Ensure services with no operations generate valid Rust

CHANGELOG.next.toml

@@ -198,3 +198,9 @@ message = "Fix inconsistent casing in services re-export."
 references = ["smithy-rs#2349"]
 meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "server" }
 author = "hlbarber"
+
+[[smithy-rs]]
+message = "Ensure the server side code generator creates valid code for services with no operations."
+references = ["smithy-rs#2351"]
+meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "server"}
+author = "hlbarber"

codegen-client-test/build.gradle.kts

@@ -92,6 +92,11 @@ val allCodegenTests = "../codegen-core/common-test-models".let { commonModels ->
             """.trimIndent(),
             imports = listOf("$commonModels/naming-obstacle-course-structs.smithy"),
         ),
+        CodegenTest(
+            "emptyservice#EmptyService",
+            "empty_service",
+            imports = listOf("$commonModels/empty-service.smithy"),
+        ),
         CodegenTest("aws.protocoltests.json#TestService", "endpoint-rules"),
         CodegenTest("com.aws.example.rust#PokemonService", "pokemon-service-client", imports = listOf("$commonModels/pokemon.smithy", "$commonModels/pokemon-common.smithy")),
         CodegenTest("com.aws.example.rust#PokemonService", "pokemon-service-awsjson-client", imports = listOf("$commonModels/pokemon-awsjson.smithy", "$commonModels/pokemon-common.smithy")),

codegen-core/common-test-models/empty-service.smithy

@@ -0,0 +1,8 @@
+namespace emptyservice
+
+use aws.protocols#restJson1
+
+@restJson1
+service EmptyService {
+    operations: []
+}

codegen-core/common-test-models/naming-obstacle-course-casing.smithy

@@ -10,14 +10,11 @@ use aws.protocols#awsJson1_1
 @awsJson1_1
 service ACRONYMInside_Service {
     operations: [
-        DoNothing,
     //    ACRONYMInside_Op
     //    ACRONYM_InsideOp
     ]
 }
 
-operation DoNothing {}
-
 // operation ACRONYMInside_Op {
 //     input: Input,
 //     output: Output,

codegen-server-test/build.gradle.kts

@@ -102,6 +102,11 @@ val allCodegenTests = "../codegen-core/common-test-models".let { commonModels ->
             extraConfig = """, "codegen": { "ignoreUnsupportedConstraints": true } """,
         ),
         CodegenTest("com.amazonaws.s3#AmazonS3", "s3"),
+        CodegenTest(
+            "emptyservice#EmptyService",
+            "empty_service",
+            imports = listOf("$commonModels/empty-service.smithy"),
+        ),
         CodegenTest(
             "com.aws.example.rust#PokemonService",
             "pokemon-service-server-sdk",

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/ServerServiceGeneratorV2.kt

@@ -10,6 +10,7 @@ import software.amazon.smithy.model.neighbor.Walker
 import software.amazon.smithy.model.shapes.OperationShape
 import software.amazon.smithy.model.shapes.StringShape
 import software.amazon.smithy.model.traits.PatternTrait
+import software.amazon.smithy.rust.codegen.core.rustlang.Attribute
 import software.amazon.smithy.rust.codegen.core.rustlang.RustReservedWords
 import software.amazon.smithy.rust.codegen.core.rustlang.RustWriter
 import software.amazon.smithy.rust.codegen.core.rustlang.Writable
@@ -184,12 +185,14 @@ class ServerServiceGeneratorV2(
             for (operationShape in operations) {
                 val fieldName = builderFieldNames[operationShape]!!
                 val operationZstTypeName = operationStructNames[operationShape]!!
-                rust(
+                rustTemplate(
                     """
                     if self.$fieldName.is_none() {
+                        use #{SmithyHttpServer}::operation::OperationShape;
                         $missingOperationsVariableName.insert(crate::operation_shape::$operationZstTypeName::NAME, ".$fieldName()");
                     }
                     """,
+                    "SmithyHttpServer" to smithyHttpServer,
                 )
             }
         }
@@ -205,6 +208,13 @@ class ServerServiceGeneratorV2(
             }
         }
 
