lib: Add beforeApply option attribute

See #299736 for the reasons.

lib/modules.nix

@@ -807,6 +807,9 @@ let
 
  in warnDeprecation opt //
  { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
+ # The merged value before applying the options `apply` function to it.
+ # In general though, `apply` should not be used, it's an anti-pattern.
+ beforeApply = res.mergedValue;
  inherit (res.defsFinal') highestPrio;
  definitions = map (def: def.value) res.defsFinal;
  files = map (def: def.file) res.defsFinal;

lib/tests/modules.sh

@@ -69,6 +69,8 @@ checkConfigOutput '^"one two"$' config.result ./shorthand-meta.nix
 
 checkConfigOutput '^true$' config.result ./test-mergeAttrDefinitionsWithPrio.nix
 
+checkConfigOutput '^true$' config.okChecks ./before-apply.nix
+
 # Check that a module argument is passed, also when a default is available
 # (but not needed)
 #

lib/tests/modules/before-apply.nix

@@ -0,0 +1,24 @@
+{ options, lib, ... }:
+
+# Tests wheter a apply function properly generates the `beforeApply` option attribute
+
+{
+ options = {
+ optionWithoutApply = lib.mkOption {
+ default = false;
+ type = lib.types.bool;
+ };
+
+ optionWithApply = lib.mkOption {
+ default = false;
+ type = lib.types.bool;
+ apply = x: !x;
+ };
+
+ okChecks = lib.mkOption {};
+ };
+ config.okChecks = builtins.addErrorContext "while evaluating the assertions" (
+ assert options.optionWithoutApply.beforeApply == options.optionWithoutApply.value;
+ assert options.optionWithApply.beforeApply == !options.optionWithApply.value;
+ true);
+}