[issue-300] Add diagnostic rule MEMPACK041 to prevent VersionTolerant on unmanaged structs

Author: kostiantyn-bessonov

src/MemoryPack.Generator/DiagnosticDescriptors.cs

@@ -332,4 +332,12 @@ internal static class DiagnosticDescriptors
         category: Category,
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true);
+
+    public static readonly DiagnosticDescriptor VersionTolerantOnUnmanagedStruct = new(
+        id: "MEMPACK041",
+        title: "Invalid usage of VersionTolerant on unmanaged struct",
+        messageFormat: "The unmanaged struct '{0}' cannot be used for VersionTolerant serialization.",
+        category: Category,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
 }

src/MemoryPack.Generator/MemoryPackGenerator.Parser.cs

@@ -294,6 +294,12 @@ public bool Validate(TypeDeclarationSyntax syntax, IGeneratorContext context, bo
 
         if (this.IsUnmanagedType)
         {
+            if (GenerateType is GenerateType.VersionTolerant)
+            {
+                context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.VersionTolerantOnUnmanagedStruct, syntax.Identifier.GetLocation(), Symbol.Name));
+                noError = false;
+            }
+
             var structLayoutFields = this.Symbol.GetAllMembers()
                 .OfType<IFieldSymbol>()
                 .Select(x =>

tests/MemoryPack.Tests/GeneratorDiagnosticsTest.cs

@@ -687,9 +687,22 @@ public Tester(int i1)
 }
 
 """);
+    }
+
+    [Fact]
+    public void MEMPACK041_UnmanagedStructCannotBeVersionTolerant()
+    {
+        Compile(41, """
+using MemoryPack;
 
+[MemoryPackable(GenerateType.VersionTolerant)]
+public partial struct Tester
+{
+    [MemoryPackOrder(0)]
+    public int I1 { get; init; }
+}
+""");
     }
 }
 
-
 #endif