modules: make apply mergeable

lib/modules.nix

@@ -744,6 +744,9 @@ let
  t' = opt.options.type;
  mergedType = t.typeMerge t'.functor;
  typesMergeable = mergedType != null;
+ applySet = if (bothHave "apply")
+ then { apply = value: opt.options.apply (res.apply value); }
+ else {};
  typeSet = if (bothHave "type") && typesMergeable
  then { type = mergedType; }
  else {};
@@ -752,7 +755,6 @@ let
  if bothHave "default" ||
  bothHave "example" ||
  bothHave "description" ||
- bothHave "apply" ||
  (bothHave "type" && (! typesMergeable))
  then
  throw "The option `${showOption loc}' in `${opt._file}' is already declared in ${showFiles res.declarations}."
@@ -772,7 +774,7 @@ let
  then [opt.pos]
  else [{ file = opt._file; line = null; column = null; }]);
  options = submodules;
- } // typeSet
+ } // applySet // typeSet
  ) { inherit loc; declarations = []; declarationPositions = []; options = []; } opts;
 
  /* Merge all the definitions of an option to produce the final

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -407,6 +407,8 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 
 - A stdenv's default set of hardening flags can now be set via its `bintools-wrapper`'s `defaultHardeningFlags` argument. A convenient stdenv adapter, `withDefaultHardeningFlags`, can be used to override an existing stdenv's `defaultHardeningFlags`.
 
+- The `apply` function passed to `mkOption` is now mergeable. This allows for accessing the previous value of the option, similar to the `prev`/`super` argument in `overlays`, `override` and `overrideAttrs`.
+
 - `libass` now uses the native CoreText backend on Darwin, which may fix subtitle rendering issues with `mpv`, `ffmpeg`, etc.
 
 - [Lilypond](https://lilypond.org/index.html) and [Denemo](https://www.denemo.org) are now compiled with Guile 3.0.