bug#29679: 26.0.90; :after-hook not compiled

[Stefan Monnier]

Package: Emacs
Version: 26.0.90

I just realized that the :after-hook option of define-derived-mode (new
in Emacs-26) is expanded in a way that introduces some problems.
More specifically a

   :after-hook (foo bar)

gets turned into something like

   (push '(foo bar) 'delayed-after-hook-forms)

which means that the code (foo bar) is not byte-compiled, hence is not
macro-expanded, and will be evaluated later via `eval` without paying
attention to the setting of `lexical-binding`.

It's not a very serious problem in practice, but if we keep the code as
we have it, it means that code byte-compiled with Emacs-26 will use the
above

   (push '(foo bar) 'delayed-after-hook-forms)

in its .elc and we'll have to preserve backward compatibility with it.
So it'd be better to fix it before Emacs-26 is released.
Here's a minimal patch for it, which I hope is safe enough for emacs-26.

OK to push to emacs-26?


        Stefan


diff --git a/lisp/emacs-lisp/derived.el b/lisp/emacs-lisp/derived.el
index 751291afa8..c0ef199424 100644
--- a/lisp/emacs-lisp/derived.el
+++ b/lisp/emacs-lisp/derived.el
@@ -285,7 +285,7 @@ define-derived-mode
          (run-mode-hooks ',hook)
          ,@(when after-hook
              `((if delay-mode-hooks
-                   (push ',after-hook delayed-after-hook-forms)
+                   (push (lambda () ,after-hook) delayed-after-hook-functions)
                  ,after-hook)))))))
 
 ;; PUBLIC: find the ultimate class of a derived mode.
diff --git a/lisp/subr.el b/lisp/subr.el
index 6db3b614d6..64521711b7 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -1844,10 +1844,10 @@ delayed-mode-hooks
 (make-variable-buffer-local 'delayed-mode-hooks)
 (put 'delay-mode-hooks 'permanent-local t)
 
-(defvar delayed-after-hook-forms nil
+(defvar delayed-after-hook-functions nil
   "List of delayed :after-hook forms waiting to be run.
 These forms come from `define-derived-mode'.")
-(make-variable-buffer-local 'delayed-after-hook-forms)
+(make-variable-buffer-local 'delayed-after-hook-functions)
 
 (defvar change-major-mode-after-body-hook nil
   "Normal hook run in major mode functions, before the mode hooks.")
@@ -1865,7 +1865,7 @@ run-mode-hooks
 Otherwise, runs hooks in the sequence: `change-major-mode-after-body-hook',
 `delayed-mode-hooks' (in reverse order), HOOKS, then runs
 `hack-local-variables', runs the hook `after-change-major-mode-hook', and
-finally evaluates the forms in `delayed-after-hook-forms' (see
+finally evaluates the functions in `delayed-after-hook-functions' (see
 `define-derived-mode').
 
 Major mode functions should use this instead of `run-hooks' when
@@ -1882,9 +1882,9 @@ run-mode-hooks
         (with-demoted-errors "File local-variables error: %s"
           (hack-local-variables 'no-mode)))
     (run-hooks 'after-change-major-mode-hook)
-    (dolist (form (nreverse delayed-after-hook-forms))
-      (eval form))
-    (setq delayed-after-hook-forms nil)))
+    (dolist (fun (nreverse delayed-after-hook-functions))
+      (funcall fun))
+    (setq delayed-after-hook-functions nil)))
 
 (defmacro delay-mode-hooks (&rest body)
   "Execute BODY, but delay any `run-mode-hooks'.