https://github.com/dotnet/csharplang/issues/2584

Progam.cs

using System;
using System.Reflection;
using System.Reflection.Emit;

class Program
{
    static void Main()
    {
        var assembly = AssemblyBuilder.DefineDynamicAssembly(new AssemblyName("a.dll"), AssemblyBuilderAccess.Run);
        var module = assembly.DefineDynamicModule("a.dll");
        var type = module.DefineType("S", default, typeof(ValueType));
        var field = type.DefineField("s", typeof(Span<int>), default);

        type.CreateType();
    }
}