.NET 7 RC 2 でやっとまっとうに動いた

FixedArray.cs

using System.Diagnostics.CodeAnalysis;
using System.Numerics;
using System.Runtime.InteropServices;

var a2 = new Array2<int>();
for (int i = 0; i < 2; i++) a2[i] = i + 1;
Console.WriteLine(sum(a2.AsReadOnlySpan()));

var a3 = new Array3<int>();
for (int i = 0; i < 3; i++) a3[i] = i + 1;
Console.WriteLine(sum(a3.AsReadOnlySpan()));

var a4 = new Array4<int>();
for (int i = 0; i < 4; i++) a4[i] = i + 1;
Console.WriteLine(sum(a4.AsReadOnlySpan()));

static T sum<T>(ReadOnlySpan<T> items) where T : INumber<T>
{
    var s = T.Zero;
    foreach (var x in items) s += x;
    return s;
}

public interface IFixedArray<T>
{
    public static abstract int Length { get; }
    Span<T> AsSpan();
    public ReadOnlySpan<T> AsReadOnlySpan() => AsSpan();
    public ref T this[int index] => ref AsSpan()[index];
}

#pragma warning disable IDE0044, IDE0051, IDE0052

struct Array2<T> : IFixedArray<T>
{
    private T _0, _1;
    public Array2(T x0, T x1) => (_0, _1) = (x0, x1);
    public static int Length => 2;
    [UnscopedRef] public Span<T> AsSpan() => MemoryMarshal.CreateSpan(ref _0, Length);
    [UnscopedRef] public ReadOnlySpan<T> AsReadOnlySpan() => AsSpan();
    [UnscopedRef] public ref T this[int index] => ref AsSpan()[index];
}

struct Array3<T> : IFixedArray<T>
{
    private T _0, _1, _2;
    public Array3(T x0, T x1, T x2) => (_0, _1, _2) = (x0, x1, x2);
    public static int Length => 3;
    [UnscopedRef] public Span<T> AsSpan() => MemoryMarshal.CreateSpan(ref _0, Length);
    [UnscopedRef] public ReadOnlySpan<T> AsReadOnlySpan() => AsSpan();
    [UnscopedRef] public ref T this[int index] => ref AsSpan()[index];
}

struct Array4<T> : IFixedArray<T>
{
    private T _0, _1, _2, _3;
    public Array4(T x0, T x1, T x2, T x3) => (_0, _1, _2, _3) = (x0, x1, x2, x3);
    public static int Length => 4;
    [UnscopedRef] public Span<T> AsSpan() => MemoryMarshal.CreateSpan(ref _0, Length);
    [UnscopedRef] public ReadOnlySpan<T> AsReadOnlySpan() => AsSpan();
    [UnscopedRef] public ref T this[int index] => ref AsSpan()[index];
}

class FixedArray
{
    public static Array2<T> Create<T>(T x0, T x1) => new(x0, x1);
    public static Array3<T> Create<T>(T x0, T x1, T x2) => new(x0, x1, x2);
    public static Array4<T> Create<T>(T x0, T x1, T x2, T x3) => new(x0, x1, x2, x3);
}