Add field forwarding in UnionStructSourceGenerator; fix incremental generator

Author: apkd

Medicine.SourceGenerator.Tests~/Program.cs

@@ -43,7 +43,9 @@
     SingletonSourceGeneratorTest.Case,
     UnionSourceGeneratorTest.Case,
     UnionSourceGeneratorTest.NoDerivedHeaderCase,
+    UnionSourceGeneratorTest.HeaderFieldForwardingCase,
     UnionNestedSourceGeneratorTest.Case,
+    UnionNestedSourceGeneratorTest.HeaderFieldForwardingCase,
     UnmanagedAccessSourceGeneratorTest.Case,
     WrapValueEnumerableSourceGeneratorTest.Case,
     ConstantsSourceGeneratorTest.Case,

Medicine.SourceGenerator.Tests~/SourceGeneratorTests/UnionNestedSourceGeneratorTest.cs

@@ -1,8 +1,13 @@
+using Microsoft.CodeAnalysis;
+
 static class UnionNestedSourceGeneratorTest
 {
     public static readonly DiagnosticTest Case =
         new("Union generator supports nested [UnionHeader] families", Run);
 
+    public static readonly DiagnosticTest HeaderFieldForwardingCase =
+        new("Union generator forwards nested header fields to [Union] structs", RunHeaderFieldForwarding);
+
     static void Run()
         => SourceGeneratorTester.AssertGeneratesSource(
             source: """
@@ -47,4 +52,81 @@ public partial struct RangeWeaponState : WeaponState.IDerivedWeapon
 """,
             generator: new UnionStructSourceGenerator()
         );
+
+    static void RunHeaderFieldForwarding()
+    {
+        var compilation = RoslynHarness.CreateCompilation(
+            Stubs.Core,
+            """
+using Medicine;
+
+[UnionHeader]
+public partial struct RootState
+{
+    public interface IDerived
+    {
+        bool IsReady(int value);
+    }
+
+    public TypeIDs TypeID;
+}
+
+[UnionHeader]
+public partial struct ChildState
+{
+    public interface IDerivedChild : RootState.IDerived
+    {
+        bool CanRun(int value);
+    }
+
+    public RootState Header;
+    public int Counter;
+}
+
+[Union(1)]
+public partial struct ChildAState : ChildState.IDerivedChild
+{
+    public ChildState Header;
+    public bool IsReady(int value) => true;
+    public bool CanRun(int value) => true;
+}
+
+static class Usage
+{
+    public static int Run()
+    {
+        var child = new ChildAState();
+        child.Counter = 9;
+        return child.Counter;
+    }
+}
+"""
+        );
+
+        var run = RoslynHarness.RunGenerators(
+            compilation,
+            [new UnionStructSourceGenerator()],
+            []
+        );
+
+        RoslynHarness.AssertDoesNotContainDiagnostic(
+            diagnostics: run.GeneratorDiagnostics,
+            id: "MED911",
+            because: "nested header-field forwarding should not throw in generator"
+        );
+
+        var missingForwardingMembers = run.CompilationDiagnostics
+            .Where(x => x.Severity == DiagnosticSeverity.Error)
+            .Where(x => x.Id is "CS1061")
+            .Where(x => x.GetMessage().Contains("Counter", StringComparison.Ordinal))
+            .ToArray();
+
+        if (missingForwardingMembers.Length == 0)
+            return;
+
+        throw new InvalidOperationException(
+            "Expected nested [Union] structs to expose nested [UnionHeader] fields via generated forwarding properties." + Environment.NewLine +
+            $"Actual diagnostics:{Environment.NewLine}{RoslynHarness.FormatDiagnostics(missingForwardingMembers)}"
+        );
+    }
 }

Medicine.SourceGenerator.Tests~/SourceGeneratorTests/UnionSourceGeneratorTest.cs

@@ -8,6 +8,9 @@ static class UnionSourceGeneratorTest
     public static readonly DiagnosticTest NoDerivedHeaderCase =
         new("Union generator supports [UnionHeader] without derived [Union] structs", RunNoDerivedHeader);
 
+    public static readonly DiagnosticTest HeaderFieldForwardingCase =
+        new("Union generator forwards header fields to [Union] structs", RunHeaderFieldForwarding);
+
     static void Run()
         => SourceGeneratorTester.AssertGeneratesSource(
             source: """
@@ -88,4 +91,74 @@ [new UnionStructSourceGenerator()],
             $"Actual diagnostics:{Environment.NewLine}{RoslynHarness.FormatDiagnostics(typeIdResolutionErrors)}"
         );
     }
+
+    static void RunHeaderFieldForwarding()
+    {
+        var compilation = RoslynHarness.CreateCompilation(
+            Stubs.Core,
+            """
+using Medicine;
+
+[UnionHeader]
+public partial struct ContractUnion
+{
+    public interface Interface
+    {
+        int Value { get; }
+    }
+
+    public TypeIDs TypeID;
+    public int Version;
+    public bool Enabled;
+}
+
+[Union(1)]
+public partial struct ContractUnionA : ContractUnion.Interface
+{
+    public ContractUnion Header;
+    public int Value => 42;
+}
+
+static class Usage
+{
+    public static int Run()
+    {
+        var item = new ContractUnionA();
+        item.Version = 7;
+        item.Enabled = true;
+        return item.Version + (item.Enabled ? 1 : 0);
+    }
+}
+"""
+        );
+
+        var run = RoslynHarness.RunGenerators(
+            compilation,
+            [new UnionStructSourceGenerator()],
+            []
+        );
+
+        RoslynHarness.AssertDoesNotContainDiagnostic(
+            diagnostics: run.GeneratorDiagnostics,
+            id: "MED911",
+            because: "header-field forwarding should not throw in generator"
+        );
+
+        var missingForwardingMembers = run.CompilationDiagnostics
+            .Where(x => x.Severity == DiagnosticSeverity.Error)
+            .Where(x => x.Id is "CS1061")
+            .Where(x =>
+                x.GetMessage().Contains("Version", StringComparison.Ordinal) ||
+                x.GetMessage().Contains("Enabled", StringComparison.Ordinal)
+            )
+            .ToArray();
+
+        if (missingForwardingMembers.Length == 0)
+            return;
+
+        throw new InvalidOperationException(
+            "Expected [Union] structs to expose [UnionHeader] fields via generated forwarding properties." + Environment.NewLine +
+            $"Actual diagnostics:{Environment.NewLine}{RoslynHarness.FormatDiagnostics(missingForwardingMembers)}"
+        );
+    }
 }