Build fails with execvpe feature enabled

[alex-gulyas]

Build errors:

• missing ffi module in unistd
• use of undeclared type Error::Sys at src/unistd.rs:396

[felippemr]

Hi!
I'd like to help you with this issue, how can I get started?

[fiveop]

This is already being tackled by #349 at the moment. Though, it is not clear whether that change will get into the tree in its current form. I'd still suggest starting with another bug. In particular the two other issues marked as good-first-bug provide lots of work.

[kamalmarhubi]

Removed the labels for now.

[Susurrus]

This is still a problem with the current release:

   Compiling nix v0.9.0-pre (file:///home/susurrus/Projects/nix)
error[E0433]: failed to resolve. Could not find `ffi` in `super`
    --> src/unistd.rs:1626:13
     |
1626 |             super::ffi::execvpe(filename.as_ptr(), args_p.as_ptr(), env_p.as_ptr())
     |             ^^^^^^^^^^^^^^^^^^^ Could not find `ffi` in `super`

error[E0433]: failed to resolve. Use of undeclared type or module `Error`
    --> src/unistd.rs:1629:13
     |
1629 |         Err(Error::Sys(Errno::last()))
     |             ^^^^^^^^^^ Use of undeclared type or module `Error`

error: aborting due to 2 previous errors

error: Could not compile `nix`.

To learn more, run the command again with --verbose.

I just tested all of the other features, and they all work fine at least.