nix-super/doc/manual/source/release-notes/rl-2.8.md

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).