Only emit JsonDerivedType attribute when not already present

Funcky.DiscriminatedUnion.SourceGeneration/Functional.cs

@@ -0,0 +1,6 @@
+namespace Funcky.DiscriminatedUnion.SourceGeneration;
+
+internal static class Functional
+{
+    public static Lazy<T> Lazy<T>(Func<T> func) => new(func);
+}

Funcky.DiscriminatedUnion.SourceGeneration/Parser.JsonPolymorphic.cs

@@ -0,0 +1,33 @@
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using static Funcky.DiscriminatedUnion.SourceGeneration.Functional;
+
+namespace Funcky.DiscriminatedUnion.SourceGeneration;
+
+internal static partial class Parser
+{
+    private const string JsonPolymorphicAttributeName = "System.Text.Json.Serialization.JsonPolymorphicAttribute";
+    private const string JsonDerivedTypeAttributeName = "System.Text.Json.Serialization.JsonDerivedTypeAttribute";
+
+    private static Func<INamedTypeSymbol, bool> GenerateJsonDerivedTypeAttribute(INamedTypeSymbol discriminatedUnion)
+    {
+        var generateJsonDerivedTypeAttributes = Lazy(() => discriminatedUnion.GetAttributes().Any(IsJsonPolymorphicAttribute));
+        var jsonDerivedTypes = Lazy(() => GetJsonDerivedTypes(discriminatedUnion));
+        return variant => generateJsonDerivedTypeAttributes.Value && !jsonDerivedTypes.Value.Contains(variant);
+    }
+
+    private static ImmutableHashSet<INamedTypeSymbol> GetJsonDerivedTypes(INamedTypeSymbol discriminatedUnion)
+        => discriminatedUnion.GetAttributes()
+            .Select(GetJsonDerivedType)
+            .Where(t => t is not null)!
+            .ToImmutableHashSet<INamedTypeSymbol>(SymbolEqualityComparer.Default);
+
+    private static bool IsJsonPolymorphicAttribute(AttributeData attribute)
+        => attribute.AttributeClass?.ToDisplayString() is JsonPolymorphicAttributeName or JsonDerivedTypeAttributeName;
+
+    private static INamedTypeSymbol? GetJsonDerivedType(AttributeData attribute)
+        => attribute.AttributeClass?.ToDisplayString() is JsonDerivedTypeAttributeName
+            && attribute.ConstructorArguments.First() is { Kind: TypedConstantKind.Type, Value: INamedTypeSymbol value }
+                ? value
+                : null;
+}

Funcky.DiscriminatedUnion.SourceGeneration/Parser.cs

@@ -6,11 +6,8 @@
 
 namespace Funcky.DiscriminatedUnion.SourceGeneration;
 
-internal static class Parser
+internal static partial class Parser
 {
-    private const string JsonPolymorphicAttributeName = "System.Text.Json.Serialization.JsonPolymorphicAttribute";
-    private const string JsonDerivedTypeAttributeName = "System.Text.Json.Serialization.JsonDerivedTypeAttribute";
-
     public static bool IsSyntaxTargetForGeneration(SyntaxNode node)
         => node is ClassDeclarationSyntax { AttributeLists.Count: > 0 }
                 or RecordDeclarationSyntax { AttributeLists.Count: > 0 };
@@ -35,7 +32,6 @@ public static bool IsSyntaxTargetForGeneration(SyntaxNode node)
 
         var (nonExhaustive, flatten, matchResultType) = ParseAttribute(typeSymbol);
         var isVariant = flatten ? IsVariantOfDiscriminatedUnionFlattened(typeSymbol, semanticModel) : IsVariantOfDiscriminatedUnion(typeSymbol, semanticModel);
-        var generateJsonDerivedTypeAttributes = typeSymbol.GetAttributes().Any(IsJsonPolymorphicAttribute);
 
         return new DiscriminatedUnion(
             Type: typeDeclaration,
@@ -44,7 +40,7 @@ public static bool IsSyntaxTargetForGeneration(SyntaxNode node)
             MatchResultTypeName: matchResultType ?? "TResult",
             MethodVisibility: nonExhaustive ? "internal" : "public",
             Variants: GetVariantTypeDeclarations(typeDeclaration, isVariant)
-                .Select(GetDiscriminatedUnionVariant(typeDeclaration, semanticModel, _ => generateJsonDerivedTypeAttributes))
+                .Select(GetDiscriminatedUnionVariant(typeDeclaration, semanticModel, GenerateJsonDerivedTypeAttribute(typeSymbol)))
                 .ToList());
     }
 
