Provide a modified xref backend for TeX buffers

In addition to providing a new `xref' backend, the patch also improves
the general handling of expl3 syntax.  Expl3 is the next-generation
LaTeX specification, and has for some time been available by default in
the LaTeX kernel.  The new syntax co-exists in many files with the
standard LaTeX2e syntax, so we try at least minimally to separate the
way modes handle the two specifications, both to reduce
visually-disturbing interference between them and also to improve the
`xref' backend.  (Bug#53749)

* lib-src/etags.c (TeX_commands): Improve parsing of commands in TeX
buffers.
(TEX_defenv): Expand list of commands to tag by default in TeX buffers.
(TeX_help):
* doc/emacs/maintaining.texi (Tag Syntax): Document new tagged commands.
(Identifier Search): Add note about semantic-symref-filepattern-alist,
auto-mode-alist, and xref-find-references.

* lisp/textmodes/tex-mode.el (tex-font-lock-suscript): Test for
underscore in expl3 files and regions, disable subscript face there.
(tex-common-initialization): Set up xref backend for in-tree TeX modes.
Detect expl3 files, and in others set up a list of expl3 regions.
(tex-expl-buffer-parse): New function called in previous.
(tex-expl-buffer-p): New variable to hold the result of previous.
(tex-expl-region-set): New function added to
'syntax-propertize-extend-region-functions' hook.
(tex-expl-region-list): New variable to hold the result of previous.
(tex--xref-backend): New function to identify the xref backend.
(tex--thing-at-point, tex-thingatpt--beginning-of-symbol)
(tex-thingatpt--end-of-symbol, tex--bounds-of-symbol-at-point):
New functions to return 'thing-at-point' for xref backend.
(tex-thingatpt-exclude-chars): New variable to do the same.
(xref-backend-identifier-at-point): New TeX backend method to provide
symbols for processing by xref.
(xref-backend-identifier-completion-table)
(xref-backend-identifier-completion-ignore-case)
(xref-backend-definitions, xref-backend-apropos): Placeholders to
call the standard 'etags' xref backend methods.
(xref-backend-references): Wrapper to call the default xref backend
method, finding as many relevant files as possible and using a bespoke
syntax-propertize-function when required.
(tex--collect-file-extensions, tex-xref-syntax-function): Helper
functions for previous.
(tex-find-references-syntax-table, tex--buffers-list)
(tex--xref-syntax-fun, tex--old-syntax-function): New variables for
the same.

4 files changed, 572 insertions, 20 deletions

diff --git a/doc/emacs/maintaining.texi b/doc/emacs/maintaining.texi
index 3c34afbaa20..0ec2385860b 100644
--- a/doc/emacs/maintaining.texi
+++ b/doc/emacs/maintaining.texi
@@ -2549,6 +2549,15 @@ identifier, showing the file name and the line where the identifier is
 referenced.  The XREF mode commands are available in this buffer, see
 @ref{Xref Commands}.
 
