Allow server decorators to inject methods on config

PR #3095 added a code-generated `${serviceName}Config` object on which
users can register layers and plugins. For example:

```rust
let config = PokemonServiceConfig::builder()
    .layer(layers)
    .http_plugin(authn_plugin)
    .model_plugin(authz_plugin)
    .build();
```

This PR makes it so that server decorators can inject methods on this
config builder object. These methods can apply arbitrary layers, HTTP
plugins, and/or model plugins. Moreover, the decorator can configure
whether invoking such method is required or not.

For example, a decorator can inject an `aws_auth` method that configures
some plugins using its input arguments. Missing invocation of this method
will result in the config failing to build:

```rust
let _: SimpleServiceConfig<
    // No layers have been applied.
    tower::layer::util::Identity,
    // One HTTP plugin has been applied.
    PluginStack<IdentityPlugin, IdentityPlugin>,
    // One model plugin has been applied.
    PluginStack<IdentityPlugin, IdentityPlugin>,
> = SimpleServiceConfig::builder()
    // This method has been injected in the config builder!
    .aws_auth("a".repeat(69).to_owned(), 69)
    // The method will apply one HTTP plugin and one model plugin,
    // configuring them with the input arguments. Configuration can be
    // declared to be fallible, in which case we get a `Result` we unwrap
    // here.
    .expect("failed to configure aws_auth")
    .build()
    // Since `aws_auth` has been marked as required, if the user misses
    // invoking it, this would panic here.
    .unwrap();
```

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

@@ -294,7 +294,7 @@ class TestWriterDelegator(
 }
 
 /**
- * Generate a newtest module
+ * Generate a new test module
  *
  * This should only be used in test code—the generated module name will be something like `tests_123`
  */

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

@@ -22,6 +22,7 @@ object ServerCargoDependency {
     val Nom: CargoDependency = CargoDependency("nom", CratesIo("7"))
     val OnceCell: CargoDependency = CargoDependency("once_cell", CratesIo("1.13"))
     val PinProjectLite: CargoDependency = CargoDependency("pin-project-lite", CratesIo("0.2"))
+    val ThisError: CargoDependency = CargoDependency("thiserror", CratesIo("1.0"))
     val Tower: CargoDependency = CargoDependency("tower", CratesIo("0.4"))
     val TokioDev: CargoDependency = CargoDependency("tokio", CratesIo("1.23.1"), scope = DependencyScope.Dev)
     val Regex: CargoDependency = CargoDependency("regex", CratesIo("1.5.5"))

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

@@ -78,6 +78,7 @@ import software.amazon.smithy.rust.codegen.server.smithy.generators.Unconstraine
 import software.amazon.smithy.rust.codegen.server.smithy.generators.UnconstrainedMapGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.UnconstrainedUnionGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.ValidationExceptionConversionGenerator
+import software.amazon.smithy.rust.codegen.server.smithy.generators.isBuilderFallible
 import software.amazon.smithy.rust.codegen.server.smithy.generators.protocol.ServerProtocol
 import software.amazon.smithy.rust.codegen.server.smithy.generators.protocol.ServerProtocolGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.protocol.ServerProtocolTestGenerator
@@ -591,11 +592,15 @@ open class ServerCodegenVisitor(
         logger.info("[rust-server-codegen] Generating a service $shape")
         val serverProtocol = protocolGeneratorFactory.protocol(codegenContext) as ServerProtocol
 
+        val configMethods = codegenDecorator.configMethods(codegenContext)
+        val isConfigBuilderFallible = configMethods.isBuilderFallible()
+
         // Generate root.
         rustCrate.lib {
             ServerRootGenerator(
                 serverProtocol,
                 codegenContext,
+                isConfigBuilderFallible,
             ).render(this)
         }
 
@@ -612,9 +617,10 @@ open class ServerCodegenVisitor(
             ServerServiceGenerator(
                 codegenContext,
                 serverProtocol,
+                isConfigBuilderFallible,
             ).render(this)
 
-            ServiceConfigGenerator(codegenContext).render(this)
+            ServiceConfigGenerator(codegenContext, configMethods).render(this)
 
