bug#74349: 30.0.92; Visiting a file under c-ts-mode loads cc-mode

[Eli Zaretskii <eliz@gnu.org>]

To reproduce:

  emacs -Q
  M-: (featurep 'cc-mode) RET
   => nil
  M-x load-library RET c-ts-mode RET
  M-: (featurep 'cc-mode) RET
   => nil
  C-x C-f src/dispnew.c RET
  M-: major-mode RET
   => c-ts-mode
  M-: (featurep 'cc-mode) RET
   => t
  M-: c-noise-macro-names RET
   => ("INLINE" "NO_INLINE" "ATTRIBUTE_NO_SANITIZE_UNDEFINED" "UNINIT" "CALLBACK" "ALIGN_STACK" "ATTRIBUTE_MALLOC" "ATTRIBUTE_DEALLOC_FREE" "ANDROID_EXPORT" "TEST_STATIC" "INLINE_HEADER_BEGIN" "INLINE_HEADER_END")

So the C file was correctly visited using c-ts-mode, but cc-mode was
still loaded, quite unexpectedly.  Moreover, the settings in our
.dir-locals.el that are defined for c-mode were applied to the buffer
which is not under c-mode.

AFAICT, this happens because of three factors:

  . Loading c-ts-mode evaluates this:

     (derived-mode-add-parents 'c-ts-mode '(c-mode))

  . We have in our .dir-locals.el settings for c-mode:

     (c-mode . ((c-file-style . "GNU")
		(c-noise-macro-names . ("INLINE" "NO_INLINE" "ATTRIBUTE_NO_SANITIZE_UNDEFINED"
					"UNINIT" "CALLBACK" "ALIGN_STACK" "ATTRIBUTE_MALLOC"
					"ATTRIBUTE_DEALLOC_FREE" "ANDROID_EXPORT" "TEST_STATIC"
					"INLINE_HEADER_BEGIN" "INLINE_HEADER_END"))
		(electric-quote-comment . nil)
		(electric-quote-string . nil)
		(indent-tabs-mode . t)
		(mode . bug-reference-prog)))

  . hack-dir-local-variables 'funcall's the mode symbol when
    processing directory-local variables for that mode

A C and Lisp backtrace showing that cc-mode was loaded as side effect
of processing .dir-locals.el is shown below.  I could understand why a
mode is funcall'ed when it is being turned on, but I didn't expect
derived-mode-add-parents to cause c-mode be turned on, and its
directory-local settings be applied, when c-ts-mode is used to visit a
file.  That's at least unexpected, and arguably a bug.

Can this be avoided, please?  Users who want to use c-ts-mode for C
files will not necessarily be happy to have cc-mode and all its
dependencies loaded, and the c-mode-related settings applied, just
because of unrelated c-mode settings in .dir-locals.el.  It bloats the
memory footprint of Emacs for no good reason, and it modifies
variables the user didn't mean to touch.

Here's the backtrace.  I obtained it by setting a breakpoint in Fload
and waiting for cc-mode to be loaded.

  (gdb) break Fload
  Breakpoint 3 at 0x84b569: file lread.c, line 1326.
  (gdb) commands
  Type commands for breakpoint(s) 3, one per line.
  End with a line saying just "end".
  >pp file
  >end
  (gdb) c
  Continuing.