+When invoked in a buffer whose major mode uses the @code{etags} backend,
+@kbd{M-?} searches files and buffers whose major mode matches that of
+the original buffer.  It guesses that mode from file extensions, so if
+@kbd{M-?} seems to be skipping relevant buffers or files, try
+customizing either the variable @code{semantic-symref-filepattern-alist}
+(if your buffer's major mode already has an entry in it), or
+@code{auto-mode-alist} (if not), thereby informing @code{xref} of the
+missing extensions (@pxref{Choosing Modes}).
+
 @vindex xref-auto-jump-to-first-xref
   If the value of the variable @code{xref-auto-jump-to-first-xref} is
 @code{t}, @code{xref-find-references} automatically jumps to the first
@@ -2767,10 +2776,32 @@ Tags for variables and functions in classes are named
 @item
 In @LaTeX{} documents, the arguments for @code{\chapter},
 @code{\section}, @code{\subsection}, @code{\subsubsection},
-@code{\eqno}, @code{\label}, @code{\ref}, @code{\cite},
-@code{\bibitem}, @code{\part}, @code{\appendix}, @code{\entry},
-@code{\index}, @code{\def}, @code{\newcommand}, @code{\renewcommand},
-@code{\newenvironment} and @code{\renewenvironment} are tags.
+@code{\eqno}, @code{\label}, @code{\ref}, @code{\Ref}, @code{\footref},
+@code{\cite}, @code{\bibitem}, @code{\part}, @code{\appendix},
+@code{\entry}, @code{\index}, @code{\def}, @code{\edef}, @code{\gdef},
+@code{\xdef}, @code{\newcommand}, @code{\renewcommand},
+@code{\newenvironment}, @code{\renewenvironment},
+@code{\DeclareRobustCommand}, @code{\newrobustcmd},
+@code{\renewrobustcmd}, @code{\providecommand},
+@code{\providerobustcmd}, @code{\NewDocumentCommand},
+@code{\RenewDocumentCommand}, @code{\ProvideDocumentCommand},
+@code{\DeclareDocumentCommand}, @code{\NewExpandableDocumentCommand},
+@code{\RenewExpandableDocumentCommand},
+@code{\ProvideExpandableDocumentCommand},
+@code{\DeclareExpandableDocumentCommand},
+@code{\NewDocumentEnvironment}, @code{\RenewDocumentEnvironment},
+@code{\ProvideDocumentEnvironment}, @code{\DeclareDocumentEnvironment},
+@code{\csdef}, @code{\csedef}, @code{\csgdef}, @code{\csxdef},
+@code{\csletcs}, @code{\cslet}, @code{\letcs}, @code{\let},
+@code{\cs_new_protected_nopar}, @code{\cs_new_protected},
+@code{\cs_new_nopar}, @code{\cs_new_eq}, @code{\cs_new},
+@code{\cs_set_protected_nopar}, @code{\cs_set_protected},
+@code{\cs_set_nopar}, @code{\cs_set_eq}, @code{\cs_set},
+@code{\cs_gset_protected_nopar}, @code{\cs_gset_protected},
+@code{\cs_gset_nopar}, @code{\cs_gset_eq}, @code{\cs_gset},
+@code{\cs_generate_from_arg_count}, and @code{\cs_generate_variant} are
+tags.  So too are the arguments of any starred variants of these
+commands.
 
 Other commands can make tags as well, if you specify them in the
 environment variable @env{TEXTAGS} before invoking @command{etags}.  The
diff --git a/etc/NEWS b/etc/NEWS
index d80e31ef7bb..4ccb158dd7d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -343,6 +343,15 @@ the 'grep' results editable.  The edits will be reflected in the buffer
 visiting the originating file.  Typing 'C-c C-c' will leave the Grep
 Edit mode.
 
+** TeX modes
+
++++
+*** New xref backend for TeX modes.
+The new backend ('tex-etags') is on by default, and improves the
+functionality of the standard 'xref' commands in TeX buffers.  You can
+restore the standard 'etags' backend with the 'M-x xref-etags-mode'
+toggle.
+
 
