emacs: add currently compiling package dirs to load-path

Co-authored-by: Tad Fisher <tadfisher@gmail.com>

pkgs/build-support/emacs/generic.nix

@@ -1,6 +1,6 @@
 # generic builder for Emacs packages
 
-{ lib, stdenv, emacs, texinfo, ... }:
+{ lib, stdenv, emacs, texinfo, writeText, ... }:
 
 with lib;
 
@@ -49,7 +49,19 @@ stdenv.mkDerivation ({
   propagatedBuildInputs = packageRequires;
   propagatedUserEnvPkgs = packageRequires;
 
-  setupHook = ./setup-hook.sh;
+  setupHook = writeText "setup-hook.sh" ''
+    source ${./emacs-funcs.sh}
+
+    if [[ ! -v emacsHookDone ]]; then
+      emacsHookDone=1
+
+      # If this is for a wrapper derivation, emacs and the dependencies are all
+      # run-time dependencies. If this is for precompiling packages into bytecode,
+      # emacs is a compile-time dependency of the package.
+      addEnvHooks "$hostOffset" addEmacsVars
+      addEnvHooks "$targetOffset" addEmacsVars
+    fi
+  '';
 
   doCheck = false;
 
@@ -63,9 +75,16 @@ stdenv.mkDerivation ({
   addEmacsNativeLoadPath = true;
 
   postInstall = ''
+    # Besides adding the output directory to the native load path, make sure
+    # the current package's elisp files are in the load path, otherwise
+    # (require 'file-b) from file-a.el in the same package will fail.
+    mkdir -p $out/share/emacs/native-lisp
+    source ${./emacs-funcs.sh}
+    addEmacsVars "$out"
+
     find $out/share/emacs -type f -name '*.el' -print0 \
       | xargs -0 -n 1 -I {} -P $NIX_BUILD_CORES sh -c \
-          "emacs --batch --eval=\"(add-to-list 'comp-eln-load-path \\\"$out/share/emacs/native-lisp/\\\")\" -f batch-native-compile {} || true"
+          "emacs --batch -f batch-native-compile {} || true"
   '';
 }