nixos/screen: fix assertion to actually execute, by moving it outside of mkIf.

[tomfitzhenry]

Fixes #312194.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[tomfitzhenry]

I see this question came up during the PR review that introduced the assertion: https://github.com/NixOS/nixpkgs/pull/262133/files/f12eabdcbe642884b37cb5d20dbf781f998061f7#r1415492863

[tomfitzhenry]

#312187 (comment) has a better suggested alternative. I'll do that.

[SuperSandro2000]

#312187 (comment) has a better suggested alternative. I'll do that.

No, that is worse. Using throw will abort the evaluation completely and not collect all assertions.

[tomfitzhenry]

@SuperSandro2000 thoughts on this being merged before the 24.05 cut?

• Con: It's technically a breaking change: any users with programs.screen.screenrc set and programs.screen.enable unset will see a failure
• Pro: It's helpful in fixing broken config.
• Pro: The assertion's purpose is to help NixOS 24.05 users.

[tomfitzhenry]

@Mic92 @wegank , thoughts re #312210 (comment) ?

[SuperSandro2000]

The code was before broken and didn't do what we expected it to do and we silently ignored peoples configs unlike what is written in the release notes.

[Atemu]

I just got hit by this assertion, why does it need to exist?

Per convention, module settings are always guarded behind the enable option's value. We usually don't assert this.

[tomfitzhenry]

I just got hit by this assertion, why does it need to exist?

#262133 introduced this assertion, and has the rationale/commentary. This PR just fixed the assertion to actually work.