A little overview of the project. I was trying to build PHP with the instructions given here: https://github.com/Microsoft/php-sdk-binary-tools#basic-usage-example - note that it mentions creating the project in the root of the drive.

After setting up VSCode properly, I still had an issue. I set the intellisense plugin's log level to DEBUG and I started getting: "Unable to retrieve file system information for <full path> error = -1". The problem seemed similar to VisualStudio's, but now the complaint seemed slightly different, but that gave me a better idea, what if I try loading the project from a different location?

Since the project is quite big, I went the quick approach and created a directory junction (a link from my user directory pointing to C:\php-sdk). When I opened VSCode from the "new" location, it started scanning stuff correctly! Next I tried VisualStudio and it started working too.

So the solution for me was to load the projects from somewhere not in the root of the drive. I don't know why this works, the machine is actually pretty new and there isn't much strange software on it. My only suspect is that I had Ubuntu on Windows (WSL) but I did not properly set it up, so for example, I could not have run "git clone" from WSL.

While on this subject, to me the logging/messages in VisualStudio seemed a little unclear; "cannot open source file" is way too general to be useful.