add NixOS support to mach and automatically re-launch mach in nix-shell

mach

@@ -106,4 +106,14 @@ if __name__ == '__main__':
         orig_command_line = forking.get_command_line
         forking.get_command_line = my_get_command_line
 
-    main(sys.argv)
+    if os.path.exists('/etc/NIXOS') and not 'IN_NIX_SHELL' in os.environ: # we're on a nixOS system, need to run mach in nix-shell
+        import subprocess
+        from shlex import quote
+        mach_dir = os.path.abspath(os.path.dirname(__file__))
+        try:
+            # sys argv already contains the ./mach part, so we just need to pass it as-is
+            subprocess.Popen(['nix-shell', mach_dir + '/etc/shell.nix', '--run', ' '.join(map(quote, sys.argv))]).wait()
+        except KeyboardInterrupt:
+            sys.exit(0)
+    else:
+        main(sys.argv)

python/servo/bootstrap.py

@@ -383,6 +383,7 @@ def get_linux_distribution():
         'debian gnu/linux',
         'fedora',
         'void',
+        'nixos',
     ]:
         raise Exception('mach bootstrap does not support %s, please file a bug' % distrib)
 
@@ -398,6 +399,11 @@ def bootstrap(context, force=False, specific=None):
     elif "linux-gnu" in host_triple():
         distrib, version = get_linux_distribution()
 
+        if distrib.lower() == 'nixos':
+            print('NixOS does not need bootstrap')
+            print('just run ./mach build')
+            return
+
         context.distro = distrib
         context.distro_version = version
         bootstrapper = LINUX_SPECIFIC_BOOTSTRAPPERS.get(specific, linux)