Poor codegen for collection expression spread (ROS<T> -> ImmutableArray<T>)

[Sergio0694]

Related to #71195

Version Used: 4.9.0-ci (a8119d1)

Steps to Reproduce:

using System;
using System.Collections.Immutable;

static unsafe ImmutableArray<byte> Foo(byte* p, int length)
{
    return [.. new ReadOnlySpan<byte>(p, length)];
}

sharplab

Expected Behavior:

ReadOnlySpan<byte> readOnlySpan = new ReadOnlySpan<byte>(p, length);
return ImmutableArray.CreateRange(readOnlySpan);

OR

ReadOnlySpan<byte> readOnlySpan = new ReadOnlySpan<byte>(p, length);
return ImmutableCollectionsMarshal.AsImmutableArray(readOnlySpan.ToArray());

Actual Behavior:

ReadOnlySpan<byte> readOnlySpan = new ReadOnlySpan<byte>(p, length);
int num = 0;
byte[] array = new byte[readOnlySpan.Length];
ReadOnlySpan<byte>.Enumerator enumerator = readOnlySpan.GetEnumerator();
while (enumerator.MoveNext())
{
    byte b = (array[num] = enumerator.Current);
    num++;
}
return ImmutableCollectionsMarshal.AsImmutableArray(array);

cc. @RikkiGibson maybe a missed case from your other optimization work? 🤔

[RikkiGibson]

I think sharplab may not have had the spread optimization present at the time this issue was filed. Seeing the following codegen now SharpLab

return ImmutableCollectionsMarshal.AsImmutableArray(new ReadOnlySpan<byte>(p, length).ToArray());

I'll close as no repro. Feel free to follow up here if I missed something. Thanks