- The garbage collector has a number of new options to allow only some
    of the garbage to be deleted. The option `--max-freed N` tells the
    collector to stop after at least *N* bytes have been deleted. The
    option `--max-links
            N` tells it to stop after the link count on `/nix/store` has dropped
    below *N*. This is useful for very large Nix stores on filesystems
    with a 32000 subdirectories limit (like `ext3`). The option
    `--use-atime` causes store paths to be deleted in order of ascending
    last access time. This allows non-recently used stuff to be deleted.
    The option `--max-atime time` specifies an upper limit to the last
    accessed time of paths that may be deleted. For instance,
    
    ``` 
        $ nix-store --gc -v --max-atime $(date +%s -d "2 months ago")
    ```
    
    deletes everything that hasn’t been accessed in two months.

  - `nix-env` now uses optimistic profile locking when performing an
    operation like installing or upgrading, instead of setting an
    exclusive lock on the profile. This allows multiple `nix-env -i / -u
    / -e` operations on the same profile in parallel. If a `nix-env`
    operation sees at the end that the profile was changed in the
    meantime by another process, it will just restart. This is generally
    cheap because the build results are still in the Nix store.

  - The option `--dry-run` is now supported by `nix-store -r` and
    `nix-build`.

  - The information previously shown by `--dry-run` (i.e., which
    derivations will be built and which paths will be substituted) is
    now always shown by `nix-env`, `nix-store -r` and `nix-build`. The
    total download size of substitutable paths is now also shown. For
    instance, a build will show something like
    
        the following derivations will be built:
          /nix/store/129sbxnk5n466zg6r1qmq1xjv9zymyy7-activate-configuration.sh.drv
          /nix/store/7mzy971rdm8l566ch8hgxaf89x7lr7ik-upstart-jobs.drv
          ...
        the following paths will be downloaded/copied (30.02 MiB):
          /nix/store/4m8pvgy2dcjgppf5b4cj5l6wyshjhalj-samba-3.2.4
          /nix/store/7h1kwcj29ip8vk26rhmx6bfjraxp0g4l-libunwind-0.98.6
          ...

  - Language features:
    
      - @-patterns as in Haskell. For instance, in a function definition
        
            f = args @ {x, y, z}: ...;
        
        `args` refers to the argument as a whole, which is further
        pattern-matched against the attribute set pattern `{x, y, z}`.
    
      - “`...`” (ellipsis) patterns. An attribute set pattern can now
        say `...` at the end of the attribute name list to specify that
        the function takes *at least* the listed attributes, while
        ignoring additional attributes. For instance,
        
            {stdenv, fetchurl, fuse, ...}: ...
        
        defines a function that accepts any attribute set that includes
        at least the three listed attributes.
    
      - New primops: `builtins.parseDrvName` (split a package name
        string like `"nix-0.12pre12876"` into its name and version
        components, e.g. `"nix"` and `"0.12pre12876"`),
        `builtins.compareVersions` (compare two version strings using
        the same algorithm that `nix-env` uses), `builtins.length`
        (efficiently compute the length of a list), `builtins.mul`
        (integer multiplication), `builtins.div` (integer division).

  - `nix-prefetch-url` now supports `mirror://` URLs, provided that the
    environment variable `NIXPKGS_ALL` points at a Nixpkgs tree.

  - Removed the commands `nix-pack-closure` and `nix-unpack-closure`.
    You can do almost the same thing but much more efficiently by doing
    `nix-store --export
            $(nix-store -qR paths) > closure` and `nix-store --import <
            closure`.

  - Lots of bug fixes, including a big performance bug in the handling
    of `with`-expressions.