@@ -123,9 +119,6 @@ private static Func<AttributeSyntax, bool> IsDiscriminatedUnionAttribute(Generat
             => context.SemanticModel.GetSymbolInfo(attribute, cancellationToken).Symbol is IMethodSymbol attributeSymbol
                 && attributeSymbol.ContainingType.ToDisplayString() == AttributeFullName;
 
-    private static bool IsJsonPolymorphicAttribute(AttributeData attribute)
-        => attribute.AttributeClass?.ToDisplayString() is JsonPolymorphicAttributeName or JsonDerivedTypeAttributeName;
-
     private sealed record DiscriminatedUnionAttributeData(bool NonExhaustive, bool Flatten, string? MatchResultType);
 
     private sealed class VariantCollectingVisitor : CSharpSyntaxWalker

Funcky.DiscriminatedUnion.Test/SourceGeneratorTest.GeneratesExpectedSourceCode_sourceFileName=JsonPolymorphic.01.verified.cs

@@ -65,3 +65,40 @@ partial record Error
         }
     }
 }
+
+[global::System.Text.Json.Serialization.JsonDerivedType(typeof(global::Shape.EquilateralTriangle), "EquilateralTriangle")]
+partial record Shape
+{
+    [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+    public abstract TResult Match<TResult>(global::System.Func<Rectangle, TResult> rectangle, global::System.Func<Circle, TResult> circle, global::System.Func<EquilateralTriangle, TResult> equilateralTriangle);
+
+    [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+    public abstract void Switch(global::System.Action<Rectangle> rectangle, global::System.Action<Circle> circle, global::System.Action<EquilateralTriangle> equilateralTriangle);
+
+    partial record Rectangle
+    {
+        [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+        public override TResult Match<TResult>(global::System.Func<Rectangle, TResult> rectangle, global::System.Func<Circle, TResult> circle, global::System.Func<EquilateralTriangle, TResult> equilateralTriangle) => rectangle(this);
+
+        [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+        public override void Switch(global::System.Action<Rectangle> rectangle, global::System.Action<Circle> circle, global::System.Action<EquilateralTriangle> equilateralTriangle) => rectangle(this);
+    }
+
+    partial record Circle
+    {
+        [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+        public override TResult Match<TResult>(global::System.Func<Rectangle, TResult> rectangle, global::System.Func<Circle, TResult> circle, global::System.Func<EquilateralTriangle, TResult> equilateralTriangle) => circle(this);
+
+        [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+        public override void Switch(global::System.Action<Rectangle> rectangle, global::System.Action<Circle> circle, global::System.Action<EquilateralTriangle> equilateralTriangle) => circle(this);
+    }
+
+    partial record EquilateralTriangle
+    {
+        [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+        public override TResult Match<TResult>(global::System.Func<Rectangle, TResult> rectangle, global::System.Func<Circle, TResult> circle, global::System.Func<EquilateralTriangle, TResult> equilateralTriangle) => equilateralTriangle(this);
+
+        [global::System.CodeDom.Compiler.GeneratedCode("Funcky.DiscriminatedUnion.SourceGeneration", "1.0.0.0")]
+        public override void Switch(global::System.Action<Rectangle> rectangle, global::System.Action<Circle> circle, global::System.Action<EquilateralTriangle> equilateralTriangle) => equilateralTriangle(this);
+    }
+}

Funcky.DiscriminatedUnion.Test/Sources/JsonPolymorphic.cs

@@ -21,3 +21,15 @@ public sealed partial record Error(string Message) : Result<T>;
         }
     }
 }
+
+[Funcky.DiscriminatedUnion]
+[System.Text.Json.Serialization.JsonDerivedType(typeof(Rectangle), typeDiscriminator: 1)]
+[System.Text.Json.Serialization.JsonDerivedType(typeof(Circle), typeDiscriminator: "⏺")]
+public abstract partial record Shape
+{
+    public sealed partial record Rectangle(double Width, double Length) : Shape;
+
+    public sealed partial record Circle(double Radius) : Shape;
+
+    public partial record EquilateralTriangle(double SideLength) : Shape;
+}