Version Used:
Version 17.9.0 Preview 1.0 [34231.268.main]

Steps to Reproduce:

static int[] GetArray(List<int> list) => list.ToArray(); 

IDE0305 recommends rewriting this to:

static int[] GetArray(List<int> list) => [.. list];

which would be fine if that still compiled down to a list.ToArray() call, but instead it compiles down to the equivalent of:

int[] array = new int[list.Count];
int num = 0;
foreach (int item in list)
{
    array[num] = item;
    num++;
}

which is significantly less efficient, especially for longer lists where the entire copy would instead be done as a vectorized memcpy invocation.

It's also worse if the list was actually empty: List<T>.ToArray() will return an empty array singleton if the count is 0, but the above code will end up always allocating a new empty array 😦

cc: @cston, @CyrusNajmabadi