Add a diagnostic when offsetof is used with DisableRuntimeMarshalling

Author: tannergooding

sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.VisitDecl.cs

@@ -1674,7 +1674,7 @@ private void VisitRecordDecl(RecordDecl recordDecl)
                 }
             }
 
-            if ((_testOutputBuilder is not null) && generateTestsClass)
+            if ((_testOutputBuilder is not null) && generateTestsClass && !_config.GenerateDisableRuntimeMarshalling)
             {
                 _testOutputBuilder.WriteIndented("/// <summary>Validates that the <see cref=\"");
                 _testOutputBuilder.Write(escapedName);

sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.VisitStmt.cs

@@ -2841,6 +2841,11 @@ private void VisitUnaryOperator(UnaryOperator unaryOperator)
 
     private void VisitOffsetOfExpr(OffsetOfExpr offsetOfExpr)
     {
+        if (_config.GenerateDisableRuntimeMarshalling)
+        {
+            AddDiagnostic(DiagnosticLevel.Warning, $"OffsetOf is unsupported when DisableRuntimeMarshalling is enabled. Generated bindings may be incomplete.", offsetOfExpr);
+        }
+
         var outputBuilder = StartCSharpCode();
 
         outputBuilder.AddUsingDirective("System.Runtime.InteropServices");