 * New Modes and Packages in Emacs 31.1
 
diff --git a/lib-src/etags.c b/lib-src/etags.c
index 556b7d701fc..7f652790261 100644
--- a/lib-src/etags.c
+++ b/lib-src/etags.c
@@ -793,11 +793,27 @@ variables set with 'set!' at top level in the file.";
 static const char *TeX_suffixes [] =
   { "bib", "clo", "cls", "ltx", "sty", "TeX", "tex", NULL };
 static const char TeX_help [] =
-"In LaTeX text, the argument of any of the commands '\\chapter',\n\
-'\\section', '\\subsection', '\\subsubsection', '\\eqno', '\\label',\n\
-'\\ref', '\\cite', '\\bibitem', '\\part', '\\appendix', '\\entry',\n\
-'\\index', '\\def', '\\newcommand', '\\renewcommand',\n\
-'\\newenvironment' or '\\renewenvironment' is a tag.\n\
+"In LaTeX text, the argument of the commands '\\chapter', '\\section',\n\
+'\\subsection', '\\subsubsection', '\\eqno', '\\label', '\\ref',\n\
+'\\Ref', '\\footref', '\\cite', '\\bibitem', '\\part', '\\appendix',\n\
+'\\entry', '\\index', '\\def', '\\edef', '\\gdef', '\\xdef',\n\
+'\\newcommand', '\\renewcommand', '\\newrobustcmd', '\\renewrobustcmd',\n\
+'\\newenvironment', '\\renewenvironment', '\\DeclareRobustCommand',\n\
+'\\providecommand', '\\providerobustcmd', '\\NewDocumentCommand',\n\
+'\\RenewDocumentCommand', '\\ProvideDocumentCommand',\n\
+'\\DeclareDocumentCommand', '\\NewExpandableDocumentCommand',\n\
+'\\RenewExpandableDocumentCommand', '\\ProvideExpandableDocumentCommand',\n\
+'\\DeclareExpandableDocumentCommand', '\\NewDocumentEnvironment',\n\
+'\\RenewDocumentEnvironment', '\\ProvideDocumentEnvironment',\n\
+'\\DeclareDocumentEnvironment','\\csdef', '\\csedef', '\\csgdef',\n\
+'\\csxdef', '\\csletcs', '\\cslet', '\\letcs', '\\let',\n\
+'\\cs_new_protected_nopar', '\\cs_new_protected', '\\cs_new_nopar',\n\
+'\\cs_new_eq', '\\cs_new', '\\cs_set_protected_nopar',\n\
+'\\cs_set_protected', '\\cs_set_nopar', '\\cs_set_eq', '\\cs_set',\n\
+'\\cs_gset_protected_nopar', '\\cs_gset_protected', '\\cs_gset_nopar',\n\
+'\\cs_gset_eq', '\\cs_gset', '\\cs_generate_from_arg_count', or\n\
+'\\cs_generate_variant' is a tag.  So is the argument of any starred\n\
+variant of these commands.\n\
 \n\
 Other commands can be specified by setting the environment variable\n\
 'TEXTAGS' to a colon-separated list like, for example,\n\
@@ -5746,9 +5762,20 @@ static linebuffer *TEX_toktab = NULL; /* Table with tag tokens */
 /* Default set of control sequences to put into TEX_toktab.
    The value of environment var TEXTAGS is prepended to this.  */
 static const char *TEX_defenv = "\
-:chapter:section:subsection:subsubsection:eqno:label:ref:cite:bibitem\
-:part:appendix:entry:index:def\
-:newcommand:renewcommand:newenvironment:renewenvironment";
+:label:ref:Ref:footref:chapter:section:subsection:subsubsection:eqno:cite\
+:bibitem:part:appendix:entry:index:def:edef:gdef:xdef:newcommand:renewcommand\
+:newenvironment:renewenvironment:DeclareRobustCommand:renewrobustcmd\
+:newrobustcmd:providecommand:providerobustcmd:NewDocumentCommand\
+:RenewDocumentCommand:ProvideDocumentCommand:DeclareDocumentCommand\
+:NewExpandableDocumentCommand:RenewExpandableDocumentCommand\
+:ProvideExpandableDocumentCommand:DeclareExpandableDocumentCommand\
+:NewDocumentEnvironment:RenewDocumentEnvironment\
+:ProvideDocumentEnvironment:DeclareDocumentEnvironment:csdef\
+:csedef:csgdef:csxdef:csletcs:cslet:letcs:let:cs_new_protected_nopar\
+:cs_new_protected:cs_new_nopar:cs_new_eq:cs_new:cs_set_protected_nopar\
+:cs_set_protected:cs_set_nopar:cs_set_eq:cs_set:cs_gset_protected_nopar\
+:cs_gset_protected:cs_gset_nopar:cs_gset_eq:cs_gset\
+:cs_generate_from_arg_count:cs_generate_variant";
 
 static void TEX_decode_env (const char *, const char *);
 
