rustpkg tries to move directory from /tmp, resulting in IO error when the destination is on an external device

[derekchiang]

Steps to reproduce:

1. cd to a path on an external device
2. rustpkg install github.com/steveklabnik/hello

You should see a task failure. I dug a bit into the code and found the problem. Basically, when the environment variable TMPDIR is not explicitly set, rustpkg uses extra::tempfile::TempDir to store temporary files. TempDir by default operates in /tmp.

Later, rustpkg does fs::rename(&clone_target, local), in which clone_target and local are paths to different devices. libuv (the default underlying IO library) seems unable to handle this case, therefore resulting in a "cross-device link not permitted" IO error. The same error can also be reproduced with Node, as described here.

[derekchiang]

I see two ways to resolve this issue:

1. Use something like ./.rust/tmp as the temporary directory by default.
2. When seeing this IO error, degrade to copying and deleting the source folder, instead of renaming.

[huonw]

Dupe of #9887.

(BTW, it's good practice to include the exact compiler/tool output to assist with searching, diagnosis and, later, checking if a bug still happens.)