To be honest I'm not sure if we really want the configFile option.

I definitely see some advantages but I'm not sure how many users would actually prefer to manage their Sway configuration via Nix (as root) instead of in their $HOME (and most users probably use $HOME ATM). And AFAIK some community members are currently trying to avoid unnecessary options.

But another problem I see is the implementation. All relevant parameters (e.g. --version and --help) still work, which is good, but (at least in theory) a user looses the ability to change/override his configuration. But on the other hand this is only a problem if someone actually uses configFile and not by default.

man sway currently states the following:

CONFIGURATION
       sway searches for a config file in the following locations, in this order:

       1.   ~/.sway/config
       2.   $XDG_CONFIG_HOME/sway/config (suggested location)
       3.   ~/.i3/config
       4.   $XDG_CONFIG_HOME/i3/config
       5.   /etc/sway/config
       6.   /etc/i3/config

       If unset, $XDG_CONFIG_HOME defaults to ~/.config.

       An error is raised when no config file is found. The recommended default configuration is usually installed to /etc/sway/config; you are encouraged to copy this to ~/.config/sway/config and edit it
       from there.

IMO it would be better if users that want to manage their configuration via Nix would simply use something like environment.etc."sway/config".text. But that's just my personal opinion.

I'm not necessarily against merging this but I'd like to get some feedback from other community members (e.g. Infinisil or Mic92) first, if you still want this merged.

The first commit is fine :)
The other two commits should use the nixos/<module> format (see CONTRIBUTING.md#submitting-changes) and I'd prefer pointing to something like $HOME/.config/sway/config (I don't like having a lot of configuration file in $HOME) but technically this isn't 100 % correct either (maybe we could just refer to $HOME or man sway).

@primeos good point, thanks! Could you please take only first commit then?

Okay, I've pushed it in e5a937b. Thanks :)