@@ -5807,19 +5834,139 @@ TeX_commands (FILE *inf)
               {
                 char *p;
                 ptrdiff_t namelen, linelen;
-                bool opgrp = false;
+                bool opgrp = false, one_esc = false, is_explthree = false;
 
                 cp = skip_spaces (cp + key->len);
+
+                /* 1. The canonical expl3 syntax looks something like this:
+                   \cs_new:Npn \__hook_tl_gput:Nn { \ERROR }.  First, if we
+                   want to tag any such commands, we include only the part
+                   before the colon (cs_new) in TEX_defenv or TEXTAGS.  Second,
+                   etags skips the argument specifier (including the colon)
+                   after the tag token, so that it doesn't become the tag name.
+                   Third, we set the boolean 'is_explthree' to true so that we
+                   can remove the argument specifier from the actual tag name
+                   (__hook_tl_gput).  This all allows us to include expl3
+                   constructs in TEX_defenv or in the environment variable
+                   TEXTAGS without requiring a change of separator, and it also
+                   allows us to find the definition of variant commands (with
+                   different argument specifiers) defined using, for example,
+                   \cs_generate_variant:Nn.  Please note that the expl3 spec
+                   requires etags to pay more attention to whitespace in the
+                   code.
+
+                   2. We also automatically remove the asterisk from starred
+                   variants of all commands, without the need to include the
+                   starred commands explicitly in TEX_defenv or TEXTAGS. */
+                if (*cp == ':')
+                  {
+                    while (!c_isspace (*cp) && *cp != TEX_opgrp)
+                      cp++;
+                    cp = skip_spaces (cp);
+                    is_explthree = true;
+                  }
+                else if (*cp == '*')
+                  cp++;
+
+                /* Skip the optional arguments to commands in the tags list so
+                   that these arguments don't end up as the name of the tag.
+                   The name will instead come from the argument in curly braces
+                   that follows the optional ones.  The '\let' command gets
+                   special treatment. */
+                while (*cp != '\0' && *cp != '%'
+                       && !streq (key->buffer, "let"))
+                  {
+                    if (*cp == '[')
+                      {
+                        while (*cp != ']' && *cp != '\0' && *cp != '%')
+                          cp++;
+                      }
+                    else if (*cp == '(')
+                      {
+                        while (*cp != ')' && *cp != '\0' && *cp != '%')
+                          cp++;
+                      }
+                    else if (*cp == ']' || *cp == ')')
+                      cp++;
+                    else
+                      break;
+                  }
                 if (*cp == TEX_opgrp)
                   {
                     opgrp = true;
                     cp++;
+                    cp = skip_spaces (cp); /* For expl3 code. */
                   }
+
+                /* Removing the TeX escape character from tag names simplifies
+                   things for editors finding tagged commands in TeX buffers.
+                   This applies to Emacs but also to the tag-finding behavior
+                   of at least some of the editors that use ctags, though in
+                   the latter case this will remain suboptimal.  The
+                   undocumented ctags option '--no-duplicates' may help. */
+                if (*cp == TEX_esc)
+                  {
+                    cp++;
+                    one_esc = true;
+                  }
+
+                /* Testing !c_isspace && !c_ispunct is simpler, but halts
+                   processing at too many places.  The list as it stands tries
+                   both to ensure that tag names will derive from macro names
+                   rather than from optional parameters to those macros, and
+                   also to return findable names while still allowing for
+                   unorthodox constructs. */
                 for (p = cp;
-                     (!c_isspace (*p) && *p != '#' &&
-                      *p != TEX_opgrp && *p != TEX_clgrp);
+                     (!c_isspace (*p) && *p != '#' && *p != '=' &&
+                      *p != '[' && *p != '(' && *p != TEX_opgrp &&
+                      *p != TEX_clgrp && *p != '"' && *p != '\'' &&
+                      *p != '%' && *p != ',' && *p != '|' && *p != '$');
                      p++)
-                  continue;
+                  /* In expl3 code we remove the argument specification from
+                     the tag name.  More generally we allow only one (deleted)
+                     escape char in a tag name, which (primarily) enables
+                     tagging a TeX command's different, possibly temporary,
+                     '\let' bindings. */
+                  if (is_explthree && *p == ':')
+                    break;
+                  else if (*p == TEX_esc)
+                    { /* Second part of test is for, e.g., \cslet. */
+                      if (!one_esc && !opgrp)
+                        {
+                          one_esc = true;
+                          continue;
+                        }
+                      else
+                        break;
+                    }
+                  else
+                    continue;
+                /* For TeX files, tags without a name are basically cruft, and
+                   in some situations they can produce spurious and confusing
+                   matches.  Try to catch as many cases as possible where a
+                   command name is of the form '\(', but avoid, as far as
+                   possible, the spurious matches. */
+                if (p == cp)
+                  {
+                    switch (*p)
+                      { /* Include =? */
+                      case '(': case '[': case '"': case '\'':
+                      case '\\': case '!': case '=': case ',':
+                      case '|': case '$':
+                        p++;
+                        break;
+                      case '{': case '}': case '<': case '>':
+                        if (!opgrp)
+                          {
+                              p++;
+                              if (*p == '\0' || *p == '%')
+                                goto tex_next_line;
+                          }
+                        break;
+                      default:
+                        break;
+                      }
+                  }
                 namelen = p - cp;
                 linelen = lb.len;
                 if (!opgrp || *p == TEX_clgrp)
@@ -5828,9 +5975,18 @@ TeX_commands (FILE *inf)
                       p++;
                     linelen = p - lb.buffer + 1;
                   }
-                make_tag (cp, namelen, true,
-                          lb.buffer, linelen, lineno, linecharno);
-                goto tex_next_line; /* We only tag a line once */
+                if (namelen)
+                  make_tag (cp, namelen, true,
+                            lb.buffer, linelen, lineno, linecharno);
+                /* Lines with more than one \def or \let are surprisingly
+                   common in TeX files, especially in the system files that
+                   form the basis of the various TeX formats.  This tags them
+                   all. */
+                /* goto tex_next_line; /\* We only tag a line once *\/ */
+                while (*cp != '\0' && *cp != '%' && *cp != TEX_esc)
+                  cp++;
+                if (*cp != TEX_esc)
+                  goto tex_next_line;
               }
         }
     tex_next_line:
diff --git a/lisp/textmodes/tex-mode.el b/lisp/textmodes/tex-mode.el
index 4bc2c840cdb..ec0c0c47a2d 100644
--- a/lisp/textmodes/tex-mode.el
+++ b/lisp/textmodes/tex-mode.el
@@ -637,6 +637,14 @@ An alternative value is \" . \", if you use a font with a narrow period."
               3 '(tex-font-lock-append-prop 'bold) 'append)))))
    "Gaudy expressions to highlight in TeX modes.")
 
