Fix error struct with default impl

CHANGELOG.next.toml

@@ -11,6 +11,14 @@
 # meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client | server | all"}
 # author = "rcoh"
 
+[[smithy-rs]]
+message = """
+Fix rendering of @error structs when fields have default values
+"""
+references = ["smithy-rs#3182"]
+meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "client"}
+author = "codypenta"
+
 [[aws-sdk-rust]]
 message = "Change `ByteStream::into_async_read` to return `AsyncBufRead`"
 references = ["smithy-rs#3164"]

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

@@ -27,6 +27,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType
 import software.amazon.smithy.rust.codegen.core.smithy.RustSymbolProvider
 import software.amazon.smithy.rust.codegen.core.smithy.customize.writeCustomizations
 import software.amazon.smithy.rust.codegen.core.smithy.generators.setterName
+import software.amazon.smithy.rust.codegen.core.smithy.isOptional
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.HttpBindingDescriptor
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.HttpLocation
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.Protocol
@@ -163,7 +164,8 @@ class ProtocolParserGenerator(
                                 val errorMessageMember = errorShape.errorMessageMember()
                                 // If the message member is optional and wasn't set, we set a generic error message.
                                 if (errorMessageMember != null) {
-                                    if (errorMessageMember.isOptional) {
+                                    val symbol = symbolProvider.toSymbol(errorMessageMember)
+                                    if (symbol.isOptional()) {
                                         rust(
                                             """
                                             if tmp.message.is_none() {

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/protocol/ProtocolParserGeneratorTest.kt

@@ -0,0 +1,71 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.rust.codegen.client.smithy.generators.protocol
+
+import org.junit.jupiter.api.Test
+import software.amazon.smithy.rust.codegen.client.testutil.clientIntegrationTest
+import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
+
+class ProtocolParserGeneratorTest {
+    private val model = """
+        ${'$'}version: "2.0"
+        namespace test
+
+        use aws.protocols#restJson1
+
+        @restJson1
+        service TestService {
+            version: "2019-12-16",
+            operations: [SomeOperation]
+            errors: [SomeTopLevelError]
+        }
+
+        @http(uri: "/SomeOperation", method: "POST")
+        operation SomeOperation {
+            input: SomeOperationInputOutput,
+            output: SomeOperationInputOutput,
+            errors: [SomeOperationError]
+        }
+
+        structure SomeOperationInputOutput {
+            payload: String,
+            a: String,
+            b: Integer
+        }
+
+        @error("server")
+        structure SomeTopLevelError {
+            @required
+            requestId: String
+
+            @required
+            message: String
+
+            code: String = "400"
+
+            context: String
+        }
+
+        @error("client")
+        structure SomeOperationError {
+            @required
+            requestId: String
+
+            @required
+            message: String
+
+            code: String = "400"
+
+            context: String
+        }
+    """
+        .asSmithyModel()
+
+    @Test
+    fun `generate an complex error structure that compiles`() {
+        clientIntegrationTest(model) { _, _ -> }
+    }
+}