bug#69191: 30.0.50; New var `major-mode-remap-defaults`, for packages

[Stefan Monnier]

Package: Emacs
Version: 30.0.50


While `major-mode-remap-alist` provides a way for users to indicate the
major mode of their choice, I think we need a similar variable for the
use of packages.

The package below accordingly adds a new `major-mode-remap-defaults`
and changes various packages to obey it or make use of it.
I think it nicely cleans the regexp duplication between CC-mode and
`c-ts-mode.el` and also makes it easier/cleaner for users to override
the changes made by `*-ts-mode.el`.

There are some FIXMEs that one might want to address, most importantly
whether we should use an indirection through `major-mode-remap-defaults`
for all TS modes or only for those for which we provide a non-TS mode.

And there's no doc update yet.


        Stefan


diff --git a/lisp/files.el b/lisp/files.el
index f67b650cb92..6b80175b157 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -3404,7 +3404,7 @@ set-auto-mode
 matches the buffer beginning against `magic-mode-alist',
 compares the file name against the entries in `auto-mode-alist',
 then matches the buffer beginning against `magic-fallback-mode-alist'.
-It also obeys `major-mode-remap-alist'.
+It also obeys `major-mode-remap-alist' and `major-mode-remap-defaults'.
 
 If `enable-local-variables' is nil, or if the file name matches
 `inhibit-local-variables-regexps', this function does not check
@@ -3550,9 +3550,22 @@ major-mode-remap-alist
 Every entry is of the form (MODE . FUNCTION) which means that in order
 to activate the major mode MODE (specified via something like
 `auto-mode-alist', file-local variables, ...) we should actually call
-FUNCTION instead."
+FUNCTION instead.
+FUNCTION can be nil to hide other entries (either in this var or in
+`major-mode-remap-defaults') and means that we should call MODE."
   :type '(alist (symbol) (function)))
 
+(defvar major-mode-remap-defaults nil
+  "Alist mapping file-specified mode to actual mode.
+This works like `major-mode-remap-alist' except it has lower priority
+and it is meant to be modified by packages rather than users.")
+
+(defun major-mode-remap (mode)
+  "Return the function to use to enable MODE."
+  (or (cdr (or (assq mode major-mode-remap-alist)
+               (assq mode major-mode-remap-defaults)))
+      mode))
+
 ;; When `keep-mode-if-same' is set, we are working on behalf of
 ;; set-visited-file-name.  In that case, if the major mode specified is the
 ;; same one we already have, don't actually reset it.  We don't want to lose
@@ -3569,7 +3582,7 @@ set-auto-mode-0
                        (eq mode (car set-auto-mode--last))
                        (eq major-mode (cdr set-auto-mode--last)))))
     (when mode
-      (funcall (alist-get mode major-mode-remap-alist mode))
+      (funcall (major-mode-remap mode))
       (unless (eq mode major-mode)
         (setq set-auto-mode--last (cons mode major-mode)))
       mode)))
diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
index 4ef17daf876..b9064d689cf 100644
--- a/lisp/progmodes/c-ts-mode.el
+++ b/lisp/progmodes/c-ts-mode.el
@@ -1439,36 +1439,33 @@ c-or-c++-ts-mode
 This function attempts to use file contents to determine whether
 the code is C or C++ and based on that chooses whether to enable
 `c-ts-mode' or `c++-ts-mode'."
+  ;; FIXME: (declare (obsolete c-or-c++-mode "30.1"))?
   (interactive)
-  (if (save-excursion
-        (save-restriction
-          (save-match-data ; Why `save-match-data'?
-            (widen)
-            (goto-char (point-min))
-            (re-search-forward c-ts-mode--c-or-c++-regexp nil t))))
-      (c++-ts-mode)
-    (c-ts-mode)))
+  (let ((mode
+         (if (save-excursion
+               (save-restriction
+                 (save-match-data       ; Why `save-match-data'?
+                   (widen)
+                   (goto-char (point-min))
+                   (re-search-forward c-ts-mode--c-or-c++-regexp nil t))))
+             'c++-ts-mode)
+         'c-ts-mode))
+    (funcall (major-mode-remap mode))))
+
 ;; The entries for C++ must come first to prevent *.c files be taken
 ;; as C++ on case-insensitive filesystems, since *.C files are C++,
 ;; not C.
 (if (treesit-ready-p 'cpp)
-    (add-to-list 'auto-mode-alist
-                 
'("\\(\\.ii\\|\\.\\(CC?\\|HH?\\)\\|\\.[ch]\\(pp\\|xx\\|\\+\\+\\)\\|\\.\\(cc\\|hh\\)\\)\\'"
-                   . c++-ts-mode)))
+    (add-to-list 'major-mode-remap-defaults
+                 '(c++-mode . c++-ts-mode)))
 
 (when (treesit-ready-p 'c)
-  (add-to-list 'auto-mode-alist
-               '("\\(\\.[chi]\\|\\.lex\\|\\.y\\(acc\\)?\\)\\'" . c-ts-mode))
-  (add-to-list 'auto-mode-alist '("\\.x[pb]m\\'" . c-ts-mode))
-  ;; image-mode's association must be before the C mode, otherwise XPM
-  ;; images will be initially visited as C files.  Also note that the
-  ;; regexp must be different from what files.el does, or else
-  ;; add-to-list will not add the association where we want it.
-  (add-to-list 'auto-mode-alist '("\\.x[pb]m\\'" . image-mode)))
-
-(if (and (treesit-ready-p 'cpp)
-         (treesit-ready-p 'c))
-    (add-to-list 'auto-mode-alist '("\\.h\\'" . c-or-c++-ts-mode)))
+  (add-to-list 'major-mode-remap-defaults '(c++-mode . c++-ts-mode))
+  (add-to-list 'major-mode-remap-defaults '(c-mode . c-ts-mode)))
+
+(when (and (treesit-ready-p 'cpp)
+           (treesit-ready-p 'c))
+  (add-to-list 'major-mode-remap-defaults '(c-or-c++-mode . c-or-c++-ts-mode)))
 