             ScopeMacroGenerator(codegenContext).render(this)
         }

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

@@ -15,6 +15,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.protocols.ProtocolMap
 import software.amazon.smithy.rust.codegen.server.smithy.ServerCodegenContext
 import software.amazon.smithy.rust.codegen.server.smithy.ServerRustSettings
 import software.amazon.smithy.rust.codegen.server.smithy.ValidationResult
+import software.amazon.smithy.rust.codegen.server.smithy.generators.ConfigMethod
 import software.amazon.smithy.rust.codegen.server.smithy.generators.ValidationExceptionConversionGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.protocol.ServerProtocolGenerator
 import java.util.logging.Logger
@@ -41,6 +42,12 @@ interface ServerCodegenDecorator : CoreCodegenDecorator<ServerCodegenContext, Se
      * Therefore, ensure that all the structure shapes returned by this method are not in the service's closure.
      */
     fun postprocessGenerateAdditionalStructures(operationShape: OperationShape): List<StructureShape> = emptyList()
+
+    /**
+     * Configuration methods that should be injected into the `${serviceName}Config` struct to allow users to configure
+     * pre-applied layers and plugins.
+     */
+    fun configMethods(codegenContext: ServerCodegenContext): List<ConfigMethod> = emptyList()
 }
 
 /**
@@ -74,10 +81,11 @@ class CombinedServerCodegenDecorator(decorators: List<ServerCodegenDecorator>) :
             decorator.postprocessValidationExceptionNotAttachedErrorMessage(accumulated)
         }
 
-    override fun postprocessGenerateAdditionalStructures(operationShape: OperationShape): List<StructureShape> {
-        return orderedDecorators.map { decorator -> decorator.postprocessGenerateAdditionalStructures(operationShape) }
-            .flatten()
-    }
+    override fun postprocessGenerateAdditionalStructures(operationShape: OperationShape): List<StructureShape> =
+        orderedDecorators.flatMap { it.postprocessGenerateAdditionalStructures(operationShape) }
+
+    override fun configMethods(codegenContext: ServerCodegenContext): List<ConfigMethod> =
+        orderedDecorators.flatMap { it.configMethods(codegenContext) }
 
     companion object {
         fun fromClasspath(

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

@@ -31,6 +31,7 @@ import software.amazon.smithy.rust.codegen.server.smithy.ServerRustModule.Output
 open class ServerRootGenerator(
     val protocol: ServerProtocol,
     private val codegenContext: ServerCodegenContext,
+    private val isConfigBuilderFallible: Boolean,
 ) {
     private val index = TopDownIndex.of(codegenContext.model)
     private val operations = index.getContainedOperations(codegenContext.serviceShape).toSortedSet(
@@ -57,6 +58,8 @@ open class ServerRootGenerator(
             }
             .join("//!\n")
 
