lisp/so-long.el: Improve support for major mode hooks

* lisp/so-long.el (so-long-remember-all, so-long-disable-minor-modes)
(so-long-override-variables): Store and use the `so-long-minor-modes'
and `so-long-variable-overrides' values seen by the original major
mode, so that buffer-local changes made in the major mode hook will be
respected.

Add documentation of this and other major mode hook usage.

lisp/so-long.el

@@ -282,6 +282,43 @@
 ;;           '((show-trailing-whitespace . nil)
 ;;             (truncate-lines . nil))))
 
+;; * Mode-specific configuration
+;; -----------------------------
+;; The `so-long-predicate' function is called in the context of the buffer's
+;; original major mode, and therefore major mode hooks can be used to control
+;; the criteria for calling `so-long' in any given mode (plus its derivatives)
+;; by setting buffer-local values for the variables in question.  This includes
+;; `so-long-predicate' itself, as well as any variables used by the predicate
+;; when determining the result.  By default this means `so-long-max-lines',
+;; `so-long-skip-leading-comments', and `so-long-threshold'.  E.g.:
+;;
+;;   (add-hook 'js-mode-hook 'my-js-mode-hook)
+;;
+;;   (defun my-js-mode-hook ()
+;;     "Custom `js-mode' behaviours."
+;;     (setq-local so-long-max-lines 100)
+;;     (setq-local so-long-threshold 1000))
+;;
+;; `so-long-variable-overrides' and `so-long-minor-modes' may also be given
+;; buffer-local values in order to apply different settings to different types
+;; of file.  For example, the Bidirectional Parentheses Algorithm does not apply
+;; to `<' and `>' characters by default, and therefore one might prefer to not
+;; set `bidi-inhibit-bpa' in XML files, on the basis that XML files with long
+;; lines are less likely to trigger BPA-related performance problems:
+;;
+;;   (add-hook 'nxml-mode-hook 'my-nxml-mode-hook)
+;;
+;;   (defun my-nxml-mode-hook ()
+;;     "Custom `nxml-mode' behaviours."
+;;     (require 'so-long)
+;;     (setq-local so-long-variable-overrides
+;;                 (remove '(bidi-inhibit-bpa . t) so-long-variable-overrides)))
+;;
+;; Finally, note that setting `so-long-target-modes' to nil buffer-locally in
+;; a major mode hook would prevent that mode from ever being targeted.  With
+;; `prog-mode' being targeted by default, specific derivatives of `prog-mode'
+;; could therefore be un-targeted if desired.
+
 ;; * Other ways of using so-long
 ;; -----------------------------
 ;; It may prove useful to automatically invoke major mode `so-long-mode' for
@@ -886,9 +923,15 @@ buffer-local."
 Stores the existing value for each entry in `so-long-variable-overrides'.
 Stores the name of each enabled mode from the list `so-long-minor-modes'.
 
+The lists themselves are also remembered, so that major mode hooks can
+provide buffer-local modifications which are still accessible after changing
+to `so-long-mode'.
+
 If RESET is non-nil, remove any existing values before storing the new ones."
   (when reset
     (setq so-long-original-values nil))
+  (so-long-remember 'so-long-variable-overrides)
+  (so-long-remember 'so-long-minor-modes)
   (dolist (ovar so-long-variable-overrides)
     (so-long-remember (car ovar)))
   (dolist (mode so-long-minor-modes)
@@ -1286,7 +1329,7 @@ Calls `so-long-disable-minor-modes' and `so-long-override-variables'."
 
 (defun so-long-disable-minor-modes ()
   "Disable any active minor modes listed in `so-long-minor-modes'."
-  (dolist (mode so-long-minor-modes)
+  (dolist (mode (so-long-original 'so-long-minor-modes))
     (when (and (boundp mode) mode)
       (funcall mode 0))))
 
@@ -1302,7 +1345,7 @@ The modes are enabled in accordance with what was remembered in `so-long'."
 
 (defun so-long-override-variables ()
   "Set the buffer-local values defined by `so-long-variable-overrides'."
-  (dolist (ovar so-long-variable-overrides)
+  (dolist (ovar (so-long-original 'so-long-variable-overrides))
     (set (make-local-variable (car ovar)) (cdr ovar))))
 
 (defun so-long-restore-variables ()
@@ -1877,7 +1920,7 @@ If it appears in `%s', you should remove it."
 ; LocalWords:  defadvice nadvice whitespace ie bos eos eobp origmode un Un setq
 ; LocalWords:  docstring auf Wiedersehen longlines alist autoload Refactored Inc
 ; LocalWords:  MERCHANTABILITY RET REGEXP VAR ELPA WS mitigations EmacsWiki eval
-; LocalWords:  rx filename filenames bidi bpa
+; LocalWords:  rx filename filenames js defun bidi bpa prog
 
 ;; So long, farewell, auf Wiedersehen, goodbye
 ;; You have to go, this code is minified