Add 'as-service-error' to simplify handling of specific error conditi…

…ons (#3333)

## Motivation and Context
aws-sdk-rust#1010

## Description
Add `as-service-error` helper function to cleanup handling of service
error

## Testing
UT

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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copy, and redistribute this contribution, under the terms of your
choice._

CHANGELOG.next.toml

@@ -63,6 +63,18 @@ references = ["smithy-rs#3252", "smithy-rs#3312"]
 meta = { "breaking" = false, "tada" = false, "bug" = true }
 author = "milesziemer"
 
+[[smithy-rs]]
+message = "Add `as_service_err()` to `SdkError` to allow checking the type of an error is without taking ownership."
+references = ["smithy-rs#3333", "aws-sdk-rust#998", "aws-sdk-rust#1010"]
+meta = { "breaking" = false, "tada" = true, "bug" = false }
+author = "rcoh"
+
+[[aws-sdk-rust]]
+message = "Add `as_service_err()` to `SdkError` to allow checking the type of an error is without taking ownership."
+references = ["smithy-rs#3333", "aws-sdk-rust#998", "aws-sdk-rust#1010"]
+meta = { "breaking" = false, "tada" = true, "bug" = false }
+author = "rcoh"
+
 [[smithy-rs]]
 message = """`requireEndpointResolver: false` is no longer required to remove the need for an endpoint resolver. Instead, `"awsSdkBuilder"` (default false), now _removes_ that requirement."""
 references = ["smithy-rs#3292"]

aws/sdk/integration-tests/s3/tests/status-200-errors.rs

@@ -6,6 +6,7 @@
 use aws_credential_types::provider::SharedCredentialsProvider;
 use aws_credential_types::Credentials;
 use aws_sdk_s3::Client;
+use aws_smithy_runtime::assert_str_contains;
 use aws_smithy_runtime::client::http::test_util::infallible_client_fn;
 use aws_smithy_types::body::SdkBody;
 use aws_smithy_types::error::metadata::ProvideErrorMetadata;
@@ -37,5 +38,6 @@ async fn status_200_errors() {
         .send()
         .await
         .expect_err("should fail");
-    assert_eq!(error.into_service_error().code(), Some("SlowDown"));
+    assert_eq!(error.as_service_error().unwrap().code(), Some("SlowDown"));
+    assert_str_contains!(format!("{:?}", error), "Please reduce your request rate");
 }

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

@@ -412,6 +412,36 @@ impl<E, R> SdkError<E, R> {
         }
     }
 
+    /// Returns a reference underlying service error `E` if there is one
+    ///
+    /// # Examples
+    /// ```no_run
+    /// # use aws_smithy_runtime_api::client::result::SdkError;
+    /// # #[derive(Debug)] enum GetObjectError { NoSuchKey(()), Other(()) }
+    /// # impl std::fmt::Display for GetObjectError {
+    /// #     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { unimplemented!() }
+    /// # }
+    /// # impl std::error::Error for GetObjectError {}
+    /// # impl GetObjectError {
+    /// #   fn is_not_found(&self) -> bool { true }
+    /// # }
+    /// # fn example() -> Result<(), GetObjectError> {
+    /// # let sdk_err = SdkError::service_error(GetObjectError::NoSuchKey(()), ());
+    /// if sdk_err.as_service_error().map(|e|e.is_not_found()) == Some(true) {
+    ///     println!("the object doesn't exist");
+    ///     // return None, or handle this error specifically
+    /// }
+    /// // ... handle other error cases, happy path, etc.
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn as_service_error(&self) -> Option<&E> {
+        match self {
+            Self::ServiceError(err) => Some(&err.source),
+            _ => None,
+        }
+    }
+
     /// Converts this error into its error source.
     ///
     /// If there is no error source, then `Err(Self)` is returned.