From 93e046722f600a0cb97934369873592899a1dea2 Mon Sep 17 00:00:00 2001
From: "Nate McMaster (AWS)" <88004173+mcmasn-amzn@users.noreply.github.com>
Date: Wed, 19 Apr 2023 08:18:13 -0700
Subject: [PATCH] Fix for Rust compiler warning about derive helpers (#2581)

## Motivation and Context
Follow-up to https://github.com/awslabs/smithy-rs/pull/2434


## Description
This provides a way to fix a compiler warning. Attributes created using
RustMetadata.additionalAttributes may trigger compiler warnings
(https://github.com/rust-lang/rust/issues/79202) such as

```
warning: derive helper attribute is used before it is introduced
    --> src/model.rs:7674:3
     |
7674 | #[serde(tag = "_type", content = "_content")]
     |   ^^^^^
7675 | #[derive(
7676 |     serde::Deserialize,
     |     ------------------ the attribute is introduced here
     |
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: for more information, see issue #79202 <https://github.com/rust-lang/rust/issues/79202>

```

## Testing
Added a unit test to validate the sort order is applied correctly to
Attributes with isDeriveHelper = true.

---------

Co-authored-by: david-perez <d@vidp.dev>
---
 .../rust/codegen/core/rustlang/RustType.kt      | 17 ++++++++++++++---
 .../codegen/core/rustlang/RustWriterTest.kt     | 13 +++++++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustType.kt b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustType.kt
index e4d27fe451..56f0b63c52 100644
--- a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustType.kt
+++ b/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) {
diff --git a/codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriterTest.kt b/codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriterTest.kt
index 3b0ef7c557..ac14bcd20f 100644
--- a/codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriterTest.kt
+++ b/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()