AFAIK this trick was never possible in .NET because Substring always 'deep copied' the relevant data into a completely new string

There is a lot of good things going on in .NET Core about this. Here is a good description of Span: http://adamsitnik.com/Span/#slicing-without-managed-heap-all...

Regex's implementation in the standard library used to do this, and I think maybe still does. Bonus points because they leaked a reference to the last string you ran a regex against (????) so if it was big you'd just eat up 50 mb of heap for a while.