 (provide 'c-ts-mode)
 (provide 'c++-ts-mode)
diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
index 64a679eacc7..6e98ce047c3 100644
--- a/lisp/progmodes/cc-mode.el
+++ b/lisp/progmodes/cc-mode.el
@@ -2902,15 +2902,19 @@ c-or-c++-mode
 the code is C or C++ and based on that chooses whether to enable
 `c-mode' or `c++-mode'."
   (interactive)
-  (if (save-excursion
-        (save-restriction
-          (save-match-data
-            (widen)
-            (goto-char (point-min))
-            (re-search-forward c-or-c++-mode--regexp
-                               (+ (point) c-guess-region-max) t))))
-      (c++-mode)
-    (c-mode)))
+  (let ((mode
+         (if (save-excursion
+               (save-restriction
+                (save-match-data
+                  (widen)
+                  (goto-char (point-min))
+                  (re-search-forward c-or-c++-mode--regexp
+                                     (+ (point) c-guess-region-max) t))))
+             'c++-mode)
+         'c-mode))
+    (funcall (if (fboundp 'major-mode-remap)
+                (major-mode-remap mode)
+              mode))))
 
 
 ;; Support for C++
diff --git a/lisp/progmodes/go-ts-mode.el b/lisp/progmodes/go-ts-mode.el
index 65adc1c55ea..296e4d0037d 100644
--- a/lisp/progmodes/go-ts-mode.el
+++ b/lisp/progmodes/go-ts-mode.el
@@ -262,6 +262,8 @@ go-ts-mode
     (treesit-major-mode-setup)))
 
 (if (treesit-ready-p 'go)
+    ;; FIXME: Should we instead put `go-mode' in `auto-mode-alist'
+    ;; and then use `major-mode-remap-defaults' to map it to `go-ts-mode'?
     (add-to-list 'auto-mode-alist '("\\.go\\'" . go-ts-mode)))
 
 (defun go-ts-mode--defun-name (node &optional skip-prefix)
diff --git a/lisp/progmodes/ruby-ts-mode.el b/lisp/progmodes/ruby-ts-mode.el
index 426ae248cac..cdfa3dca498 100644
--- a/lisp/progmodes/ruby-ts-mode.el
+++ b/lisp/progmodes/ruby-ts-mode.el
@@ -1211,18 +1211,8 @@ ruby-ts-mode
   (setq-local syntax-propertize-function #'ruby-ts--syntax-propertize))
 
 (if (treesit-ready-p 'ruby)
-    ;; Copied from ruby-mode.el.
-    (add-to-list 'auto-mode-alist
-                 (cons (concat "\\(?:\\.\\(?:"
-                               "rbw?\\|ru\\|rake\\|thor"
-                               "\\|jbuilder\\|rabl\\|gemspec\\|podspec"
-                               "\\)"
-                               "\\|/"
-                               "\\(?:Gem\\|Rake\\|Cap\\|Thor"
-                               "\\|Puppet\\|Berks\\|Brew"
-                               "\\|Vagrant\\|Guard\\|Pod\\)file"
-                               "\\)\\'")
-                       'ruby-ts-mode)))
+    (add-to-list 'major-mode-remap-defaults
+                 '(ruby-mode . ruby-ts-mode)))
 
 (provide 'ruby-ts-mode)
 
diff --git a/lisp/textmodes/tex-mode.el b/lisp/textmodes/tex-mode.el
index 5c5ca573f38..d14953ef1c7 100644
--- a/lisp/textmodes/tex-mode.el
+++ b/lisp/textmodes/tex-mode.el
@@ -1032,17 +1032,13 @@ tex--redirect-to-submode
                  ;; `tex--guess-mode' really tries to guess the *type* of file,
                  ;; so we still need to consult `major-mode-remap-alist'
                  ;; to see which mode to use for that type.
-                 (alist-get mode major-mode-remap-alist mode))))))
+                 (major-mode-remap mode))))))
 
-;; The following three autoloaded aliases appear to conflict with
-;; AUCTeX.  We keep those confusing aliases for those users who may
-;; have files annotated with -*- LaTeX -*- (e.g. because they received
+;; Support files annotated with -*- LaTeX -*- (e.g. because they received
 ;; them from someone using AUCTeX).
-;; FIXME: Turn them into autoloads so that AUCTeX can override them
-;; with its own autoloads?  Or maybe rely on `major-mode-remap-alist'?
-;;;###autoload (defalias 'TeX-mode #'tex-mode)
-;;;###autoload (defalias 'plain-TeX-mode #'plain-tex-mode)
-;;;###autoload (defalias 'LaTeX-mode #'latex-mode)
+;;;###autoload (add-to-list 'major-mode-remap-defaults '(TeX-mode . tex-mode))
+;;;###autoload (add-to-list 'major-mode-remap-defaults '(plain-TeX-mode . 
plain-tex-mode))
+;;;###autoload (add-to-list 'major-mode-remap-defaults '(LaTeX-mode . 
latex-mode))
 
 ;;;###autoload
 (define-derived-mode plain-tex-mode tex-mode "TeX"