How C# lambdas with a single ref-type immutable closure can be optimized

gistfile1.cs

using System;

static class LiftingClosureToDelegateTarget {
  static void Main() {
    // normal lambda
    {
      var str = GetString();
      Func<string> f = () => str;
      Console.WriteLine(f());
    }

    // what is generated (two allocations: closure and delegate)
    {
      var closure = new Closure();
      closure._str = GetString();
      Func<string> f = new Func<string>(closure.LambdaBody);
      Console.WriteLine(f());
    }

    // what CAN be generated if captured variable is immutable (single allocation: delegate)
    // delegate instance contains both 'str' reference and a function pointer to static method
    {
      var str = GetString();
      Func<string> f = new Func<string>(str.LambdaBodyAsExtensionMethod);
      Console.WriteLine(f());
    }

    // the same with reflection
    {
      var str = GetString();
      Func<string> f = (Func<string>) Delegate.CreateDelegate(
        type: typeof (Func<string>),
        firstArgument: str, // <-- all magic is here
        method: typeof (LiftingClosureToDelegateTarget).GetMethod("LambdaBodyAsExtensionMethod"));
      Console.WriteLine(f());
    }
  }

  private static string GetString() {
    return "abc";
  }

  private sealed class Closure {
    public string _str;
    public string LambdaBody() { return _str; }
  }

  public static string LambdaBodyAsExtensionMethod(this string str) {
    return str;
  }
}