Subject: 24.1; (wishlist) Easy way to make a file-local variable also permanent-local

From: yary <[email-masked]>
Date: Sat, 19 Oct 2013 18:15:04 -0400

There are occasional needs to preserve buffer-local variables across
major-mode changes. In the specific case of directory-local variables,
an automated solution looks to be feasible, see "bug#15577: 24.3;
dir-local variables not applied when switching major-mode"

File-local variables on the other hand are not as easy to categorize as
to their intent- for a particular mode, vs the file contents in general.

I propose adding `permanent' as another special "variable name" (along
the lines of `mode', `eval', `coding', and `unibyte') to let people
mark the file variables they want to remain between mode changes. For
example:

; mode: Lisp
; fill-column: 75
; comment-column: 50
; permanent: fill-column tab-width

would set & keep fill-column at 75 between mode changes, and would
preserve between major-mode switches any changes to tab-width.

(There is one edge case with this proposal: what would `permanent: eval'
do? Re-evaluate all `eval' file variables after a mode change? And
`permanent: mode' would have to be ignored or give a warning.)

In GNU Emacs 24.1.1 (i386-mingw-nt5.1.2600)
 of 2012-06-10 on MARVIN
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include'

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENU
  value of $XMODIFIERS: nil
  locale-coding-system: cp1252
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t