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# Release 2.12 (2022-12-06)

- On Linux, Nix can now run builds in a user namespace where they run as root (UID 0) and have 65,536 UIDs available. This is primarily useful for running containers such as `systemd-nspawn` inside a Nix build. For an example, see `tests/systemd-nspawn/nix`.

  A build can enable this by setting the derivation attribute:

      requiredSystemFeatures = [ "uid-range" ];

  The `uid-range` system feature requires the `auto-allocate-uids` setting to be enabled.

- Nix can now automatically pick UIDs for builds, removing the need to create `nixbld*` user accounts. See `auto-allocate-uids`.

- On Linux, Nix has experimental support for running builds inside a cgroup. See `use-cgroups`.

- `<nix/fetchurl.nix>` now accepts an additional argument `impure` which defaults to `false`. If it is set to `true`, the `hash` and `sha256` arguments will be ignored and the resulting derivation will have `__impure` set to `true`, making it an impure derivation.

- If `builtins.readFile` is called on a file with context, then only the parts of the context that appear in the content of the file are retained. This avoids a lot of spurious errors where strings end up having a context just because they are read from a store path (#7260).

- `nix build --json` now prints some statistics about top-level derivations, such as CPU statistics when cgroups are enabled.