  Thread 1 hit Breakpoint 3, Fload (file=XIL(0x80000000094bf9d8),
      noerror=XIL(0), nomessage=XIL(0x30), nosuffix=XIL(0),
      must_suffix=XIL(0x30)) at lread.c:1326
  1326      file_stream stream UNINIT;
  "cc-mode"
  (gdb) bt
  #0  Fload (file=XIL(0x80000000094bf9d8), noerror=XIL(0), nomessage=XIL(0x30),
      nosuffix=XIL(0), must_suffix=XIL(0x30)) at lread.c:1326
  #1  0x0084d064 in save_match_data_load (file=XIL(0x80000000094bf9d8),
      noerror=XIL(0), nomessage=XIL(0x30), nosuffix=XIL(0),
      must_suffix=XIL(0x30)) at lread.c:1783
  #2  0x007fcf6d in load_with_autoload_queue (file=XIL(0x80000000094bf9d8),
      noerror=XIL(0), nomessage=XIL(0x30), nosuffix=XIL(0),
      must_suffix=XIL(0x30)) at eval.c:2382
  #3  0x007fd29b in Fautoload_do_load (fundef=XIL(0xc0000000094bf998),
      funname=XIL(0), macro_only=XIL(0)) at eval.c:2428
  #4  0x007fff77 in funcall_subr (subr=0xe5d100 <Sautoload_do_load>, numargs=1,
      args=0xa0916b8) at eval.c:3165
  #5  0x0086d77d in exec_byte_code (fun=XIL(0xa0000000094c03a8),
      args_template=513, nargs=1, args=0xa091768) at bytecode.c:812
  #6  0x008007f3 in funcall_lambda (fun=XIL(0xa00000000df14188), nargs=1,
      arg_vector=0xa091560) at eval.c:3252
  #7  0x007ff8b9 in funcall_general (fun=XIL(0xa00000000df14188), numargs=1,
      args=0xa091560) at eval.c:3044
  #8  0x007ffbe2 in Ffuncall (nargs=2, args=0xa091558) at eval.c:3093
  #9  0x007fef39 in run_hook_wrapped_funcall (nargs=2, args=0xa091558)
      at eval.c:2872
  #10 0x007ff2f2 in run_hook_with_args (nargs=2, args=0xa091558,
      funcall=0x7feef1 <run_hook_wrapped_funcall>) at eval.c:2953
  #11 0x007fef88 in Frun_hook_wrapped (nargs=2, args=0xa091558) at eval.c:2887
  #12 0x0080038b in funcall_subr (subr=0xe5d2c0 <Srun_hook_wrapped>, numargs=2,
      args=0xa091558) at eval.c:3184
  #13 0x0086d77d in exec_byte_code (fun=XIL(0xa000000009bc4278),
      args_template=769, nargs=1, args=0xa091560) at bytecode.c:812
  #14 0x008007f3 in funcall_lambda (fun=XIL(0xa000000009556de8), nargs=2,
      arg_vector=0x607e9f0) at eval.c:3252
  #15 0x007ff8b9 in funcall_general (fun=XIL(0xa000000009556de8), numargs=2,
      args=0x607e9f0) at eval.c:3044
  #16 0x007ffbe2 in Ffuncall (nargs=3, args=0x607e9e8) at eval.c:3093
  #17 0x007f099a in Ffuncall_interactively (nargs=3, args=0x607e9e8)
      at callint.c:250
  #18 0x0080038b in funcall_subr (subr=0xe5c840 <Sfuncall_interactively>,
      numargs=3, args=0x607e9e8) at eval.c:3184
  #19 0x007ff852 in funcall_general (fun=XIL(0xa000000000e5c840), numargs=3,
      args=0x607e9e8) at eval.c:3040
  #20 0x007ffbe2 in Ffuncall (nargs=4, args=0x607e9e0) at eval.c:3093
  #21 0x007fed45 in Fapply (nargs=3, args=0x607ebf8) at eval.c:2765
  #22 0x007f0fea in Fcall_interactively (function=XIL(0x85c18c8),
      record_flag=XIL(0), keys=XIL(0xa00000000b9f6798)) at callint.c:342
  #23 0x007fff77 in funcall_subr (subr=0xe5c880 <Scall_interactively>,
      numargs=3, args=0xa091078) at eval.c:3165
  #24 0x0086d77d in exec_byte_code (fun=XIL(0xa000000009bc6ba0),
      args_template=1025, nargs=1, args=0x607f7a0) at bytecode.c:812
  #25 0x008007f3 in funcall_lambda (fun=XIL(0xa000000009bc6ba0), nargs=1,
      arg_vector=0x607f798) at eval.c:3252
  #26 0x007ff8b9 in funcall_general (fun=XIL(0xa000000009bc6ba0), numargs=1,
      args=0x607f798) at eval.c:3044
  #27 0x007ffbe2 in Ffuncall (nargs=2, args=0x607f790) at eval.c:3093
  #28 0x00705721 in command_loop_1 () at keyboard.c:1550
  #29 0x007fa8d1 in internal_condition_case (bfun=0x704b31 <command_loop_1>,
      handlers=XIL(0x90), hfun=0x703b8a <cmd_error>) at eval.c:1613
  #30 0x00704596 in command_loop_2 (handlers=XIL(0x90)) at keyboard.c:1168
  #31 0x007f9956 in internal_catch (tag=XIL(0x12720),
      func=0x70455f <command_loop_2>, arg=XIL(0x90)) at eval.c:1292
  #32 0x00704501 in command_loop () at keyboard.c:1146
  #33 0x007035ea in recursive_edit_1 () at keyboard.c:754
  #34 0x00703888 in Frecursive_edit () at keyboard.c:837
  #35 0x006fe9a9 in main (argc=2, argv=0x7b225f8) at emacs.c:2635

