Avoid explicitly emitting Unit type within Union

This commit addresses #1546. Previously, the Unit type in a Union was
rendered as an enum variant whose inner data was crate::model::Unit.
The way such a variant appears in Rust code feels a bit odd as it
usually does not carry inner data for `()`. We now render a Union
member of type Unit to an enum variant without inner data.

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/UnionGenerator.kt

@@ -5,6 +5,7 @@
 
 package software.amazon.smithy.rust.codegen.core.smithy.generators
 
+import software.amazon.smithy.codegen.core.Symbol
 import software.amazon.smithy.codegen.core.SymbolProvider
 import software.amazon.smithy.model.Model
 import software.amazon.smithy.model.shapes.MemberShape
@@ -49,16 +50,18 @@ class UnionGenerator(
     private val sortedMembers: List<MemberShape> = shape.allMembers.values.sortedBy { symbolProvider.toMemberName(it) }
 
     fun render() {
-        renderUnion()
-    }
-
-    private fun renderUnion() {
         writer.documentShape(shape, model)
         writer.deprecatedShape(shape)
 
         val unionSymbol = symbolProvider.toSymbol(shape)
         val containerMeta = unionSymbol.expectRustMetadata()
         containerMeta.render(writer)
+
+        renderUnion(unionSymbol)
+        renderImplBlock(unionSymbol)
+    }
+
+    private fun renderUnion(unionSymbol: Symbol) {
         writer.rustBlock("enum ${unionSymbol.name}") {
             sortedMembers.forEach { member ->
                 val memberSymbol = symbolProvider.toSymbol(member)
@@ -67,7 +70,7 @@ class UnionGenerator(
                 documentShape(member, model, note = note)
                 deprecatedShape(member)
                 memberSymbol.expectRustMetadata().renderAttributes(this)
-                write("${symbolProvider.toMemberName(member)}(#T),", symbolProvider.toSymbol(member))
+                writer.renderVariant(symbolProvider, member, memberSymbol)
             }
             if (renderUnknownVariant) {
                 docs("""The `Unknown` variant represents cases where new union variant was received. Consider upgrading the SDK to the latest available version.""")
@@ -82,6 +85,9 @@ class UnionGenerator(
                 rust("Unknown,")
             }
         }
+    }
+
+    private fun renderImplBlock(unionSymbol: Symbol) {
         writer.rustBlock("impl ${unionSymbol.name}") {
             sortedMembers.forEach { member ->
                 val memberSymbol = symbolProvider.toSymbol(member)
@@ -91,11 +97,7 @@ class UnionGenerator(
                 if (sortedMembers.size == 1) {
                     Attribute.Custom("allow(irrefutable_let_patterns)").render(this)
                 }
-                rust("/// Tries to convert the enum instance into [`$variantName`](#T::$variantName), extracting the inner #D.", unionSymbol, memberSymbol)
-                rust("/// Returns `Err(&Self)` if it can't be converted.")
-                rustBlock("pub fn as_$funcNamePart(&self) -> std::result::Result<&#T, &Self>", memberSymbol) {
-                    rust("if let ${unionSymbol.name}::$variantName(val) = &self { Ok(val) } else { Err(self) }")
-                }
+                writer.renderAsVariant(member, variantName, funcNamePart, unionSymbol, memberSymbol)
                 rust("/// Returns true if this is a [`$variantName`](#T::$variantName).", unionSymbol)
                 rustBlock("pub fn is_$funcNamePart(&self) -> bool") {
                     rust("self.as_$funcNamePart().is_ok()")
@@ -114,7 +116,44 @@ class UnionGenerator(
         const val UnknownVariantName = "Unknown"
     }
 }
+
 fun unknownVariantError(union: String) =
     "Cannot serialize `$union::${UnionGenerator.UnknownVariantName}` for the request. " +
         "The `Unknown` variant is intended for responses only. " +
         "It occurs when an outdated client is used after a new enum variant was added on the server side."
+
+private fun RustWriter.renderVariant(symbolProvider: SymbolProvider, member: MemberShape, memberSymbol: Symbol) {
+    if (member.target.name == "Unit") {
+        write("${symbolProvider.toMemberName(member)},")
+    } else {
+        write("${symbolProvider.toMemberName(member)}(#T),", memberSymbol)
+    }
+}
+
+private fun RustWriter.renderAsVariant(
+    member: MemberShape,
+    variantName: String,
+    funcNamePart: String,
+    unionSymbol: Symbol,
+    memberSymbol: Symbol,
+) {
+    if (member.target.name == "Unit") {
+        rust(
+            "/// Tries to convert the enum instance into [`$variantName`], extracting the inner `()`.",
+        )
+        rust("/// Returns `Err(&Self)` if it can't be converted.")
+        rustBlock("pub fn as_$funcNamePart(&self) -> std::result::Result<(), &Self>") {
+            rust("if let ${unionSymbol.name}::$variantName = &self { Ok(()) } else { Err(self) }")
+        }
+    } else {
+        rust(
+            "/// Tries to convert the enum instance into [`$variantName`](#T::$variantName), extracting the inner #D.",
+            unionSymbol,
+            memberSymbol,
+        )
+        rust("/// Returns `Err(&Self)` if it can't be converted.")
+        rustBlock("pub fn as_$funcNamePart(&self) -> std::result::Result<&#T, &Self>", memberSymbol) {
+            rust("if let ${unionSymbol.name}::$variantName(val) = &self { Ok(val) } else { Err(self) }")
+        }
+    }
+}

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/UnionGeneratorTest.kt

@@ -124,6 +124,17 @@ class UnionGeneratorTest {
         writer.compileAndTest()
     }
 
+    @Test
+    fun `unit types should not appear in generated enum`() {
+        val writer = generateUnion("union MyUnion { a: Unit, b: String }", unknownVariant = true)
+        writer.compileAndTest(
+            """
+            let a = MyUnion::A;
+            assert!(a.as_a() == Ok(()));
+            """,
+        )
+    }
+
     private fun generateUnion(modelSmithy: String, unionName: String = "MyUnion", unknownVariant: Boolean = true): RustWriter {
         val model = "namespace test\n$modelSmithy".asSmithyModel()
         val provider: SymbolProvider = testSymbolProvider(model)