lib/modules: Init lib.mkDefinition

lib/default.nix

@@ -446,6 +446,7 @@ let
         fixupOptionType
         mkIf
         mkAssert
+        mkDefinition
         mkMerge
         mkOverride
         mkOptionDefault

lib/modules.nix

@@ -1097,10 +1097,16 @@ let
         # Process mkMerge and mkIf properties.
         defs' = concatMap (
           m:
-          map (value: {
-            inherit (m) file;
-            inherit value;
-          }) (addErrorContext "while evaluating definitions from `${m.file}':" (dischargeProperties m.value))
+          map (
+            value:
+            if value._type or null == "definition" then
+              value
+            else
+              {
+                inherit (m) file;
+                inherit value;
+              }
+          ) (addErrorContext "while evaluating definitions from `${m.file}':" (dischargeProperties m.value))
         ) defs;
 
         # Process mkOverride properties.
@@ -1365,6 +1371,11 @@ let
     inherit contents;
   };
 
+  /**
+    Return a definition with file location information.
+  */
+  mkDefinition = args@{ file, value, ... }: args // { _type = "definition"; };
+
   mkOverride = priority: content: {
     _type = "override";
     inherit priority content;
@@ -2095,6 +2106,7 @@ private
     mkBefore
     mkChangedOptionModule
     mkDefault
+    mkDefinition
     mkDerivedConfig
     mkFixStrictness
     mkForce

lib/tests/modules.sh

@@ -673,6 +673,14 @@ checkConfigError 'The option .conflictingPathOptionType. in .*/pathWith.nix. is
 # types.pathWith { inStore = true; absolute = false; }
 checkConfigError 'In pathWith, inStore means the path must be absolute' config.impossiblePathOptionType ./pathWith.nix
 
+# mkDefinition
+# check that mkDefinition 'file' is printed in the error message
+checkConfigError 'Cannot merge definitions.*\n\s*- In .file.*\n\s*- In .other.*' config.conflict ./mkDefinition.nix
+checkConfigError 'A definition for option .viaOptionDefault. is not of type .boolean.*' config.viaOptionDefault ./mkDefinition.nix
+checkConfigOutput '^true$' config.viaConfig ./mkDefinition.nix
+checkConfigOutput '^true$' config.mkMerge ./mkDefinition.nix
+checkConfigOutput '^true$' config.mkForce ./mkDefinition.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass

lib/tests/modules/mkDefinition.nix

@@ -0,0 +1,71 @@
+{ lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    mkDefinition
+    mkOptionDefault
+    ;
+in
+{
+  imports = [
+    {
+      _file = "file";
+      options.conflict = mkOption {
+        default = 1;
+      };
+      config.conflict = mkDefinition {
+        file = "other";
+        value = mkOptionDefault 42;
+      };
+    }
+    {
+      # Check that mkDefinition works within 'config'
+      options.viaConfig = mkOption { };
+      config.viaConfig = mkDefinition {
+        file = "other";
+        value = true;
+      };
+    }
+    {
+      # Check mkMerge can wrap mkDefinitions
+      # Not the other way around
+      options.mkMerge = mkOption {
+        type = lib.types.bool;
+      };
+      config.mkMerge = lib.mkMerge [
+        (mkDefinition {
+          file = "a.nix";
+          value = true;
+        })
+        (mkDefinition {
+          file = "b.nix";
+          value = true;
+        })
+      ];
+    }
+    {
+      # Check mkDefinition can use mkForce on the value
+      # Not the other way around
+      options.mkForce = mkOption {
+        type = lib.types.bool;
+        default = false;
+      };
+      config.mkForce = mkDefinition {
+        file = "other";
+        value = lib.mkForce true;
+      };
+    }
+    {
+      # Currently expects an error
+      # mkDefinition doesn't work on option default
+      # This is a limitation and might be resolved in the future
+      options.viaOptionDefault = mkOption {
+        type = lib.types.bool;
+        default = mkDefinition {
+          file = "other";
+          value = true;
+        };
+      };
+    }
+  ];
+}

nixos/doc/manual/development/option-def.section.md

@@ -123,3 +123,65 @@ they were declared in separate modules. This can be done using
     ];
 }
 ```
+
+## Free-floating definitions {#sec-option-definitions-definitions}
+
+:::{.note}
+The module system internally transforms module syntax into definitions. This always happens internally.
+:::
+
+It is possible to create first class definitions which are not transformed _again_ into definitions by the module system.
+
+Usually the file location of a definition is implicit and equal to the file it came from.
+However, when manipulating definitions, it may be useful for them to be completely self-contained (or "free-floating").
+
+A free-floating definition is created with `mkDefinition { file = ...; value = ...; }`.
+
+Preserving the file location creates better error messages, for example when copying definitions from one option to another.
+
+Other properties like `mkOverride` `mkMerge` `mkAfter` can be used in the `value` attribute but not on the entire definition.
+
+This is what would work
+
+```nix
+mkDefinition {
+   value = mkForce 42;
+   file = "somefile.nix";
+}
+```
+
+While this would NOT work.
+
+```nix
+mkForce (mkDefinition {
+   value = 42;
+   file = "somefile.nix";
+})
+```
+
+The following shows an example configuration that yields an error with the custom position information:
+
+```nix
+{
+  _file = "file.nix";
+  options.foo = mkOption {
+    default = 13;
+  };
+  config.foo = lib.mkDefinition {
+    file = "custom place";
+    # mkOptionDefault creates a conflict with the option foo's `default = 1` on purpose
+    # So we see the error message below contains the conflicting values and different positions
+    value = lib.mkOptionDefault 42;
+  };
+}
+```
+
+evaluating the module yields the following error:
+
+```
+error: Cannot merge definitions of `foo'. Definition values:
+- In `file.nix': 13
+- In `custom place': 42
+```
+
+To set the file location for all definitions in a module, you may add the `_file` module syntax attribute, which has a similar effect to using `mkDefinition` on all definitions in the module, without the hassle.

nixos/doc/manual/redirects.json

@@ -1664,6 +1664,9 @@
   "sec-option-definitions-merging": [
     "index.html#sec-option-definitions-merging"
   ],
+  "sec-option-definitions-definitions": [
+    "index.html#sec-option-definitions-definitions"
+  ],
   "sec-assertions": [
     "index.html#sec-assertions"
   ],