta = { "breaking" = false, "tada" = false, "bug" = false }
 author = "parker-timmerman"
+
+[[smithy-rs]]
+message = "Code generation will abort if the `ignoreUnsupportedConstraints` codegen flag has no effect, that is, if all constraint traits used in your model are well-supported. Please remove the flag in such case."
+references = ["smithy-rs#2539"]
+meta = { "breaking" = true, "tada" = false, "bug" = false, "target" = "server" }
+author = "david-perez"

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

@@ -158,8 +158,6 @@ data class LogMessage(val level: Level, val message: String)
 data class ValidationResult(val shouldAbort: Boolean, val messages: List<LogMessage>) :
     Throwable(message = messages.joinToString("\n") { it.message })
 
-private val unsupportedConstraintsOnMemberShapes = allConstraintTraits - RequiredTrait::class.java
-
 /**
  * Validate that all constrained operations have the shape [validationExceptionShapeId] shape attached to their errors.
  */
@@ -280,7 +278,7 @@ fun validateUnsupportedConstraints(
         .toSet()
 
     val messages =
-        unsupportedLengthTraitOnStreamingBlobShapeSet.map {
+        (unsupportedLengthTraitOnStreamingBlobShapeSet.map {
             it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints)
         } +
             unsupportedConstraintShapeReachableViaAnEventStreamSet.map {
@@ -289,7 +287,17 @@ fun validateUnsupportedConstraints(
             unsupportedRangeTraitOnShapeSet.map { it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints) } +
             mapShapeReachableFromUniqueItemsListShapeSet.map {
                 it.intoLogMessage(codegenConfig.ignoreUnsupportedConstraints)
-            }
+            }).toMutableList()
+
+    if (messages.isEmpty() && codegenConfig.ignoreUnsupportedConstraints) {
+        messages += LogMessage(
+            Level.SEVERE,
+            """
+            The `ignoreUnsupportedConstraints` flag in the `codegen` configuration is set to `true`, but it has no 
+            effect. All the constraint traits used in the model are well-supported, please remove this flag.
+            """.trimIndent().replace("\n", " ")
+        )
+    }
 
     return ValidationResult(shouldAbort = messages.any { it.level == Level.SEVERE }, messages)
 }

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

@@ -19,6 +19,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.transformers.EventStreamN
 import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
 import software.amazon.smithy.rust.codegen.core.util.lookup
 import software.amazon.smithy.rust.codegen.server.smithy.customizations.SmithyValidationExceptionConversionGenerator
+import java.io.File
 import java.util.logging.Level
 
 internal class ValidateUnsupportedConstraintsAreNotUsedTest {
@@ -37,7 +38,7 @@ internal class ValidateUnsupportedConstraintsAreNotUsedTest {
         """
 
     private fun validateModel(model: Model, serverCodegenConfig: ServerCodegenConfig = ServerCodegenConfig()): ValidationResult {
-        val service = model.lookup<ServiceShape>("test#TestService")
+        val service = model.serviceShapes.first()
         return validateUnsupportedConstraints(model, service, serverCodegenConfig)
     }
 
@@ -100,7 +101,7 @@ internal class ValidateUnsupportedConstraintsAreNotUsedTest {
         """.trimIndent().replace("\n", " ")
     }
 
-    val constrainedShapesInEventStreamModel =
+    private val constrainedShapesInEventStreamModel =
         """
         $baseModel
 
@@ -242,4 +243,25 @@ internal class ValidateUnsupportedConstraintsAreNotUsedTest {
         validationResult.messages shouldHaveAtLeastSize 1
         validationResult.messages.shouldForAll { it.level shouldBe Level.WARNING }
     }
+
+    @Test
+    fun `it should abort when ignoreUnsupportedConstraints is true and all used constraints are supported`() {
+        val allConstraintTraitsAreSupported = File("../codegen-core/common-test-models/constraints.smithy")
+            .readText()
+            .asSmithyModel()
+
+        val validationResult = validateModel(
+            allConstraintTraitsAreSupported,
+            ServerCodegenConfig().copy(ignoreUnsupportedConstraints = true),
+        )
+
+        validationResult.messages shouldHaveSize 1
+        validationResult.shouldAbort shouldBe true
+        validationResult.messages[0].message shouldContain(
+            """
+            The `ignoreUnsupportedConstraints` flag in the `codegen` configuration is set to `true`, but it has no 
+            effect. All the constraint traits used in the model are well-supported, please remove this flag.
+            """.trimIndent().replace("\n", " ")
+            )
+    }
 }