programs.screen's assertion doesn't run, possibly due to confusing semantics around mkIf?

[tomfitzhenry]

Steps To Reproduce

Steps to reproduce the behavior:

1. Build a machine with:

porgrams.screen.enable = false;
programs.screen.screenrc = "foobar";

Expected behavior

This should fail to build, due to this assertion:

nixpkgs/nixos/modules/programs/screen.nix

Lines 27 to 32 in 005028d

	assertions = [
	{
	assertion = cfg.screenrc != null -> cfg.enable;
	message = "`programs.screen.screenrc` has been configured, but `programs.screen.enable` is not true";
	}
	];

But the assertion doesn't assert.

As a means of diagnosis, if I remove "lib.mkIf cfg.enable" from line 33, the assertion asserts.

Very confusing!

Additional context

I noticed this oddity between lib.mkIf, //, and assertions, while addressing a PR review comment in an unrelated PR: #312187 (comment)

Notify maintainers

@vrthra

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[tomfitzhenry]

Oh, I think I see what's going on.

{ foo = 1; } // mkIf true { bar = 1; }; is mixing a plain attrset (LHS) with a NixOS option if attrset (RHS), which is an invalid operation.

This assertion just won't work.

There's presumably a way to do this in https://github.com/NixOS/nixpkgs/blob/master/lib/modules.nix that does the right thing.

[tomfitzhenry]

There's presumably a way to do this in https://github.com/NixOS/nixpkgs/blob/master/lib/modules.nix that does the right thing.

I found a way, but it's gross.

Disclaimer: I've never explored the innards of options/module merging, so my understanding/terminology is going to be way off.

The idea is to lift the plain attrset into the NixOS module value space (?) via lib.mkIf true, and then use lib.mkMerge as an alternative to //.

config = lib.mkMerge [
  (lib.mkIf true { assertions = foo; })
  (lib.mkIf cfg.enable { blah = 2; })
];

It works!