As [http://lists.fedoraproject.org/pipermail/packaging/2012-June/008489.html discussed in the packaging list]:

I've just [https://bugzilla.redhat.com/show_bug.cgi?id=827289 submitted a review request] for [http://nixos.org/nix/ Nix], a purely
functional package manager.

Nix uses the /nix directory by default for essential operations -- its
package store, for instance, is located at /nix/store by default, and
it's local state in /nix/var.

These can be overridden at compile time, however, doing so makes it
[http://hydra.nixos.org/build/2657151/download/1/manual/#id523917 impossible to use pre-built binaries] provided in Nix channels
(repositories).

As such, it seems that this is a justifiable case for creating a new
directory under root -- cf. the introduction of /run ([http://docs.fedoraproject.org/en-US/Fedora/15/html/Release_Notes/sect-Release_Notes-Changes_for_SysAdmin.html#id2993828 documented
in Fedora 15's release notes], but Toshio informed me the explanation was incorrect):

This change is compliant with the Filesystem Hierarchy Standard,
which allows distributions to create new directories in the root
hierarchy as long as there is careful consideration of the
consequences.

I posit that compatibility with a vast amount of pre-built binaries,
and the reduced usefulness of the tool without this compatibility
(anyone who has used MacPorts, with its lack of pre-built binaries,
would sympathize), merit having the directory added.

Another concern is that this is an additional package manager, and that tends to be frowned upon. However, in Nix's favor, its packages are installed into the Nix store (which is by default in an entirely new directory, the whole reason for this request), and most packages can be used by users and get symlinked into the user's home directory into $HOME/.nix-profile -- so there is no danger of having different package managers collide over file ownerships. The situation is probably more similar to the alternate package managers we do ship, like python-pip and zeroinstall-injector