Stack based object pool C# .NET tailored for performance and simplicity

StackPool.cs

using System;
using System.Threading;
using static System.Console;

public class Test
{
	public static void Main()
	{
		var p = new StackPool<S>();
		var s1 = Rent(p, "55");
		WriteLine(s1);
		
		var s2 = Rent(p, "66");
		WriteLine(s2);
		
		p.Return(s1);
		WriteLine("returning "+s1);
		p.Return(s2);
		WriteLine("returning "+s2);
		
		var s3 = Rent(p, "77");
		WriteLine(s3);
	}
	
	private static S Rent(StackPool<S> p, string s) =>
	  p.Rent()?.Init(s) ?? new S(s);
	
	public class S
	{
		public static int N;
		private string _s;
		public S Init(string s) 
		{ _s = s; return this; }
		public S(string s) { _s = s; ++N; }
		public override string ToString() => 
		_s+"("+N+")";
	}
}

public sealed class StackPool<T> where T : class
{
	public T Rent() =>
	  Interlocked.Exchange(ref _s, _s?.Tail)
	  ?.Head;
	  	
	public void Return(T x) =>
	  Interlocked.Exchange(ref _s, 
	  	new Stack(x, _s));
	
	private Stack _s;
	private sealed class Stack
	{
		public T Head;
		public Stack Tail;
		public Stack(T h, Stack t)
		{
			Head = h;
			Tail = t;
		}
	}
}