Release 2.2 (2019-01-11)

This is primarily a bug fix release. It also has the following changes:

In derivations that use structured attributes (i.e. that specify set the __structuredAttrs attribute to true to cause all attributes to be passed to the builder in JSON format), you can now specify closure checks per output, e.g.:

outputChecks."out" = {
  # The closure of 'out' must not be larger than 256 MiB.
  maxClosureSize = 256 * 1024 * 1024;

  # It must not refer to C compiler or to the 'dev' output.
  disallowedRequisites = [ stdenv.cc "dev" ];
};

outputChecks."dev" = {
  # The 'dev' output must not be larger than 128 KiB.
  maxSize = 128 * 1024;
};

The derivation attribute requiredSystemFeatures is now enforced for local builds, and not just to route builds to remote builders. The supported features of a machine can be specified through the configuration setting system-features.

By default, system-features includes kvm if /dev/kvm exists. For compatibility, it also includes the pseudo-features nixos-test, benchmark and big-parallel which are used by Nixpkgs to route builds to particular Hydra build machines.
Sandbox builds are now enabled by default on Linux.
The new command nix doctor shows potential issues with your Nix installation.
The fetchGit builtin function now uses a caching scheme that puts different remote repositories in distinct local repositories, rather than a single shared repository. This may require more disk space but is faster.
The dirOf builtin function now works on relative paths.
Nix now supports SRI hashes, allowing the hash algorithm and hash to be specified in a single string. For example, you can write:
```
import <nix/fetchurl.nix> {
  url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
  hash = "sha256-XSLa0FjVyADWWhFfkZ2iKTjFDda6mMXjoYMXLRSYQKQ=";
};
```
instead of
```
import <nix/fetchurl.nix> {
  url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
  sha256 = "5d22dad058d5c800d65a115f919da22938c50dd6ba98c5e3a183172d149840a4";
};
```
In fixed-output derivations, the outputHashAlgo attribute is no longer mandatory if outputHash specifies the hash.

nix hash-file and nix hash-path now print hashes in SRI format by default. They also use SHA-256 by default instead of SHA-512 because that's what we use most of the time in Nixpkgs.
Integers are now 64 bits on all platforms.
The evaluator now prints profiling statistics (enabled via the NIX_SHOW_STATS and NIX_COUNT_CALLS environment variables) in JSON format.
The option --xml in nix-store --query has been removed. Instead, there now is an option --graphml to output the dependency graph in GraphML format.
All nix-* commands are now symlinks to nix. This saves a bit of disk space.
nix repl now uses libeditline or libreadline.

Nix Reference Manual

Release 2.2 (2019-01-11)