Support literate config / doom +org tangle

Adds `tangleArgs`. When set, runs `doom +org tangle` in a symlinked copy
of DOOMDIR, using that instead of the original for everything else.

I expect this to be usable as an alternative to `doom :config literate`,
which (because it does its work from `doom-before-sync-hook`) has no
effect.

We currently (somewhat hackishly) run `doom` with just an `init.el`
enabling `:lang org` (the bare minimum for Doom to provide the `+org
tangle` command), not a full profile. This means the user can tangle
their init.el, but also that Doom uses org-mode from upstream emacs, not
its pinned version. If this turns out to be a problem, we should be able
to build an actual Doom profile and `doom` wrapper first.

README.md

@@ -184,6 +184,20 @@ support use without flakes.
   efficient but considered experimental in Nix (subject to changes which might
   break fetches).
 
+- `tangleArgs`: When set, Unstraightened runs `doom +org tangle` in `doomDir`.
+  See `doom +org tangle --help` for the arguments you can use here, and see the
+  [Org-mode manual](https://orgmode.org/manual/Extracting-Source-Code.html) for
+  more information.
+
+> [!NOTE]
+> The `:config literate` module has no effect when using Unstraightened.
+> Use this instead.
+
+> [!NOTE]
+> Currently, this feature uses the version of Org-mode that comes with Emacs
+> when tangling, not the version installed by Doom. This limitation could be
+> lifted: file an issue if this limitation is a problem for you.
+
 There are a few other settings but they are not typically useful. See the
 source.
 

default.nix

@@ -33,6 +33,8 @@
     fd
     ripgrep
   ],
+  # Args to pass to `doom +org tangle`.
+  tangleArgs ? null,
 
   callPackage,
   callPackages,
@@ -47,13 +49,45 @@
   makeBinaryWrapper,
   stdenv,
   stdenvNoCC,
+  toInit,
+  writeTextDir,
 }:
 let
   inherit (lib) optionalAttrs optionalString;
   inherit (import ./fetch-overrides.nix) extraPins extraUrls;
 
   nonEmptyProfileName = if profileName != "" then profileName else "nix";
 
+  tangleDoomDir = writeTextDir "init.el" (toInit {
+    lang.org = true;
+  });
+
+  # Preprocess DOOMDIR with `doom +org tangle` if requested.
+  doomDir' =
+    if tangleArgs != null then
+      runCommandLocal "tangled-doomdir"
+        {
+          inherit
+            tangleArgs
+            doomDir
+            doomSource
+            runtimeShell
+            ;
+          EMACS = lib.getExe emacs;
+          DOOMDIR = tangleDoomDir;
+          # Enable this to troubleshoot failures at this step.
+          #DEBUG = "1";
+        }
+        ''
+          mkdir $out doomlocaldir
+          export DOOMLOCALDIR=$PWD/doomlocaldir
+          cd $out
+          ln -s $doomDir/* ./
+          $runtimeShell $doomSource/bin/doom +org tangle $tangleArgs
+        ''
+    else
+      doomDir;
+
   # Step 1: determine which Emacs packages to pull in.
   #
   # Inputs: Doom, original DOOMDIR (only init.el and packages.el are used).
@@ -69,7 +103,7 @@ let
     name = "doom-intermediates";
     inherit doomSource emacs;
     extraArgs = {
-      DOOMDIR = "${doomDir}";
+      DOOMDIR = "${doomDir'}";
     };
     script = ./build-helpers/dump;
     scriptArgs = "-o $out";
@@ -358,11 +392,11 @@ let
     buildCommandPath = ./build-helpers/build-doom-profile.sh;
 
     inherit
-      doomDir
       doomIntermediates
       doomSource
       runtimeShell
       ;
+    doomDir = doomDir';
     profileName = nonEmptyProfileName;
     noProfileHack = profileName == "";
     buildProfileLoader = ./build-helpers/build-profile-loader;

doomdir/config.org

@@ -0,0 +1,5 @@
+#+title: Config
+
+#+begin_src emacs-lisp :tangle init.el
+(doom! :lang org)
+#+end_src

flake.nix

@@ -58,7 +58,7 @@
           # directly instead of having pkgs.callPackage do it.
           inherit (emacs-overlay.overlays.package { } pkgs) emacsPackagesFor;
           mergedArgs = args // {
-            inherit emacsPackagesFor;
+            inherit emacsPackagesFor toInit;
             doomSource = doomemacs;
           };
         in
@@ -126,6 +126,12 @@
             doomLocalDir = "~/.local/share/nix-doom-unstraightened";
             profileName = "";
           }).doomEmacs;
+        doom-emacs-tangle =
+          (doomFromPackages pkgs {
+            doomDir = ./doomdir;
+            doomLocalDir = "~/.local/share/nix-doom-unstraightened";
+            tangleArgs = ".";
+          }).doomEmacs;
         cachix-packages =
           let
             doomDirs = pkgs.callPackages ./build-helpers/doomdirs.nix {

home-manager.nix

@@ -129,6 +129,16 @@ in
         description = "Extra packages to add to Doom's $PATH.";
       };
 
+      tangleArgs = mkOption {
+        default = null;
+        example = ".";
+        type = types.orNull types.str;
+        defaultText = literalExpression null;
+        description = ''
+          When set, run `doom +org tangle $tangleArgs` in `doomDir`.
+        '';
+      };
+
       # Home Manager-specific options.
       provideEmacs = mkOption {
         type = types.bool;
@@ -172,6 +182,7 @@ in
             experimentalFetchTree
             extraPackages
             extraBinPackages
+            tangleArgs
             ;
         };
       in