+        var missingOperationsUnusedMut = writable("")
+        var unusedExpected = writable("")
+        if (operations.isEmpty()) {
+            missingOperationsUnusedMut = writable { Attribute.AllowUnusedMut.render(this) }
+            unusedExpected = writable { Attribute.AllowUnusedVariables.render(this) }
+        }
+
         rustTemplate(
             """
             /// Constructs a [`$serviceName`] from the arguments provided to the builder.
@@ -216,14 +226,15 @@ class ServerServiceGeneratorV2(
             pub fn build(self) -> Result<$serviceName<#{SmithyHttpServer}::routing::Route<$builderBodyGenericTypeName>>, MissingOperationsError>
             {
                 let router = {
-                    use #{SmithyHttpServer}::operation::OperationShape;
+                    #{MissingOperationsUnusedMut:W}
                     let mut $missingOperationsVariableName = std::collections::HashMap::new();
                     #{NullabilityChecks:W}
                     if !$missingOperationsVariableName.is_empty() {
                         return Err(MissingOperationsError {
                             operation_names2setter_methods: $missingOperationsVariableName,
                         });
                     }
+                    #{UnusedExpected:W}
                     let $expectMessageVariableName = "this should never panic since we are supposed to check beforehand that a handler has been registered for this operation; please file a bug report under https://github.com/awslabs/smithy-rs/issues";
 
                     #{PatternInitializations:W}
@@ -240,6 +251,8 @@ class ServerServiceGeneratorV2(
             "RoutesArrayElements" to routesArrayElements,
             "SmithyHttpServer" to smithyHttpServer,
             "PatternInitializations" to patternInitializations(),
+            "MissingOperationsUnusedMut" to missingOperationsUnusedMut,
+            "UnusedExpected" to unusedExpected,
         )
     }
 
@@ -319,14 +332,23 @@ class ServerServiceGeneratorV2(
     /** Returns a `Writable` containing the builder struct definition and its implementations. */
     private fun builder(): Writable = writable {
         val builderGenerics = listOf(builderBodyGenericTypeName, builderPluginGenericTypeName).joinToString(", ")
+        var allBuilderFields = builderFields + "plugin: $builderPluginGenericTypeName"
+        var allowDeadCodePlugin = writable("")
+
+        // With no operations there is no use of the `Body` type variable and `plugin: Plugin` becomes dead code
+        if (operations.isEmpty()) {
+            allBuilderFields += "_body: std::marker::PhantomData<$builderBodyGenericTypeName>"
+            allowDeadCodePlugin = writable { Attribute.AllowDeadCode.render(this) }
+        }
+
         rustTemplate(
             """
             /// The service builder for [`$serviceName`].
             ///
             /// Constructed via [`$serviceName::builder_with_plugins`] or [`$serviceName::builder_without_plugins`].
+            #{AllowDeadCodePlugin:W}
             pub struct $builderName<$builderGenerics> {
-                ${builderFields.joinToString(", ")},
-                plugin: $builderPluginGenericTypeName,
+                ${allBuilderFields.joinToString(",")}
             }
 
             impl<$builderGenerics> $builderName<$builderGenerics> {
@@ -342,6 +364,7 @@ class ServerServiceGeneratorV2(
             "Setters" to builderSetters(),
             "BuildMethod" to buildMethod(),
             "BuildUncheckedMethod" to buildUncheckedMethod(),
+            "AllowDeadCodePlugin" to allowDeadCodePlugin,
             *codegenScope,
         )
     }
@@ -381,6 +404,13 @@ class ServerServiceGeneratorV2(
     private fun serviceStruct(): Writable = writable {
         documentShape(service, model)
 
+        var builderFields = notSetFields + writable("plugin")
+        // With no operations there is no use of the `Body` type variable
+        if (operations.isEmpty()) {
+            builderFields += writable("_body: std::marker::PhantomData")
+        }
+        val joinedBuilderFields = builderFields.join(",")
+
         rustTemplate(
             """
             ///
@@ -400,8 +430,7 @@ class ServerServiceGeneratorV2(
                 /// multiple plugins.
                 pub fn builder_with_plugins<Body, Plugin>(plugin: Plugin) -> $builderName<Body, Plugin> {
                     $builderName {
-                        #{NotSetFields:W},
-                        plugin
+                        #{BuilderFields:W}
                     }
                 }
 
@@ -470,7 +499,7 @@ class ServerServiceGeneratorV2(
                 }
             }
             """,
-            "NotSetFields" to notSetFields.join(", "),
+            "BuilderFields" to joinedBuilderFields,
             "Router" to protocol.routerType(),
             "Protocol" to protocol.markerStruct(),
             *codegenScope,