+(defvar-local tex-expl-region-list nil
+  "List of region boundaries where expl3 syntax is active.
+It will be nil in buffers visiting files which use expl3 syntax
+throughout, for example, expl3 classes or packages.")
+
+(defvar-local tex-expl-buffer-p nil
+  "Non-nil in buffers using expl3 syntax throughout.")
+
 (defun tex-font-lock-suscript (pos)
   (unless (or (memq (get-text-property pos 'face)
                     '(font-lock-constant-face font-lock-builtin-face
@@ -646,7 +654,17 @@ An alternative value is \" . \", if you use a font with a narrow period."
                     (pos pos))
                 (while (eq (char-before pos) ?\\)
                   (setq pos (1- pos) odd (not odd)))
-                odd))
+                odd)
+              ;; Check if POS is in an expl3 syntax region or an expl3 buffer
+              (when (eq (char-after pos) ?_)
+                (or tex-expl-buffer-p
+                    (and
+                     tex-expl-region-list
+                     (catch 'result
+                       (dolist (range tex-expl-region-list)
+                         (and (> pos (car range))
+                              (< pos (cdr range))
+                              (throw 'result t))))))))
     (if (eq (char-after pos) ?_)
         `(face subscript display (raise ,(car tex-font-script-display)))
       `(face superscript display (raise ,(cadr tex-font-script-display))))))
@@ -1290,8 +1308,16 @@ Entering SliTeX mode runs the hook `text-mode-hook', then the hook
                 #'tex--prettify-symbols-compose-p)
   (setq-local syntax-propertize-function
               (syntax-propertize-rules latex-syntax-propertize-rules))
+  ;; Don't add extra processing to `syntax-propertize' in files where
+  ;; expl3 syntax is always active.
+  :after-hook (progn (tex-expl-buffer-parse)
+                     (unless tex-expl-buffer-p
+                       (add-hook 'syntax-propertize-extend-region-functions
+                                 #'tex-expl-region-set nil t)))
   ;; TABs in verbatim environments don't do what you think.
   (setq-local indent-tabs-mode nil)
+  ;; Set up xref backend in TeX buffers.
+  (add-hook 'xref-backend-functions #'tex--xref-backend nil t)
   ;; Other vars that should be buffer-local.
   (make-local-variable 'tex-command)
   (make-local-variable 'tex-start-of-header)
@@ -1937,6 +1963,36 @@ Mark is left at original location."
                 (forward-sexp 1))))))
       (message "%s words" count))))
 
