Deprecation warnings should not be used in Nixpkgs because they spam
innocent "nix-env -qa" users with (in this case) dozens of messages
that they can't do anything about.
This also reverts commit 2ca883338389b7ab995924a0cab0211993bdf1da.
Allows one or more directories to be mounted as a read-only file system.
This makes it convenient to run volatile containers that do not retain
application state.
This gives a nicer error message than (say)
while evaluating the option `fileSystems':
while evaluating the attribute ‘isDefined’ at /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:323:5:
while evaluating ‘filterOverrides’ at /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:395:21, called from /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:307:18:
while evaluating ‘concatMap’ at /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/lists.nix:79:18, called from /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:401:8:
while evaluating ‘concatMap’ at /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/lists.nix:79:18, called from /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:302:17:
while evaluating anonymous function at /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:302:28, called from undefined position:
while evaluating ‘dischargeProperties’ at /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:365:25, called from /nix/store/r8z4vvl2qzg31zm4ra6awlx5b22k7gf9-nixos-16.09.tar.gz/lib/modules.nix:303:62:
value is a list while a Boolean was expected
They both reference the toJSON builtin, so we get semantic identifiers
that express the intent of the generation.
Both should be able to map each nix value (minus functions) to the
destination config files.
Includes two invocation unit tests.
