On ReFS, Windows supports a block copy operation, like copy on write but at a sub-file level.

https://learn.microsoft.com/en-us/windows/win32/fileio/block-cloning
https://learn.microsoft.com/en-us/windows/win32/api/winioctl/ni-winioctl-fsctl_duplicate_extents_to_file

This has existed for several years, but is suddenly more interesting now because of the announcement of dev drive.

We should enlighten File.Copy to use this on Windows when possible, so that MSBuild, Nuget etc work extra fast on dev drive.