Merge #12796: nixos docs: show references to packages

(version 2) A better implementation of #10039, after #12357.
This time I did more thorough checking.

See commit messages for details.

nixos/modules/programs/ssh.nix

···
 
       askPassword = mkOption {
         type = types.str;
+        default = "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
         description = ''Program used by SSH to ask for passwords.'';
       };
 
···
       '';
 
     environment.variables.SSH_ASKPASS = optionalString config.services.xserver.enable askPassword;
-
-    programs.ssh.askPassword = mkDefault "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
 
   };
 }

nixos/modules/programs/venus.nix

···
       };
 
       outputTheme = mkOption {
+        default = "${pkgs.venus}/themes/classic_fancy";
         type = types.path;
         description = ''
           Directory containing a config.ini file which is merged with this one.
···
         serviceConfig.Group = "${cfg.group}";
         startAt = cfg.dates;
       };
-
-    services.venus.outputTheme = mkDefault "${pkgs.venus}/themes/classic_fancy";
 
   };
 }

nixos/modules/services/amqp/activemq/default.nix

···
         '';
       };
       configurationDir = mkOption {
+        default = "${activemq}/conf";
         description = ''
           The base directory for ActiveMQ's configuration.
           By default, this directory is searched for a file named activemq.xml,
···
           ${cfg.extraJavaOptions} ActiveMQBroker "${cfg.configurationURI}"
       '';
     };
-
-    services.activemq.configurationDir = mkDefault "${activemq}/conf";
 
   };
 

nixos/modules/services/misc/nixos-manual.nix

···
       nixpkgs.system = config.nixpkgs.system;
     };
 
-  eval = evalModules {
-    modules = [ versionModule ] ++ baseModules;
-    args = (config._module.args) // { modules = [ ]; };
-  };
-
+  /* For the purpose of generating docs, evaluate options with each derivation
+    in `pkgs` (recursively) replaced by a fake with path "\${pkgs.attribute.path}".
+    It isn't perfect, but it seems to cover a vast majority of use cases.
+    Caveat: even if the package is reached by a different means,
+    the path above will be shown and not e.g. `${config.services.foo.package}`. */
   manual = import ../../../doc/manual {
     inherit pkgs;
     version = config.system.nixosVersion;
     revision = config.system.nixosRevision;
-    options = eval.options;
+    options =
+      let
+        scrubbedEval = evalModules {
+          modules = [ versionModule ] ++ baseModules;
+          args = (config._module.args) // { modules = [ ]; };
+          specialArgs = { pkgs = scrubDerivations "pkgs" pkgs; };
+        };
+        scrubDerivations = namePrefix: pkgSet: mapAttrs
+          (name: value:
+            let wholeName = "${namePrefix}.${name}"; in
+            if isAttrs value then
+              scrubDerivations wholeName value
+              // (optionalAttrs (isDerivation value) { outPath = "\${${wholeName}}"; })
+            else value
+          )
+          pkgSet;
+      in scrubbedEval.options;
   };
 
   entry = "${manual.manual}/share/doc/nixos/index.html";
···
 
     services.nixosManual.browser = mkOption {
       type = types.path;
+      default = "${pkgs.w3m-nox}/bin/w3m";
       description = ''
         Browser used to show the manual.
       '';
···
 
     services.mingetty.helpLine = mkIf cfg.showManual
       "\nPress <Alt-F${toString cfg.ttyNumber}> for the NixOS manual.";
-
-    services.nixosManual.browser = mkDefault "${pkgs.w3m-nox}/bin/w3m";
 
   };
 

nixos/modules/services/misc/subsonic.nix

···
 
       transcoders = mkOption {
         type = types.listOf types.path;
+        default = [ "${pkgs.ffmpeg}/bin/ffmpeg" ];
         description = ''
           List of paths to transcoder executables that should be accessible
           from Subsonic. Symlinks will be created to each executable inside
···
     };
 
     users.extraGroups.subsonic.gid = config.ids.gids.subsonic;
-
-    services.subsonic.transcoders = mkDefault [ "${pkgs.ffmpeg}/bin/ffmpeg" ];
-
   };
 }

nixos/modules/services/monitoring/grafana.nix

···
 
     staticRootPath = mkOption {
       description = "Root path for static assets.";
+      default = "${cfg.package.out}/share/grafana/public";
       type = types.str;
     };
 
···
       home = cfg.dataDir;
       createHome = true;
     };
-
-    services.grafana.staticRootPath = mkDefault "${cfg.package}/share/grafana/public";
-
   };
 }

nixos/modules/services/networking/dnscrypt-proxy.nix

···
         default = "opendns";
         type = types.nullOr types.string;
         description = ''
-          The name of the upstream DNSCrypt resolver to use.
+          The name of the upstream DNSCrypt resolver to use. See
+          <literal>${resolverListFile}</literal> for alternative resolvers
+          (e.g., if you are concerned about logging and/or server
+          location).
         '';
       };
       customResolver = mkOption {

nixos/modules/services/web-servers/phpfpm.nix

···
 
       phpIni = mkOption {
         type = types.path;
-        description = "PHP configuration file to use.";
+        default = "${cfg.phpPackage}/etc/php-recommended.ini";
+        description = "php.ini file to use.";
       };
 
       poolConfigs = mkOption {
···
         PIDFile = pidFile;
       };
     };
-
-    services.phpfpm.phpIni = mkDefault "${cfg.phpPackage}/etc/php-recommended.ini";
 
   };
 }

nixos/modules/services/x11/display-managers/lightdm.nix

···
 
       background = mkOption {
         type = types.str;
+        default = "${pkgs.nixos-artwork}/share/artwork/gnome/Gnome_Dark.png";
         description = ''
           The background image or color to use.
         '';
···
     };
 
     users.extraGroups.lightdm.gid = config.ids.gids.lightdm;
-
-    services.xserver.displayManager.lightdm.background = mkDefault "${pkgs.nixos-artwork}/share/artwork/gnome/Gnome_Dark.png";
-
     services.xserver.tty     = null; # We might start multiple X servers so let the tty increment themselves..
     services.xserver.display = null; # We specify our own display (and logfile) in xserver-wrapper up there
   };