Calculate permutations recursively

Permutations.cs

private static IEnumerable<string> Permutations(string s)
            => Permutations("", s).Select(e => e.Item1);

        private static IEnumerable<(string, string)> Permutations(string acc, string rest)
        {
            if (string.IsNullOrEmpty(rest))
                return new[] {(acc, rest)};

            return rest.SelectMany(c =>
                Permutations(
                    acc + c.ToString(),
                    rest.RemoveChar(c.ToString())
                ));
        }

        [Fact]
        void as_a_function()
        {
            var result = Permutations("abc");

            result.Should().BeEquivalentTo(new List<string>
            {
                "abc",
                "acb",
                "bac",
                "bca",
                "cab",
                "cba"
            });
        }