+(defun tex-expl-buffer-parse ()
+  "Identify buffers using expl3 syntax throughout."
+  (save-excursion
+    (goto-char (point-min))
+    (when (tex-search-noncomment
+           (re-search-forward
+            "\\\\\\(?:ExplFile\\|ProvidesExpl\\|__xparse_file\\)"
+            nil t))
+      (setq tex-expl-buffer-p t))))
+
+(defun tex-expl-region-set (_beg _end)
+  "Create a list of regions where expl3 syntax is active.
+This function updates the list whenever `syntax-propertize' runs, and
+stores it in the buffer-local variable `tex-expl-region-list'.  The list
+will always be nil when the buffer visits an expl3 file, for example, an
+expl3 class or package, where the entire file uses expl3 syntax."
+  (unless syntax-ppss--updated-cache;; Stop forward search running twice.
+    (setq tex-expl-region-list nil)
+    ;; Leaving this test here allows users to set `tex-expl-buffer-p'
+    ;; independently of the mode's automatic detection of an expl3 file.
+    (unless tex-expl-buffer-p
+      (goto-char (point-min))
+      (let ((case-fold-search nil))
+        (while (tex-search-noncomment
+                (search-forward "\\ExplSyntaxOn" nil t))
+          (let ((new-beg (point))
+                (new-end (or (tex-search-noncomment
+                              (search-forward "\\ExplSyntaxOff" nil t))
+                             (point-max))))
+            (push (cons new-beg new-end) tex-expl-region-list)))))))
 
 
 ;;; Invoking TeX in an inferior shell.
@@ -3743,6 +3799,306 @@ There might be text before point."
       (process-send-region tex-chktex--process (point-min) (point-max))
       (process-send-eof tex-chktex--process))))
 
