Generate "warnings as errors" ruleset from error code definitions in Roslyn source code

CSharpErrorCodes.cs

void Main()
{
    string errorCodesFileUrl = "https://raw.githubusercontent.com/dotnet/roslyn/master/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs";
    string errorCodesFileContent = new WebClient().DownloadString(errorCodesFileUrl);
    var syntaxTree = CSharpSyntaxTree.ParseText(errorCodesFileContent);
    var root = syntaxTree.GetRoot();
    var enumDeclaration =
        root.DescendantNodes()
            .OfType<EnumDeclarationSyntax>()
            .First(e => e.Identifier.ValueText == "ErrorCode");
    var errorCodes =
        enumDeclaration
            .Members
            .Select(m => new ErrorCode(m))
            .ToList();
    
    var doc =
        new XDocument(
            new XElement("RuleSet",
                new XAttribute("Name", "C# Warnings as Errors"),
                new XAttribute("ToolsVersion", "15.0"),
                new XElement("Rules",
                    new XAttribute("AnalyzerId", "Microsoft.CodeAnalysis.CSharp"),
                    new XAttribute("RuleNamespace", "Microsoft.CodeAnalysis.CSharp"),
                    from e in errorCodes
                    where e.Level == "WRN"
                    select new XElement("Rule",
                        new XAttribute("Id", e.Code),
                        new XAttribute("Action", "Error")))));
    doc.Save(Console.Out);
}

class ErrorCode
{
    public ErrorCode(EnumMemberDeclarationSyntax member)
    {
        string name = member.Identifier.ValueText;
        if (name == "Void" || name == "Unknown")
        {
            Name = name;
            Value = 0;
            Level = "UNK";
        }
        else
        {
            Name = name.Substring(4);
            Value = int.Parse(member.EqualsValue?.Value?.GetText()?.ToString() ?? "0");
            Level = name.Substring(0, 3);
        }
    }
    
    public string Name { get; }
    public int Value { get; }
    public string Level { get; }
    public string Code => $"CS{Value:D4}";
}