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Release 2.8 (2022-04-19)

  • New experimental command: nix fmt, which applies a formatter defined by the formatter.<system> flake output to the Nix expressions in a flake.

  • Various Nix commands can now read expressions from standard input using --file -.

  • New experimental builtin function builtins.fetchClosure that copies a closure from a binary cache at evaluation time and rewrites it to content-addressed form (if it isn't already). Like builtins.storePath, this allows importing pre-built store paths; the difference is that it doesn't require the user to configure binary caches and trusted public keys.

This function is only available if you enable the experimental feature fetch-closure.

  • New experimental feature: impure derivations. These are derivations that can produce a different result every time they're built. Here is an example:
stdenv.mkDerivation {
  name = "impure";
  __impure = true; ## marks this derivation as impure
  buildCommand = "date > $out";
}

Running nix build twice on this expression will build the derivation twice, producing two different content-addressed store paths. Like fixed-output derivations, impure derivations have access to the network. Only fixed-output derivations and impure derivations can depend on an impure derivation.

  • nix store make-content-addressable has been renamed to nix store make-content-addressed.

  • The nixosModule flake output attribute has been renamed consistent with the .default renames in Nix 2.7.

  • nixosModule → nixosModules.default

As before, the old output will continue to work, but nix flake check will issue a warning about it.

  • nix run is now stricter in what it accepts: members of the apps flake output are now required to be apps (as defined in the manual), and members of packages or legacyPackages must be derivations (not apps).