+
+;;; Xref backend
+
+;; Here we lightly adapt the default etags backend for xref so that
+;; the main xref user commands (including `xref-find-definitions',
+;; `xref-find-apropos', and `xref-find-references' [on M-., C-M-., and
+;; M-?, respectively]) work in TeX buffers.  The only methods we
+;; actually modify are `xref-backend-identifier-at-point' and
+;; `xref-backend-references'.  Many of the complications here, and in
+;; `etags' itself, are due to the necessity of parsing both the old
+;; TeX syntax and the new expl3 syntax, which will continue to appear
+;; together in documents for the foreseeable future.  Synchronizing
+;; Emacs and `etags' this way aims to improve the user experience "out
+;; of the box."
+
+(defvar tex-thingatpt-exclude-chars '(?\\ ?\{ ?\})
+  "Exclude these chars by default from TeX thing-at-point.
+
+The TeX `xref-backend-identifier-at-point' method uses the characters
+listed in this variable to decide on the default search string to
+present to the user who calls an `xref' command.  These characters
+become part of a regexp which always excludes them from that default
+string.  For the `xref' commands to function properly in TeX buffers, at
+least the TeX escape and the two TeX grouping characters should be
+listed here.  Should your TeX documents contain other characters which
+you want to exclude by default, then you can add them to the list,
+though you may wish to consult the functions
+`tex-thingatpt--beginning-of-symbol' and `tex-thingatpt--end-of-symbol'
+to see what the regexp already contains.  If your documents contain
+non-standard escape and grouping characters, then you can replace the
+three listed here with your own, thereby allowing the three standard
+characters to appear by default in search strings.  Please be aware,
+however, that the `etags' program only recognizes `\\' (92) and `!' (33)
+as escape characters in TeX documents, and if it detects the latter it
+also uses `<>' as the TeX grouping construct rather than `{}'.  Setting
+the escape and grouping chars to anything other than `\\=\\{}' or `!<>'
+will not be useful without changes to `etags', at least for commands
+that search tags tables, such as \\[xref-find-definitions] and \
+\\[xref-find-apropos].
+
+Should you wish to change the defaults, please also be aware that,
+without further modifications to tex-mode.el, the usual text-parsing
+routines for `font-lock' and the like won't work correctly, as the
+default escape and grouping characters are currently hard coded in many
+places.")
+
+;; Populate `semantic-symref-filepattern-alist' for the in-tree modes;
+;; AUCTeX is doing the same for its modes.
+(with-eval-after-load 'semantic/symref/grep
+  (defvar semantic-symref-filepattern-alist)
+  (push '(latex-mode "*.[tT]e[xX]" "*.ltx" "*.sty" "*.cl[so]"
+                     "*.bbl" "*.drv" "*.hva")
+        semantic-symref-filepattern-alist)
+  (push '(plain-tex-mode "*.[tT]e[xX]" "*.ins")
+        semantic-symref-filepattern-alist)
+  (push '(doctex-mode "*.dtx") semantic-symref-filepattern-alist))
+
+(defun tex--xref-backend () 'tex-etags)
+
+(cl-defmethod xref-backend-identifier-at-point ((_backend (eql 'tex-etags)))
+  (require 'etags)
+  (tex--thing-at-point))
+
+;; The detection of `_' and `:' is a primitive method for determining
+;; whether point is on an expl3 construct.  It may fail in some
+;; instances.
+(defun tex--thing-at-point ()
+  "Demarcate `thing-at-point' for the TeX `xref' backend."
+  (let ((bounds (tex--bounds-of-symbol-at-point)))
+    (when bounds
+      (let ((texsym (buffer-substring-no-properties (car bounds) (cdr bounds))))
+        (if (and (not (string-match-p "reference" (symbol-name this-command)))
+                 (seq-contains-p texsym ?_)
+                 (seq-contains-p texsym ?:))
+            (seq-take texsym (seq-position texsym ?:))
+          texsym)))))
+
+(defun tex-thingatpt--beginning-of-symbol ()
+  (and
+   (re-search-backward (concat "[]["
+                               (mapconcat #'regexp-quote
+                                          (mapcar #'char-to-string
+                                                  tex-thingatpt-exclude-chars))
+                               "\"*`'#=&()%,|$[:cntrl:][:blank:]]"))
+   (forward-char)))
+
+(defun tex-thingatpt--end-of-symbol ()
+  (and
+   (re-search-forward (concat "[]["
+                              (mapconcat #'regexp-quote
+                                          (mapcar #'char-to-string
+                                                  tex-thingatpt-exclude-chars))
+                              "\"*`'#=&()%,|$[:cntrl:][:blank:]]"))
+   (backward-char)))
+
+(defun tex--bounds-of-symbol-at-point ()
+  "Simplify `bounds-of-thing-at-point' for TeX `xref' backend."
+  (let ((orig (point)))
+    (ignore-errors
+      (save-excursion
+        (tex-thingatpt--end-of-symbol)
+        (tex-thingatpt--beginning-of-symbol)
+        (let ((beg (point)))
+          (if (<= beg orig)
+              (let ((real-end
