Nix 2.8 (2022-04-19) - Lix Reference Manual

# 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).