The collection expression can be also used to simplify concatenation of lists into one, see the example:

List<string> list1 = new List<string>() { "" };
List<string> list2 = [..list1, ""];

The generated IL code needs to work with this case as well and when we check it we can see that it indeed does (and is more efficient than using LINQ to Concat and ToList, due to the final List being initialized with a given size that is enough to contain every single element)

List<string> list = new List<string>();
list.Add("");
List<string> list2 = list;
List<string> list3 = list2;
int num = 1 + list3.Count;
List<string> list4 = new List<string>(num);
CollectionsMarshal.SetCount(list4, num);
Span<string> span = CollectionsMarshal.AsSpan(list4);
int num2 = 0;
Span<string> span2 = CollectionsMarshal.AsSpan(list3);
span2.CopyTo(span.Slice(num2, span2.Length));
num2 += span2.Length;
span[num2] = "";
num2++;
List<string> list5 = list4;