Add decorator and setting to set hints.mostly-unused

.changelog/hints-mostly-unused.md

@@ -0,0 +1,23 @@
+---
+applies_to:
+  - aws-sdk-rust
+  - client
+authors:
+  - landonxjames
+references:
+  - smithy-rs#4208
+breaking: false
+new_feature: true
+bug_fix: false
+---
+Add the ability to insert `hints.mostly-unused = true` in Cargo.toml. Enable this hint for the below crates:
+- aws-sd-cloudformation
+- aws-sdk-dynamodb
+- aws-sdk-ec2
+- aws-sdk-s3
+- aws-sdk-sns
+- aws-sdk-sqs
+- aws-sdk-ssm
+- aws-sdk-sts
+
+See more information about this hint at https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/

aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/AwsCodegenDecorator.kt

@@ -7,6 +7,8 @@ package software.amazon.smithy.rustsdk
 
 import software.amazon.smithy.rust.codegen.client.smithy.customizations.DocsRsMetadataDecorator
 import software.amazon.smithy.rust.codegen.client.smithy.customizations.DocsRsMetadataSettings
+import software.amazon.smithy.rust.codegen.client.smithy.customizations.ManifestHintsDecorator
+import software.amazon.smithy.rust.codegen.client.smithy.customizations.ManifestHintsSettings
 import software.amazon.smithy.rust.codegen.client.smithy.customize.ClientCodegenDecorator
 import software.amazon.smithy.rust.codegen.client.smithy.customize.CombinedClientCodegenDecorator
 import software.amazon.smithy.rustsdk.customize.AwsDisableStalledStreamProtection
@@ -22,6 +24,7 @@ import software.amazon.smithy.rustsdk.customize.dsql.DsqlDecorator
 import software.amazon.smithy.rustsdk.customize.ec2.Ec2Decorator
 import software.amazon.smithy.rustsdk.customize.glacier.GlacierDecorator
 import software.amazon.smithy.rustsdk.customize.onlyApplyTo
+import software.amazon.smithy.rustsdk.customize.onlyApplyToList
 import software.amazon.smithy.rustsdk.customize.rds.RdsDecorator
 import software.amazon.smithy.rustsdk.customize.route53.Route53Decorator
 import software.amazon.smithy.rustsdk.customize.s3.S3Decorator
@@ -109,6 +112,18 @@ val DECORATORS: List<ClientCodegenDecorator> =
                 ),
             ),
         ),
+        ManifestHintsDecorator(ManifestHintsSettings(mostlyUnused = true)).onlyApplyToList(
+            listOf(
+                "com.amazonaws.cloudformation#CloudFormation",
+                "com.amazonaws.dynamodb#DynamoDB_20120810",
+                "com.amazonaws.ec2#AmazonEC2",
+                "com.amazonaws.s3#AmazonS3",
+                "com.amazonaws.sns#AmazonSimpleNotificationService",
+                "com.amazonaws.sqs#AmazonSQS",
+                "com.amazonaws.ssm#AmazonSSM",
+                "com.amazonaws.sts#AWSSecurityTokenServiceV20110615",
+            ),
+        ),
     ).flatten()
 
 class AwsCodegenDecorator : CombinedClientCodegenDecorator(DECORATORS) {

aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/customize/ServiceSpecificDecorator.kt

@@ -17,6 +17,14 @@ fun ClientCodegenDecorator.onlyApplyTo(serviceId: String): List<ClientCodegenDec
         },
     )
 
+/** Only apply this decorator to the given list of service IDs */
+fun ClientCodegenDecorator.onlyApplyToList(serviceIds: List<String>): List<ClientCodegenDecorator> =
+    serviceIds.map {
+        ConditionalDecorator(this) { _, serviceShapeId ->
+            serviceShapeId == ShapeId.from(it)
+        }
+    }
+
 /** Apply this decorator to all services but the one identified by ID */
 fun ClientCodegenDecorator.applyExceptFor(serviceId: String): List<ClientCodegenDecorator> =
     listOf(

aws/sdk/build.gradle.kts

@@ -494,8 +494,8 @@ fun Project.registerDowngradeFor(
             val crateNameToLastKnownWorkingVersions =
                 mapOf(
                     "minicbor" to "0.24.2",
-                    "libfuzzer-sys" to "0.4.7" // TODO(https://github.com/rust-fuzz/libfuzzer/issues/126)
-                    )
+                    "libfuzzer-sys" to "0.4.7", // TODO(https://github.com/rust-fuzz/libfuzzer/issues/126)
+                )
 
             crateNameToLastKnownWorkingVersions.forEach { (crate, version) ->
                 // doesn't matter even if the specified crate does not exist in the lockfile

aws/sdk/gradle.properties

@@ -2,5 +2,13 @@
 # Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 # SPDX-License-Identifier: Apache-2.0
 #
-
 aws.services=
+aws.services.hints.mostlyUnused=\
+  aws-sd-cloudformation,\
+  aws-sdk-dynamodb,\
+  aws-sdk-ec2,\
+  aws-sdk-s3,\
+  aws-sdk-sns,\
+  aws-sdk-sqs,\
+  aws-sdk-ssm,\
+  aws-sdk-sts

codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/ClientRustSettings.kt

@@ -115,14 +115,29 @@ data class ClientCodegenConfig(
         ) = if (node.isPresent) {
             ClientCodegenConfig(
                 formatTimeoutSeconds = coreCodegenConfig.formatTimeoutSeconds,
-                flattenCollectionAccessors = node.get().getBooleanMemberOrDefault("flattenCollectionAccessors", DEFAULT_FLATTEN_ACCESSORS),
+                flattenCollectionAccessors =
+                    node.get()
+                        .getBooleanMemberOrDefault("flattenCollectionAccessors", DEFAULT_FLATTEN_ACCESSORS),
                 debugMode = coreCodegenConfig.debugMode,
                 renameExceptions = node.get().getBooleanMemberOrDefault("renameErrors", DEFAULT_RENAME_EXCEPTIONS),
-                includeFluentClient = node.get().getBooleanMemberOrDefault("includeFluentClient", DEFAULT_INCLUDE_FLUENT_CLIENT),
-                addMessageToErrors = node.get().getBooleanMemberOrDefault("addMessageToErrors", DEFAULT_ADD_MESSAGE_TO_ERRORS),
-                includeEndpointUrlConfig = node.get().getBooleanMemberOrDefault("includeEndpointUrlConfig", DEFAULT_INCLUDE_ENDPOINT_URL_CONFIG),
-                enableUserConfigurableRuntimePlugins = node.get().getBooleanMemberOrDefault("enableUserConfigurableRuntimePlugins", DEFAULT_ENABLE_USER_CONFIGURABLE_RUNTIME_PLUGINS),
-                nullabilityCheckMode = NullableIndex.CheckMode.valueOf(node.get().getStringMemberOrDefault("nullabilityCheckMode", DEFAULT_NULLABILITY_CHECK_MODE)),
+                includeFluentClient =
+                    node.get()
+                        .getBooleanMemberOrDefault("includeFluentClient", DEFAULT_INCLUDE_FLUENT_CLIENT),
+                addMessageToErrors =
+                    node.get()
+                        .getBooleanMemberOrDefault("addMessageToErrors", DEFAULT_ADD_MESSAGE_TO_ERRORS),
+                includeEndpointUrlConfig =
+                    node.get()
+                        .getBooleanMemberOrDefault("includeEndpointUrlConfig", DEFAULT_INCLUDE_ENDPOINT_URL_CONFIG),
+                enableUserConfigurableRuntimePlugins =
+                    node.get().getBooleanMemberOrDefault(
+                        "enableUserConfigurableRuntimePlugins",
+                        DEFAULT_ENABLE_USER_CONFIGURABLE_RUNTIME_PLUGINS,
+                    ),
+                nullabilityCheckMode =
+                    NullableIndex.CheckMode.valueOf(
+                        node.get().getStringMemberOrDefault("nullabilityCheckMode", DEFAULT_NULLABILITY_CHECK_MODE),
+                    ),
             )
         } else {
             ClientCodegenConfig(

codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/customizations/ManifestHintsDecorator.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.rust.codegen.client.smithy.customizations
+
+import software.amazon.smithy.rust.codegen.client.smithy.ClientCodegenContext
+import software.amazon.smithy.rust.codegen.client.smithy.customize.ClientCodegenDecorator
+import software.amazon.smithy.rust.codegen.core.smithy.generators.ManifestCustomizations
+
+/**
+ * See https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/ for more information
+ */
+data class ManifestHintsSettings(
+    val mostlyUnused: Boolean? = null,
+)
+
+fun ManifestHintsSettings.asMap(): Map<String, Any> {
+    val inner =
+        listOfNotNull(
+            mostlyUnused?.let { "mostly-unused" to it },
+        ).toMap()
+    return mapOf("hints" to inner)
+}
+
+/**
+ * Write compilation hints metdata settings into Cargo.toml.
+ *
+ * # Notes
+ * This decorator is not used by default, code generators must manually configure and include it in their builds.
+ */
+class ManifestHintsDecorator(private val manifestHintsSettings: ManifestHintsSettings) : ClientCodegenDecorator {
+    override val name: String = "ManifestHintsDecorator"
+    override val order: Byte = 0
+
+    override fun crateManifestCustomizations(codegenContext: ClientCodegenContext): ManifestCustomizations {
+        return manifestHintsSettings.asMap()
+    }
+}

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

@@ -52,9 +52,13 @@ open class CoreCodegenConfig(
         fun fromNode(node: Optional<ObjectNode>): CoreCodegenConfig =
             if (node.isPresent) {
                 CoreCodegenConfig(
-                    formatTimeoutSeconds = node.get().getNumberMemberOrDefault("formatTimeoutSeconds", DEFAULT_FORMAT_TIMEOUT_SECONDS).toInt(),
+                    formatTimeoutSeconds =
+                        node.get()
+                            .getNumberMemberOrDefault("formatTimeoutSeconds", DEFAULT_FORMAT_TIMEOUT_SECONDS).toInt(),
                     debugMode = node.get().getBooleanMemberOrDefault("debugMode", DEFAULT_DEBUG_MODE),
-                    flattenCollectionAccessors = node.get().getBooleanMemberOrDefault("flattenCollectionAccessors", DEFAULT_FLATTEN_MODE),
+                    flattenCollectionAccessors =
+                        node.get()
+                            .getBooleanMemberOrDefault("flattenCollectionAccessors", DEFAULT_FLATTEN_MODE),
                 )
             } else {
                 CoreCodegenConfig(
@@ -123,12 +127,14 @@ open class CoreRustSettings(
                             "contain any service shapes",
                     )
                 }
+
                 services.size > 1 -> {
                     throw CodegenException(
                         "Cannot infer service to generate because the model contains " +
                             "multiple service shapes: " + services,
                     )
                 }
+
                 else -> {
                     val service = services[0]
                     LOGGER.info("Inferring service to generate as: $service")