Cloner.cs

#if SYSTEM_SOURCEGEN_ENABLED

using System.Linq;
using Mono.Cecil;
using Unity.Entities;
using Unity.Entities.CodeGen;

class Cloner : EntitiesILPostProcessor
{
    protected override bool PostProcessImpl(TypeDefinition[] componentSystemTypes)
    {
        var madeChange = false;
        foreach (var c in componentSystemTypes)
        {
            var typeMethods = c.Methods.ToArray();
            foreach (var sourceMethod in typeMethods.Where(m => m.CustomAttributes.Any(
                attribute => attribute.AttributeType.Name == nameof(DOTSCompilerPatchedMethodAttribute))))
            {
                var destinationMethodName = sourceMethod.CustomAttributes.First(attribute => attribute.AttributeType.Name == nameof(DOTSCompilerPatchedMethodAttribute))
                    .ConstructorArguments.First().Value.ToString();

                var matchingMethods = c.Methods.Where(m => m.Name == destinationMethodName);
                var destinationMethod = matchingMethods.Single();
                foreach (var displayClass in destinationMethod.Body.Variables.Select(v => v.VariableType).OfType<TypeDefinition>().Where(IsDisplayClass))
                    destinationMethod.DeclaringType.NestedTypes.Remove(displayClass);

                destinationMethod.Body = sourceMethod.Body;
                c.Methods.Remove(sourceMethod);

                var sequencePoints = destinationMethod.DebugInformation.SequencePoints;
                sequencePoints.Clear();
                foreach (var sp in sourceMethod.DebugInformation.SequencePoints)
                    sequencePoints.Add(sp);
                destinationMethod.DebugInformation.Scope = sourceMethod.DebugInformation.Scope;

                if (destinationMethod.HasGenericParameters && sourceMethod.HasGenericParameters)
                {
                    destinationMethod.GenericParameters.Clear();
                    foreach (var genericSourceParam in sourceMethod.GenericParameters)
                        destinationMethod.GenericParameters.Add(genericSourceParam);
                }

                madeChange = true;
            }
        }
        return madeChange;
    }

    static bool IsDisplayClass(TypeDefinition arg) => arg.Name.Contains("<>");

    protected override bool PostProcessUnmanagedImpl(TypeDefinition[] unmanagedComponentSystemTypes)
    {
        return false;
    }
}

#endif