# Release X.Y (202?-??-??)

* `<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 that context that appear in the content of the file are retained.
  This avoids a lot of spurious errors where some benign strings end-up having
  a context just because they are read from a store path
  ([#7260](https://github.com/NixOS/nix/pull/7260)).

* Nix can now automatically pick UIDs for builds, removing the need to
  create `nixbld*` user accounts. These UIDs are allocated starting at
  872415232 (0x34000000) on Linux and 56930 on macOS.

  This is an experimental feature. To enable it, add the following to
  `nix.conf`:

  ```
  extra-experimental-features = auto-allocate-uids
  auto-allocate-uids = true
  ```

* On Linux, Nix can now run builds in a user namespace where the build
  runs as root (UID 0) and has 65,536 UIDs available. This is
  primarily useful for running containers such as `systemd-nspawn`
  inside a Nix build. For an example, see
  https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.

  A build can enable this by requiring the `uid-range` system feature,
  i.e. by setting the derivation attribute

  ```
  requiredSystemFeatures = [ "uid-range" ];
  ```

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

* On Linux, Nix has experimental support for running builds inside a
  cgroup. It can be enabled by adding

  ```
  extra-experimental-features = cgroups
  use-cgroups = true
  ```

  to `nix.conf`. Cgroups are required for derivations that require the
  `uid-range` system feature.

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