simple stack-based C# string

StackString.cs

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

/// <summary>
/// Stack-based String
/// </summary>
[InterpolatedStringHandler]
[SkipLocalsInit]
public unsafe struct StackString
{
	public const int Capacity = 128;

	public fixed char Buffer[Capacity];
	public int Length { get; private set; }

	public StackString() { Buffer[0] = '\0'; Length = 0; }
	public StackString(int literalLength, int formattedCount) : this() { }
	public StackString(in string str) : this() { Append(str); }
	public StackString(in ReadOnlySpan<char> str) : this() { Append(str); }

	public static implicit operator StackString(in string str) => new(str);
	public static implicit operator StackString(in ReadOnlySpan<char> str) => new(str);

	public ReadOnlySpan<char> Span => MemoryMarshal.CreateReadOnlySpan(ref Buffer[0], Length);

	private Span<char> Avail
	{
		get
		{
			if (Length < Capacity - 1)
				return MemoryMarshal.CreateSpan(ref Buffer[0], Capacity)[Length..^1];
			return Span<char>.Empty;
		}
	}

	public void Append(ReadOnlySpan<char> s)
	{
		if (Avail.Length > 0)
		{
			if (s.Length > Avail.Length)
				s = s[0..Avail.Length];
			s.CopyTo(Avail);
			Length += s.Length;
			Buffer[Length] = '\0';
		}
	}

	public void AppendLiteral(string s) => Append(s);

	public void AppendFormatted(int value) => AppendFormatted(value, null);
	public void AppendFormatted(int value, string? format)
	{
		value.TryFormat(Avail, out int len, format ?? default);
		Length += len;
		Buffer[Length] = '\0';
	}

	public void AppendFormatted(long value) => AppendFormatted(value, null);
	public void AppendFormatted(long value, string? format)
	{
		value.TryFormat(Avail, out int len, format ?? default);
		Length += len;
		Buffer[Length] = '\0';
	}

	public void AppendFormatted(float value) => AppendFormatted(value, null);
	public void AppendFormatted(float value, string? format)
	{
		value.TryFormat(Avail, out int len, format ?? default);
		Length += len;
		Buffer[Length] = '\0';
	}

	public void AppendFormatted(double value) => AppendFormatted(value, null);
	public void AppendFormatted(double value, string? format)
	{
		value.TryFormat(Avail, out int len, format ?? default);
		Length += len;
		Buffer[Length] = '\0';
	}

	public void AppendFormatted<T>(T t) => AppendFormatted(t, null);
	public void AppendFormatted<T>(T t, string? format)
	{
		if (t is string vStr)
		{
			Append(vStr);
		}
		else
		{
			throw new NotImplementedException();
		}

		Buffer[Length] = '\0';
	}

	public override string ToString()
	{
		return Span.ToString();
	}
}