Permutations of a string, with separate pending and permutations

PermutationsItertive2.cs

        private static IEnumerable<string> PermutationsIterative(string s)
        {
            var pending = new List<(string permutation, string rest)>
            {
                ("", s)
            };
            var permutations = new List<string>();

            while (pending.Any())
            {
                pending = pending
                    .SelectMany(p =>
                        p.rest.Select(c =>
                        (
                            p.permutation + c,
                            p.rest.RemoveChar(c)
                        ))).ToList();

                permutations = permutations.Union(
                        pending
                            .Where(p => p.rest == "")
                            .Select(p => p.permutation))
                    .ToList();
            }

            return permutations;
        }