head-tail sum perf

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Running;

public class Program
{
    static void Main() => BenchmarkRunner.Run<Benchmark>();
}

[MemoryDiagnoser]
public class Benchmark
{
    List<int> list = Enumerable.Range(1, 1_000).ToList();

    [Benchmark]
    public int LinqSum()
    {
       return list.Sum(); 
    }

    [Benchmark]
    public int ListDeconstruct()
    {
        int Sum(List<int> list)
        {
            switch (list.Count)
            {
                case 0:
                    return 0;
                default:
                    var (head, tail) = list;
                    return head + Sum(tail);
            }
        }

        return Sum(list);
    }

    [Benchmark]
    public int EnumerableDeconstruct()
    {
        int Sum(IEnumerable<int> list)
        {
            switch (list.Count())
            {
                case 0:
                    return 0;
                default:
                    var (head, tail) = list;
                    return head + Sum(tail);
            }
        }

        return Sum(list);
    }    

    [Benchmark]
    public int SpanDeconstruct()
    {
        int Sum(Span<int> list)
        {
            switch (list.Length)
            {
                case 0:
                    return 0;
                default:
                    var (head, tail) = list;
                    return head + Sum(tail);
            }
        }

        return Sum(CollectionsMarshal.AsSpan(list));
    }
}

static class E
{
    public static void Deconstruct<T>(this List<T> list, out T head, out List<T> tail)
    {
        head = list.FirstOrDefault();
        tail = new List<T>(list.Skip(1));
    }

    public static void Deconstruct<T>(this IEnumerable<T> list, out T head, out IEnumerable<T> tail)
    {
        head = list.FirstOrDefault();
        tail = list.Skip(1);
    }    

    public static void Deconstruct<T>(this Span<T> list, out T head, out Span<T> tail)
    {
        head = list.Length == 0 ? default : list[0];
        tail = list.Slice(1);
    }    
}

results.md

BenchmarkDotNet=v0.12.0, OS=Windows 10.0.18362
Intel Core i5-2300 CPU 2.80GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET Core SDK=5.0.100-alpha.1.20060.7
  [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.6002, CoreFX 5.0.20.6002), X64 RyuJIT
  DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.6002, CoreFX 5.0.20.6002), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
LinqSum	10.55 us	0.073 us	0.065 us	-	-	-	40 B
ListDeconstruct	10,023.12 us	150.070 us	140.375 us	1000.0000	406.2500	-	5642989 B
EnumerableDeconstruct	87.94 us	1.103 us	1.032 us	15.2588	-	-	48000 B
SpanDeconstruct	17.50 us	0.152 us	0.142 us	-	-	-	-