SpanLexer.cs

ref struct Lexer
{
    private Span<char> _buffer;
    private int _loc;
    
    public Lexer(Span<char> buffer)
    {
       _buffer = buffer;
       _loc = 0;
    }
    /// Move past the next token, incrementing _loc
    public void EatToken() { ... }
}

public static LexHelpers
{
   public static EatHelper(ref Lexer lex) => lex.EatToken(); // Pass by ref to preserve mutation
}