lib.modules: add mkDerivedConfig

mkDerivedConfig : Option a -> (a -> Definition b) -> Definition b

Create config definitions with the same priority as the definition of another option.
This should be used for option definitions where one option sets the value of another as a convenience.
For instance a config file could be set with a `text` or `source` option, where text translates to a `source`
value using `mkDerivedConfig options.text (pkgs.writeText "filename.conf")`.

It takes care of setting the right priority using `mkOverride`.

lib/default.nix

@@ -119,7 +119,7 @@ let
       mkFixStrictness mkOrder mkBefore mkAfter mkAliasDefinitions
       mkAliasAndWrapDefinitions fixMergeModules mkRemovedOptionModule
       mkRenamedOptionModule mkMergedOptionModule mkChangedOptionModule
-      mkAliasOptionModule doRename;
+      mkAliasOptionModule mkDerivedConfig doRename;
     inherit (self.options) isOption mkEnableOption mkSinkUndeclaredOptions
       mergeDefaultOption mergeOneOption mergeEqualOption getValues
       getFiles optionAttrSetToDocList optionAttrSetToDocList'

lib/modules.nix

@@ -956,6 +956,26 @@ rec {
     use = id;
   };
 
+  /* mkDerivedConfig : Option a -> (a -> Definition b) -> Definition b
+
+    Create config definitions with the same priority as the definition of another option.
+    This should be used for option definitions where one option sets the value of another as a convenience.
+    For instance a config file could be set with a `text` or `source` option, where text translates to a `source`
+    value using `mkDerivedConfig options.text (pkgs.writeText "filename.conf")`.
+
+    It takes care of setting the right priority using `mkOverride`.
+  */
+  # TODO: make the module system error message include information about `opt` in
+  # error messages about conflicts. E.g. introduce a variation of `mkOverride` which
+  # adds extra location context to the definition object. This will allow context to be added
+  # to all messages that report option locations "this value was derived from <full option name>
+  # which was defined in <locations>". It can provide a trace of options that contributed
+  # to definitions.
+  mkDerivedConfig = opt: f:
+    mkOverride
+      (opt.highestPrio or defaultPriority)
+      (f opt.value);
+
   doRename = { from, to, visible, warn, use, withPriority ? true }:
     { config, options, ... }:
     let

nixos/modules/system/etc/etc.nix

@@ -172,9 +172,8 @@ in
             target = mkDefault name;
             source = mkIf (config.text != null) (
               let name' = "etc-" + baseNameOf name;
-              in mkOverride
-                (options.text.highestPrio or lib.modules.defaultPriority)
-                (pkgs.writeText name' config.text));
+              in mkDerivedConfig options.text (pkgs.writeText name')
+            );
           };
 
         }));