bundix --magic

Fetching rake 12.3.1
Installing rake 12.3.1
Fetching concurrent-ruby 1.0.5
Installing concurrent-ruby 1.0.5
Fetching i18n 0.9.5
Installing i18n 0.9.5
Fetching json 1.8.6
Installing json 1.8.6 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/loaner/code/k2_web/vendor/bundle/ruby/2.3.0/gems/json-1.8.6/ext/json/ext/generator
/nix/store/5svvwcf445rfb2irn0n64lwi89wa1q41-ruby-2.3.8/bin/ruby -r ./siteconf20190103-85567-1yhcidy.rb extconf.rb
creating Makefile

current directory: /Users/loaner/code/k2_web/vendor/bundle/ruby/2.3.0/gems/json-1.8.6/ext/json/ext/generator
make "DESTDIR=" clean

current directory: /Users/loaner/code/k2_web/vendor/bundle/ruby/2.3.0/gems/json-1.8.6/ext/json/ext/generator
make "DESTDIR="
compiling generator.c
In file included from generator.c:1:
In file included from ./../fbuffer/fbuffer.h:5:
In file included from /nix/store/5svvwcf445rfb2irn0n64lwi89wa1q41-ruby-2.3.8/include/ruby-2.3.0/ruby.h:33:
In file included from /nix/store/5svvwcf445rfb2irn0n64lwi89wa1q41-ruby-2.3.8/include/ruby-2.3.0/ruby/ruby.h:36:
/nix/store/5svvwcf445rfb2irn0n64lwi89wa1q41-ruby-2.3.8/include/ruby-2.3.0/ruby/defines.h:26:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
         ^~~~~~~~~
1 error generated.
make: *** [Makefile:239: generator.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/loaner/code/k2_web/vendor/bundle/ruby/2.3.0/gems/json-1.8.6 for inspection.
Results logged to /Users/loaner/code/k2_web/vendor/bundle/ruby/2.3.0/extensions/x86_64-darwin-17/2.3.0/json-1.8.6/gem_make.out

shell.nix

# This allows overriding nixpkgs by passing `--arg pkgs ...`
{ pkgs ? import ./devtools/lib/nix/nixpkgs.nix {}
, lib ? pkgs.lib
}:

with pkgs;
with lib;

let

  ruby = ruby_2_3;


in gccStdenv.mkDerivation {
  name = "development-environment";
  buildInputs = [
    bundix

    ruby

    (bundlerEnv {
      inherit ruby;

      gemdir = ./.;

      groups = [ "default" "production" "development" "test" ];
      name = "dev-env";
    })

    # Gem dependencies
    libxml2        # dependency of the nokogiri Gem
    postgresql     # dependency of the pg Gem
    sqlite         # dependency of the sqlite3 Gem

    openssl
  ] ++ optional stdenv.isDarwin [
    docker-machine
  ] ++ optional stdenv.isLinux [
    # TODO: make wkhtmltopdf compatible with Mac!
    wkhtmltopdf
  ];

  phases = ["nobuildPhase"];

  nobuildPhase = ''
    echo
    echo "This derivation is not meant to be built, aborting";
    echo
    exit 1
  '';
}