From d565a0528d13f3b6981d817048593be08e52db10 Mon Sep 17 00:00:00 2001
From: david-perez <d@vidp.dev>
Date: Fri, 3 Jun 2022 14:28:11 +0200
Subject: [PATCH] Only generate `@error` `message` getter when defined in the
 model

We are currently generating a useless `message` getter that returns
`None` even when the `message` field is not defined in the model.
Granted, in the client this field is _almost always_ there, because the
`AddErrorMessage` model transformer is applied unless the user opts out
in their `smithy-build.json`.
---
 .../smithy/generators/error/ErrorGenerator.kt | 27 ++++++++++---------
 .../smithy/transformers/AddErrorMessage.kt    |  6 ++---
 2 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/error/ErrorGenerator.kt b/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/error/ErrorGenerator.kt
index da4c699f49..c3e495827e 100644
--- a/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/error/ErrorGenerator.kt
+++ b/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/error/ErrorGenerator.kt
@@ -69,13 +69,6 @@ class ErrorGenerator(
         val symbol = symbolProvider.toSymbol(shape)
         val messageShape = shape.errorMessageMember()
         val errorKindT = RuntimeType.errorKind(symbolProvider.config().runtimeConfig)
-        val (returnType, message) = messageShape?.let {
-            if (symbolProvider.toSymbol(messageShape).isOptional()) {
-                "Option<&str>" to "self.${symbolProvider.toMemberName(it)}.as_deref()"
-            } else {
-                "&str" to "self.${symbolProvider.toMemberName(it)}.as_ref()"
-            }
-        } ?: "Option<&str>" to "None"
         writer.rustBlock("impl ${symbol.name}") {
             val retryKindWriteable = shape.modeledRetryKind(error)?.writable(symbolProvider.config().runtimeConfig)
             if (retryKindWriteable != null) {
@@ -84,12 +77,20 @@ class ErrorGenerator(
                     retryKindWriteable(this)
                 }
             }
-            rust(
-                """
-                /// Returns the error message.
-                pub fn message(&self) -> $returnType { $message }
-                """
-            )
+            if (messageShape != null) {
+                val (returnType, message) = if (symbolProvider.toSymbol(messageShape).isOptional()) {
+                    "Option<&str>" to "self.${symbolProvider.toMemberName(messageShape)}.as_deref()"
+                } else {
+                    "&str" to "self.${symbolProvider.toMemberName(messageShape)}.as_ref()"
+                }
+
+                rust(
+                    """
+                    /// Returns the error message.
+                    pub fn message(&self) -> $returnType { $message }
+                    """
+                )
+            }
 
             /*
              * If we're generating for a server, the `name` method is added to enable
diff --git a/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/transformers/AddErrorMessage.kt b/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/transformers/AddErrorMessage.kt
index 797884d86a..834a9313f5 100644
--- a/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/transformers/AddErrorMessage.kt
+++ b/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/transformers/AddErrorMessage.kt
@@ -18,11 +18,11 @@ import java.util.logging.Logger
 fun StructureShape.errorMessageMember(): MemberShape? = this.getMember("message").or { this.getMember("Message") }.orNull()
 
 /**
- * Ensure that all errors have error messages
+ * Ensure that all errors have error messages.
  *
  * Not all errors are modeled with an error message field. However, in many cases, the server can still send an error.
  * If an error, specifically, a structure shape with the error trait does not have a member `message` or `Message`,
- * this transformer will add a `message` member targetting a string.
+ * this transformer will add a `message` member targeting a string.
  *
  * This ensures that we always generate a modeled error message field enabling end users to easily extract the error
  * message when present.
@@ -33,7 +33,7 @@ object AddErrorMessage {
     private val logger = Logger.getLogger("AddErrorMessage")
 
     /**
-     * Ensure that all errors have error messages
+     * Ensure that all errors have error messages.
      */
     fun transform(model: Model): Model {
         return ModelTransformer.create().mapShapes(model) { shape ->