std::fs::copy is unsupported

[anatawa12]

When we call std::fs::copy on macos, we encountered unsupported operation: can't call foreign function `fclonefileat` on OS `macos` error.

I think std::fs::copy is a basic API commonly used so I reported here as miri error message suggests.

I searched for fclonefileat but I could not found existing issue

[RalfJung]

Thanks for the report!

Yes it would be reasonable to support this; no idea how tricky it would be though. I don't think we support fs::copy on any OS so far, so implementing it on Linux would likely be the first step.

[RalfJung]

Looks like on Linux,fs::copy falls back to io::copy, so this might already work.