Make RuntimePlugin & RuntimePlugins Send/Sync (#2771)

## Motivation and Context
We will eventually want to store additional runtime plugins on fluent
builders, clients, etc. To make this work, the RuntimePlugin trait needs
to assert Send/Sync

## Description
Small tweaks + test on the RuntimePlugin trait

## Testing
CI


----

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

Author: rcoh

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

@@ -51,7 +51,7 @@ open class OperationGenerator(
                     """
                     pub(crate) fn register_default_runtime_plugins(
                         runtime_plugins: #{RuntimePlugins},
-                        operation: #{Box}<dyn #{RuntimePlugin} + #{Send} + #{Sync}>,
+                        operation: impl #{RuntimePlugin} + 'static,
                         handle: #{Arc}<crate::client::Handle>,
                         config_override: #{Option}<crate::config::Builder>,
                     ) -> #{RuntimePlugins} {
@@ -180,7 +180,7 @@ open class OperationGenerator(
                     ) -> #{RuntimePlugins} {
                         #{register_default_runtime_plugins}(
                             runtime_plugins,
-                            #{Box}::new(Self::new()) as _,
+                            Self::new(),
                             handle,
                             config_override
                         )

rust-runtime/aws-smithy-runtime-api/src/client/runtime_plugin.rs

@@ -6,16 +6,16 @@
 use crate::client::interceptors::InterceptorRegistrar;
 use aws_smithy_types::config_bag::{ConfigBag, FrozenLayer};
 use std::fmt::Debug;
+use std::sync::Arc;
 
 pub type BoxError = Box<dyn std::error::Error + Send + Sync>;
-pub type BoxRuntimePlugin = Box<dyn RuntimePlugin + Send + Sync>;
 
 /// RuntimePlugin Trait
 ///
 /// A RuntimePlugin is the unit of configuration for augmenting the SDK with new behavior
 ///
 /// Runtime plugins can set configuration and register interceptors.
-pub trait RuntimePlugin: Debug {
+pub trait RuntimePlugin: Debug + Send + Sync {
     fn config(&self) -> Option<FrozenLayer> {
         None
     }
@@ -25,40 +25,44 @@ pub trait RuntimePlugin: Debug {
     }
 }
 
-impl RuntimePlugin for BoxRuntimePlugin {
+#[derive(Debug, Clone)]
+struct SharedRuntimePlugin(Arc<dyn RuntimePlugin>);
+
+impl SharedRuntimePlugin {
+    fn new(plugin: impl RuntimePlugin + 'static) -> Self {
+        Self(Arc::new(plugin))
+    }
+}
+
+impl RuntimePlugin for SharedRuntimePlugin {
     fn config(&self) -> Option<FrozenLayer> {
-        self.as_ref().config()
+        self.0.config()
     }
 
     fn interceptors(&self, interceptors: &mut InterceptorRegistrar) {
-        self.as_ref().interceptors(interceptors)
+        self.0.interceptors(interceptors)
     }
 }
 
-#[derive(Default)]
+#[derive(Default, Clone, Debug)]
 pub struct RuntimePlugins {
-    client_plugins: Vec<BoxRuntimePlugin>,
-    operation_plugins: Vec<BoxRuntimePlugin>,
+    client_plugins: Vec<SharedRuntimePlugin>,
+    operation_plugins: Vec<SharedRuntimePlugin>,
 }
 
 impl RuntimePlugins {
     pub fn new() -> Self {
         Default::default()
     }
 
-    pub fn with_client_plugin(
-        mut self,
-        plugin: impl RuntimePlugin + Send + Sync + 'static,
-    ) -> Self {
-        self.client_plugins.push(Box::new(plugin));
+    pub fn with_client_plugin(mut self, plugin: impl RuntimePlugin + 'static) -> Self {
+        self.client_plugins.push(SharedRuntimePlugin::new(plugin));
         self
     }
 
-    pub fn with_operation_plugin(
-        mut self,
-        plugin: impl RuntimePlugin + Send + Sync + 'static,
-    ) -> Self {
-        self.operation_plugins.push(Box::new(plugin));
+    pub fn with_operation_plugin(mut self, plugin: impl RuntimePlugin + 'static) -> Self {
+        self.operation_plugins
+            .push(SharedRuntimePlugin::new(plugin));
         self
     }
 
@@ -106,4 +110,10 @@ mod tests {
     fn can_add_runtime_plugin_implementors_to_runtime_plugins() {
         RuntimePlugins::new().with_client_plugin(SomeStruct);
     }
+
+    #[test]
+    fn runtime_plugins_are_send_sync() {
+        fn assert_send_sync<T: Send + Sync>() {}
+        assert_send_sync::<RuntimePlugins>();
+    }
 }