Fix for Rust compiler warning about derive helpers

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustType.kt

@@ -385,11 +385,17 @@ data class RustMetadata(
         this.copy(derives = derives - withoutDerives.toSet())
 
     fun renderAttributes(writer: RustWriter): RustMetadata {
-        additionalAttributes.forEach {
+        val (deriveHelperAttrs, otherAttrs) = additionalAttributes.partition { it.isDeriveHelper }
+        otherAttrs.forEach {
             it.render(writer)
         }
+
         Attribute(derive(derives)).render(writer)
 
+        // Derive helper attributes must come after derive, see https://github.com/rust-lang/rust/issues/79202
+        deriveHelperAttrs.forEach {
+            it.render(writer)
+        }
         return this
     }
 
@@ -450,17 +456,22 @@ enum class AttributeKind {
  * [Attributes](https://doc.rust-lang.org/reference/attributes.html) are general free form metadata
  * that are interpreted by the compiler.
  *
+ * If the attribute is a "derive helper", such as  `#[serde]`, set `isDeriveHelper` to `true` so it is sorted correctly after
+ * the derive attribute is rendered. (See https://github.com/rust-lang/rust/issues/79202 for why sorting matters.)
+ *
  * For example:
  * ```rust
+ * #[allow(missing_docs)] // <-- this is an attribute, and it is not a derive helper
  * #[derive(Clone, PartialEq, Serialize)] // <-- this is an attribute
- * #[serde(serialize_with = "abc")] // <-- this is an attribute
+ * #[serde(serialize_with = "abc")] // <-- this attribute is a derive helper because the `Serialize` derive uses it
  * struct Abc {
  *   a: i64
  * }
  * ```
  */
-class Attribute(val inner: Writable) {
+class Attribute(val inner: Writable, val isDeriveHelper: Boolean = false) {
     constructor(str: String) : this(writable(str))
+    constructor(str: String, isDeriveHelper: Boolean) : this(writable(str), isDeriveHelper)
     constructor(runtimeType: RuntimeType) : this(runtimeType.writable)
 
     fun render(writer: RustWriter, attributeKind: AttributeKind = AttributeKind.Outer) {

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriterTest.kt

@@ -133,6 +133,19 @@ class RustWriterTest {
         sut.toString().shouldContain("#[foo]\n/// here's an attribute")
     }
 
+    @Test
+    fun `attributes with derive helpers must come after derives`() {
+        val attr = Attribute("foo", isDeriveHelper = true)
+        val metadata = RustMetadata(
+            derives = setOf(RuntimeType.Debug),
+            additionalAttributes = listOf(Attribute.AllowDeprecated, attr),
+            visibility = Visibility.PUBLIC,
+        )
+        val sut = RustWriter.root()
+        metadata.render(sut)
+        sut.toString().shouldContain("#[allow(deprecated)]\n#[derive(std::fmt::Debug)]\n#[foo]")
+    }
+
     @Test
     fun `deprecated attribute without any field`() {
         val sut = RustWriter.root()