`os::copy_file` does not clean after failure.

[dpc]

If you eg. give os::copy_file source argument that is a directory it will leave an empty destination file. I belive this is not good, and may lead to terrible, hard to debug bugs in software using it.

[hatahet]

Does this happen on *nix, Windows, or either?

[hatahet]

Also seems to fail if the source and destination files are the same. The resulting file is empty.

[hatahet]

#9958

[dpc]

I've tested this on Linux.

[emberian]

std::io::fs::open appears to not does this? #10175 modified some behavior around this. @dpc can you try and reproduce on master?

[steveklabnik]

We no longer have os::copy_file, and as @cmr asks, I think this just works now. @dpc if you think this is still an issue, please reproduce and let us know :)