+        val unwrapConfigBuilder = if (isConfigBuilderFallible) ".expect(\"config failed to build\")" else ""
+
         writer.rustTemplate(
             """
             //! A fast and customizable Rust implementation of the $serviceName Smithy service.
@@ -75,7 +78,10 @@ open class ServerRootGenerator(
             //! ## async fn dummy() {
             //! use $crateName::{$serviceName, ${serviceName}Config};
             //!
-            //! ## let app = $serviceName::builder(${serviceName}Config::builder().build()).build_unchecked();
+            //! ## let app = $serviceName::builder(
+            //! ##     ${serviceName}Config::builder()
+            //! ##         .build()$unwrapConfigBuilder
+            //! ## ).build_unchecked();
             //! let server = app.into_make_service();
             //! let bind: SocketAddr = "127.0.0.1:6969".parse()
             //!     .expect("unable to parse the server bind address and port");
@@ -92,7 +98,10 @@ open class ServerRootGenerator(
             //! use $crateName::$serviceName;
             //!
             //! ## async fn dummy() {
-            //! ## let app = $serviceName::builder(${serviceName}Config::builder().build()).build_unchecked();
+            //! ## let app = $serviceName::builder(
+            //! ##     ${serviceName}Config::builder()
+            //! ##         .build()$unwrapConfigBuilder
+            //! ## ).build_unchecked();
             //! let handler = LambdaHandler::new(app);
             //! lambda_http::run(handler).await.unwrap();
             //! ## }
@@ -118,7 +127,7 @@ open class ServerRootGenerator(
             //! let http_plugins = HttpPlugins::new()
             //!         .push(LoggingPlugin)
             //!         .push(MetricsPlugin);
-            //! let config = ${serviceName}Config::builder().build();
+            //! let config = ${serviceName}Config::builder().build()$unwrapConfigBuilder;
             //! let builder: $builderName<Body, _, _, _> = $serviceName::builder(config);
             //! ```
             //!
@@ -183,13 +192,13 @@ open class ServerRootGenerator(
             //!
             //! ## Example
             //!
-            //! ```rust
+            //! ```rust,no_run
             //! ## use std::net::SocketAddr;
             //! use $crateName::{$serviceName, ${serviceName}Config};
             //!
             //! ##[#{Tokio}::main]
             //! pub async fn main() {
-            //!    let config = ${serviceName}Config::builder().build();
+            //!    let config = ${serviceName}Config::builder().build()$unwrapConfigBuilder;
             //!    let app = $serviceName::builder(config)
             ${builderFieldNames.values.joinToString("\n") { "//!        .$it($it)" }}
             //!        .build()
@@ -236,6 +245,23 @@ open class ServerRootGenerator(
     fun render(rustWriter: RustWriter) {
         documentation(rustWriter)
 
-        rustWriter.rust("pub use crate::service::{$serviceName, ${serviceName}Config, ${serviceName}ConfigBuilder, ${serviceName}Builder, MissingOperationsError};")
+        // Only export config builder error if fallible.
+        val configErrorReExport = if (isConfigBuilderFallible) {
+            "${serviceName}ConfigError,"
+        } else {
+            ""
+        }
+        rustWriter.rust(
+            """
+            pub use crate::service::{
+                $serviceName,
+                ${serviceName}Config,
+                ${serviceName}ConfigBuilder,
+                $configErrorReExport
+                ${serviceName}Builder,
+                MissingOperationsError
+            };
+            """
+        )
     }
 }

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

@@ -33,6 +33,7 @@ import software.amazon.smithy.rust.codegen.server.smithy.ServerRustModule.Output
 class ServerServiceGenerator(
     private val codegenContext: ServerCodegenContext,
     private val protocol: ServerProtocol,
+    private val isConfigBuilderFallible: Boolean,
 ) {
     private val runtimeConfig = codegenContext.runtimeConfig
     private val smithyHttpServer = ServerCargoDependency.smithyHttpServer(runtimeConfig).toType()
@@ -107,6 +108,11 @@ class ServerServiceGenerator(
             val docHandler = DocHandlerGenerator(codegenContext, operationShape, "handler", "///")
             val handler = docHandler.docSignature()
             val handlerFixed = docHandler.docFixedSignature()
+            val unwrapConfigBuilder = if (isConfigBuilderFallible) {
+                ".expect(\"config failed to build\")"
+            } else {
+                ""
+            }
             rustTemplate(
                 """
                 /// Sets the [`$structName`](crate::operation_shape::$structName) operation.
@@ -123,7 +129,7 @@ class ServerServiceGenerator(
                 ///
                 #{Handler:W}
                 ///
-                /// let config = ${serviceName}Config::builder().build();
+                /// let config = ${serviceName}Config::builder().build()$unwrapConfigBuilder;
                 /// let app = $serviceName::builder(config)
                 ///     .$fieldName(handler)
                 ///     /* Set other handlers */
@@ -186,7 +192,7 @@ class ServerServiceGenerator(
                 ///
                 #{HandlerFixed:W}
                 ///
-                /// let config = ${serviceName}Config::builder().build();
+                /// let config = ${serviceName}Config::builder().build()$unwrapConfigBuilder;
                 /// let svc = #{Tower}::util::service_fn(handler);
                 /// let app = $serviceName::builder(config)
                 ///     .${fieldName}_service(svc)