  Lisp Backtrace:
  "autoload-do-load" (0xa0916b8)
  "dir-locals-collect-variables" (0xa091630)
  "hack-dir-local--get-variables" (0xa0915c0)
  0xdf14188 PVEC_CLOSURE
  "run-hook-wrapped" (0xa091558)
  "hack-dir-local-variables" (0xa0914d8)
  "hack-local-variables" (0xa091490)
  "run-mode-hooks" (0xa091448)
  "c-ts-mode" (0xa0913f8)
  "set-auto-mode-0" (0xa0913a8)
  "set-auto-mode--apply-alist" (0xa091328)
  "set-auto-mode" (0xa0912d0)
  "normal-mode" (0xa091280)
  "after-find-file" (0xa091220)
  "find-file-noselect-1" (0xa091190)
  "find-file-noselect" (0xa091108)
  "find-file" (0x607e9f0)
  "funcall-interactively" (0x607e9e8)
  "call-interactively" (0xa091078)
  "command-execute" (0x607f798)

In GNU Emacs 30.0.92 (build 22, i686-pc-mingw32) of 2024-11-13 built on
 ELIZ-PC
Windowing system distributor 'Microsoft Corp.', version 10.0.22631
System Description: Microsoft Windows 10 Enterprise (v10.0.2009.22631.4460)

Configured using:
 'configure -C --prefix=/d/usr --with-wide-int
 --enable-checking=yes,glyphs --without-native-compilation 'CFLAGS=-O0
 -gdwarf-4 -g3''

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG LCMS2 LIBXML2 MODULES NOTIFY W32NOTIFY
PDUMPER PNG RSVG SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP XPM ZLIB

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

Major mode: ELisp/l

Minor modes in effect:
  bug-reference-prog-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-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
  minibuffer-regexp-mode: t
  line-number-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa epg rfc6068
epg-config gnus-util time-date subr-x mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils cl-extra shortdoc
text-property-search comp-common rx derived help-fns radix-tree
help-mode vc-git diff-mode track-changes easy-mmode vc-dispatcher
bug-reference byte-opt gv bytecomp byte-compile cc-mode cc-fonts
cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
cl-seq c++-ts-mode c-ts-mode c-ts-common treesit cl-loaddefs cl-lib
thingatpt find-func rmc iso-transl tooltip cconv eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
touch-screen dos-w32 ls-lisp disp-table term/w32-win w32-win w32-vars
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode lisp-mode prog-mode register
page tab-bar menu-bar rfn-eshadow isearch easymenu timer select
scroll-bar mouse jit-lock font-lock syntax font-core term/tty-colors
frame minibuffer nadvice seq simple cl-generic indonesian philippine
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite emoji-zwj charscript
charprop case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads w32notify w32 lcms2 multi-tty move-toolbar make-network-process
emacs)

Memory information:
((conses 16 103247 13845) (symbols 48 10527 0) (strings 16 31840 3566)
 (string-bytes 1 855779) (vectors 16 16474)
 (vector-slots 8 171529 10546) (floats 8 88 4) (intervals 40 673 162)
 (buffers 896 13))