AsciiBuffer8.cs

using System.Buffers.Text;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.InteropServices;

var x = 0x12345678;
Console.WriteLine($"{x.Hex()}");

public static class Format
{
    public static AsciiBuffer8 Hex(this int x)
    {
        AsciiBuffer8 buffer = default;
        Utf8Formatter.TryFormat(x, buffer.AsSpan(), out _, new('x', 8));
        return buffer;
    }
}

/// <summary>
/// 8-character ASCII buffer.
/// </summary>
public struct AsciiBuffer8 : ISpanFormattable
{
    private ulong _buffer;

    [UnscopedRef]
    public Span<byte> AsSpan() => MemoryMarshal.AsBytes(new Span<ulong>(ref _buffer));

    public readonly string ToString(string? format, IFormatProvider? formatProvider)
        => string.Create(8, this, Format);

    public readonly bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format, IFormatProvider? provider)
    {
        if (destination.Length < 8)
        {
            charsWritten = 0;
            return false;
        }

        Format(destination, this);

        charsWritten = 8;
        return true;
    }

    private static void Format(Span<char> destination, AsciiBuffer8 buffer)
    {
        var span = buffer.AsSpan();

        for (int i = 0; i < span.Length; ++i)
            destination[i] = (char)span[i];
    }
}