Implement `StdError::source()` for Error enum (#2564)

## Motivation and Context
This is an attempt at fixing
https://github.com/awslabs/aws-sdk-rust/issues/784.

The service-level `Error` enum implements `std::error::Error` but does
not implement its `source()` method. This means that an error library
like `anyhow` or `eyre` won't be able to display the root cause of an
error, which is especially problematic for the `Unhandled` variant.

## Description
I modified `ServiceErrorGenerator` in the `codegen-client` crate and
replaced the line that output `impl std::error::Error for Error {}` with
an impl block that implements the `source()` method by delegating to the
inner error structure.

## Testing
I've added a simple unit test to `ServiceErrorGeneratorTest`.

## 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

----

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

CHANGELOG.next.toml

@@ -11,6 +11,18 @@
 # meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client | server | all"}
 # author = "rcoh"
 
+[[aws-sdk-rust]]
+message = "Implement std::error::Error#source() properly for the service meta Error enum"
+references = ["aws-sdk-rust#784"]
+meta = { "breaking" = false, "tada" = false, "bug" = false }
+author = "abusch"
+
+[[smithy-rs]]
+message = "Implement std::error::Error#source() properly for the service meta Error enum"
+references = ["aws-sdk-rust#784"]
+meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client"}
+author = "abusch"
+
 [[aws-sdk-rust]]
 message = "The outputs for event stream operations (for example, S3's SelectObjectContent) now implement the `Sync` auto-trait."
 references = ["smithy-rs#2496"]

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

@@ -80,7 +80,16 @@ class ServiceErrorGenerator(
                         errors.map { it.id },
                     )
                 }
-            rust("impl #T for Error {}", RuntimeType.StdError)
+            rustBlock("impl #T for Error", RuntimeType.StdError) {
+                rustBlock("fn source(&self) -> std::option::Option<&(dyn #T + 'static)>", RuntimeType.StdError) {
+                    rustBlock("match self") {
+                        allErrors.forEach {
+                            rust("Error::${symbolProvider.toSymbol(it).name}(inner) => inner.source(),")
+                        }
+                        rust("Error::Unhandled(inner) => inner.source()")
+                    }
+                }
+            }
             writeCustomizations(customizations, ErrorSection.ServiceErrorAdditionalTraitImpls(allErrors))
         }
         crate.lib { rust("pub use error_meta::Error;") }

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/error/ServiceErrorGeneratorTest.kt

@@ -6,45 +6,48 @@
 package software.amazon.smithy.rust.codegen.client.smithy.generators.error
 
 import org.junit.jupiter.api.Test
+import software.amazon.smithy.model.shapes.StructureShape
 import software.amazon.smithy.rust.codegen.client.testutil.clientIntegrationTest
 import software.amazon.smithy.rust.codegen.core.rustlang.rust
 import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
 import software.amazon.smithy.rust.codegen.core.testutil.integrationTest
+import software.amazon.smithy.rust.codegen.core.testutil.unitTest
+import software.amazon.smithy.rust.codegen.core.util.lookup
 
 internal class ServiceErrorGeneratorTest {
+    private val model = """
+        namespace com.example
+
+        use aws.protocols#restJson1
+
+        @restJson1
+        service HelloService {
+            operations: [SayHello],
+            version: "1"
+        }
+
+        @http(uri: "/", method: "POST")
+        operation SayHello {
+            input: EmptyStruct,
+            output: EmptyStruct,
+            errors: [SorryBusy, CanYouRepeatThat, MeDeprecated]
+        }
+
+        structure EmptyStruct { }
+
+        @error("server")
+        structure SorryBusy { }
+
+        @error("client")
+        structure CanYouRepeatThat { }
+
+        @error("client")
+        @deprecated
+        structure MeDeprecated { }
+    """.asSmithyModel()
+
     @Test
     fun `top level errors are send + sync`() {
-        val model = """
-            namespace com.example
-
-            use aws.protocols#restJson1
-
-            @restJson1
-            service HelloService {
-                operations: [SayHello],
-                version: "1"
-            }
-
-            @http(uri: "/", method: "POST")
-            operation SayHello {
-                input: EmptyStruct,
-                output: EmptyStruct,
-                errors: [SorryBusy, CanYouRepeatThat, MeDeprecated]
-            }
-
-            structure EmptyStruct { }
-
-            @error("server")
-            structure SorryBusy { }
-
-            @error("client")
-            structure CanYouRepeatThat { }
-
-            @error("client")
-            @deprecated
-            structure MeDeprecated { }
-        """.asSmithyModel()
-
         clientIntegrationTest(model) { codegenContext, rustCrate ->
             rustCrate.integrationTest("validate_errors") {
                 rust(
@@ -60,4 +63,25 @@ internal class ServiceErrorGeneratorTest {
             }
         }
     }
+
+    @Test
+    fun `generates combined error enums`() {
+        clientIntegrationTest(model) { _, rustCrate ->
+            rustCrate.moduleFor(model.lookup<StructureShape>("com.example#CanYouRepeatThat")) {
+                unitTest(
+                    name = "generates_combined_error_enums",
+                    test = """
+                        use std::error::Error as StdError;
+                        use crate::Error;
+                        use crate::operation::say_hello::SayHelloError;
+
+                        // Unhandled variants properly delegate source.
+                        let error = Error::from(SayHelloError::unhandled("some other error"));
+                        let source = error.source().expect("source should not be None");
+                        assert_eq!(format!("{}", source), "some other error");
+                    """,
+                )
+            }
+        }
+    }
 }