gistfile1.txt

// This is a .NET 3.5 console app, compiled with the C# 6 compiler from CTP6
using System;

namespace System.Runtime.CompilerServices
{
    public class FormattableStringFactory
    {
        public static FormattableString Create(string messageFormat, params object[] args)
        {
            return new FormattableString(messageFormat, args);
        }

        public static FormattableString Create(string messageFormat, DateTime bad, params object[] args)
        {
            var realArgs = new object[args.Length + 1];
            realArgs[0] = "Please don't use DateTime";
            Array.Copy(args, 0, realArgs, 1, args.Length);
            return new FormattableString(messageFormat, realArgs);
        }
    }
}

namespace System
{
    public class FormattableString
    {
        private readonly string messageFormat;
        private readonly object[] args;

        public FormattableString(string messageFormat, object[] args)
        {
            this.messageFormat = messageFormat;
            this.args = args;
        }
        public override string ToString()
        {
            return string.Format(messageFormat, args);
        }
    }
}

namespace ConsoleApp35
{
    class Program
    {
        static void Main(string[] args)
        {
            int x = 10;
            FormattableString fs = $"{nameof(x)}={x}";
            Console.WriteLine(fs);
            
            fs = $"{DateTime.Now}: {nameof(x)}={x}";
            Console.WriteLine(fs);
        }
    }
}

@jskeet, FormattableString nneds to implement IFormattable.