Disallow partial struct declarations for blittable structs

src/ZeroSerializerGenerator.cs

@@ -683,6 +683,20 @@ private static bool TryGetBlittableStructByteCount(INamedTypeSymbol candidateTyp
 
     private static bool IsEligibleForBlittable(INamedTypeSymbol structType)
     {
+        foreach (SyntaxReference syntaxReference in structType.DeclaringSyntaxReferences)
+        {
+            if (syntaxReference.GetSyntax() is TypeDeclarationSyntax declaration)
+            {
+                foreach (SyntaxToken modifier in declaration.Modifiers)
+                {
+                    if (modifier.IsKind(SyntaxKind.PartialKeyword))
+                    {
+                        return false;
+                    }
+                }
+            }
+        }
+
         foreach (ISymbol member in structType.GetMembers())
         {
             if (member is IFieldSymbol field)

tests/Diagnostics/DiagnosticTests.cs

@@ -234,6 +234,29 @@ await CSharpSourceGeneratorVerifier<ZeroSerializerGenerator>.VerifySourceGenerat
         );
     }
 
+    [Fact]
+    public async Task ZEROS002_Violation_PartialStructWithLayout()
+    {
+        string source = @"
+using System.Runtime.InteropServices;
+using ZeroSerializer;
+
+[{|#0:StructLayout(LayoutKind.Sequential, Pack = 1)|}]
+[ZeroSerializer]
+public partial struct MyPartialStruct
+{
+    public int Value { get; set; }
+}
+";
+
+        await CSharpSourceGeneratorVerifier<ZeroSerializerGenerator>.VerifySourceGeneratorAsync(
+            source,
+            new DiagnosticResult("ZEROS002", DiagnosticSeverity.Warning)
+                .WithLocation(0)
+                .WithMessage("Struct 'MyPartialStruct' is marked with StructLayout(LayoutKind.Sequential, Pack = 1) but does not meet the requirements to be a blittable struct")
+        );
+    }
+
     [Fact]
     public async Task ZEROS003_Violation_UnmarkedBlittableNestedStruct()
     {