LineEnumerator.cs

public ref struct LineEnumerator(ReadOnlySpan<char> text)
{
    private ReadOnlySpan<char> text = text == default ? "" : text;

    public bool MoveNext()
    {
        if (text == default)
            return false;

        var nextEnd = text.IndexOf('\n');
        if (nextEnd != -1)
        {
            Current = text[..(nextEnd + 1)];
            currentWithoutLineEnding = default;
            text = text[(nextEnd + 1)..];
        }
        else
        {
            Current = text;
            currentWithoutLineEnding = text;
            text = default;
        }

        return true;
    }

    public ReadOnlySpan<char> Current { get; private set; }

    private ReadOnlySpan<char> currentWithoutLineEnding;
    public ReadOnlySpan<char> CurrentWithoutLineEnding
    {
        get
        {
            if (currentWithoutLineEnding == default)
            {
                var hasCrlf = Current is [.., '\r', _];
                currentWithoutLineEnding = Current[..^(hasCrlf ? 2 : 1)];
            }

            return currentWithoutLineEnding;
        }
    }
}

public readonly ref struct LineEnumerable(ReadOnlySpan<char> text)
{
    private readonly ReadOnlySpan<char> text = text;

    public LineEnumerator GetEnumerator() => new(text);
}

public static class LineEnumerableExtensions
{
    public static LineEnumerable EnumerateLines(this ReadOnlySpan<char> text) => new(text);
    public static LineEnumerable EnumerateLines(this string text) => new(text);
}