Simplify setting straight-base-dir

Instead of passing the path down from the profile loader to init.el,
just substitute the path into init.el.

This requires moving DOOMDIR generation into the same derivation as
profile build to avoid a circular dependency.

doom.nix

@@ -46,7 +46,6 @@
   lib,
   runCommand,
   runtimeShell,
-  substitute,
   writeText,
   makeBinaryWrapper,
 }:
@@ -216,48 +215,44 @@ let
   # the set from step 2.
   emacsWithPackages = doomEmacsPackages.emacsWithPackages (epkgs: (map (p: epkgs.${p}) (builtins.attrNames doomPackageSet)));
 
-  # Step 4: build a final DOOMDIR with packages.el from step 1.
-  finalInitFile = substitute {
-    src = ./init.el;
-    replacements = ["--replace" "@user-init@" "${doomDir}/init.el" ];
-  };
-  finalDoomDir = runCommand "doom-dir" {} ''
-    mkdir $out
-    ln -s ${doomDir}/* $out/
-    # yasnippet logs an error at startup if snippets/ does not exist.
-    if ! [[ -e $out/snippets ]]; then
-      mkdir $out/snippets
-    fi
-    ln -sf ${finalInitFile} $out/init.el
-    ln -sf ${doomIntermediates}/packages.el $out/
-  '';
-
-  # Step 5: build a Doom profile and profile loader using Emacs from step 3 and
-  # DOOMDIR from step 4.
+  # Step 4: build a DOOMDIR, Doom profile and profile loader using Emacs from
+  # step 3 and packages.el from step 1.
   #
-  # Create both in the same derivation: we want the path to the generated
-  # profile in the loader (so building the loader depends on the profile), but
-  # I'm currently using the loader to set up Doom to write the profile to the
-  # right place (so building the profile depends on the loader).
+  # Do this all in one derivation because these refer back to each other:
+  # - init.el in DOOMDIR refers to the straight.el build cache generated along
+  #   with the profile
+  # - The path to the generated profile is included in the loader
+  # - Generating the profile depends on the loader
 
   # XXX runCommandLocal? (See doomIntermediates.)
   doomProfile = runCommand "doom-profile"
     {
       env = {
         EMACS = lib.getExe emacsWithPackages;
-        DOOMDIR = finalDoomDir;
         # Enable this to troubleshoot failures at this step.
         #DEBUG = "1";
       };
       # Required to avoid Doom erroring out at startup.
       nativeBuildInputs = [ git ];
     } ''
-    mkdir $out $out/loader $out/profile $out/straight
+    mkdir $out $out/loader $out/doomdir $out/profile $out/straight
+    ln -s ${doomDir}/* $out/doomdir/
+    # yasnippet logs an error at startup if snippets/ does not exist.
+    if ! [[ -e $out/doomdir/snippets ]]; then
+      mkdir $out/doomdir/snippets
+    fi
+    rm $out/doomdir/init.el
+    substitute ${./init.el} $out/doomdir/init.el \
+      --replace @user-init@ "${doomDir}/init.el" \
+      --replace @straight-base-dir@ $out
+    ln -sf ${doomIntermediates}/packages.el $out/doomdir/
+    export DOOMDIR=$out/doomdir
+
     export DOOMPROFILELOADFILE=$out/loader/init.el
     # DOOMLOCALDIR must be writable, Doom creates some subdirectories.
     export DOOMLOCALDIR=$(mktemp -d)
     ${runtimeShell} ${doomSource}/bin/doomscript ${./build-helpers/build-profile-loader} \
-      -n "${profileName}" -b "$out" -d "${finalDoomDir}" ${optionalString noProfileHack "-u"}
+      -n "${profileName}" -b "$out" -d "$out/doomdir" ${optionalString noProfileHack "-u"}
 
     # With DOOMPROFILE set, doom-state-dir and friends are HOME-relative.
     export HOME=$(mktemp -d)