+                     (progn
+                       (tex-thingatpt--end-of-symbol)
+                       (point))))
+                (cond ((and (<= orig real-end) (< beg real-end))
+                       (cons beg real-end))
+                      ((and (= orig real-end) (= beg real-end))
+                       (cons beg (1+ beg)))))))))));; For 1-char TeX commands.
+
+(cl-defmethod xref-backend-identifier-completion-table ((_backend
+                                                         (eql 'tex-etags)))
+  (xref-backend-identifier-completion-table 'etags))
+
+(cl-defmethod xref-backend-identifier-completion-ignore-case ((_backend
+                                                               (eql
+                                                                'tex-etags)))
+  (xref-backend-identifier-completion-ignore-case 'etags))
+
+(cl-defmethod xref-backend-definitions ((_backend (eql 'tex-etags)) symbol)
+  (xref-backend-definitions 'etags symbol))
+
+(cl-defmethod xref-backend-apropos ((_backend (eql 'tex-etags)) pattern)
+  (xref-backend-apropos 'etags pattern))
+
+;; The `xref-backend-references' method requires more code than the
+;; others for at least two main reasons: TeX authors have typically been
+;; free in their invention of new file types with new suffixes, and they
+;; have also tended sometimes to include non-symbol characters in
+;; command names.  When combined with the default Semantic Symbol
+;; Reference API, these two characteristics of TeX code mean that a
+;; command like `xref-find-references' would often fail to find any hits
+;; for a symbol at point, including the one under point in the current
+;; buffer, or it would find only some instances and skip others.
+
+(defun tex-find-references-syntax-table ()
+  (let ((st (if (boundp 'TeX-mode-syntax-table)
+                 (make-syntax-table TeX-mode-syntax-table)
+               (make-syntax-table tex-mode-syntax-table))))
+    st))
+
+(defvar tex--xref-syntax-fun nil)
+
+(defun tex-xref-syntax-function (str beg end)
+  "Provide a bespoke `syntax-propertize-function' for \\[xref-find-references]."
+  (let* (grpb tempstr
+              (shrtstr (if end
+                           (progn
+                             (setq tempstr (seq-take str (1- (length str))))
+                             (if beg
+                                 (setq tempstr (seq-drop tempstr 1))
+                               tempstr))
+                         (seq-drop str 1)))
+              (grpa (if (and beg end)
+                        (prog1
+                            (list 1 "_")
+                          (setq grpb (list 2 "_")))
+                      (list 1 "_")))
+              (re (concat beg (regexp-quote shrtstr) end))
+              (temp-rule (if grpb
+                             (list re grpa grpb)
+                           (list re grpa))))
+    ;; Simple benchmarks suggested that the speed-up from compiling this
+    ;; function was nearly nil, so `eval' and its non-byte-compiled
+    ;; function remain.
+    (setq tex--xref-syntax-fun (eval
+                                `(syntax-propertize-rules ,temp-rule)))))
+
+(defun tex--collect-file-extensions ()
+  "Gather TeX file extensions from `auto-mode-alist'."
+  (let* ((mlist (when (rassq major-mode auto-mode-alist)
+                  (seq-filter
+                   (lambda (elt)
+                     (eq (cdr elt) major-mode))
+                   auto-mode-alist)))
+         (lcsym (intern-soft (downcase (symbol-name major-mode))))
+         (lclist (and lcsym
+                      (not (eq lcsym major-mode))
+                      (rassq lcsym auto-mode-alist)
+                      (seq-filter
+                       (lambda (elt)
+                         (eq (cdr elt) lcsym))
+                       auto-mode-alist)))
+         (shortsym (when (stringp mode-name)
+                     (intern-soft (concat (string-trim-right mode-name "/.*")
+                                          "-mode"))))
+         (lcshortsym (when (stringp mode-name)
+                       (intern-soft (downcase
+                                     (concat
+                                      (string-trim-right mode-name "/.*")
+                                      "-mode")))))
+         (shlist (and shortsym
+                      (not (eq shortsym major-mode))
+                      (not (eq shortsym lcsym))
+                      (rassq shortsym auto-mode-alist)
+                      (seq-filter
+                       (lambda (elt)
+                         (eq (cdr elt) shortsym))
+                       auto-mode-alist)))
+         (lcshlist (and lcshortsym
+                        (not (eq lcshortsym major-mode))
+                        (not (eq lcshortsym lcsym))
+                        (rassq lcshortsym auto-mode-alist)
+                        (seq-filter
+                         (lambda (elt)
+                           (eq (cdr elt) lcshortsym))
+                         auto-mode-alist)))
+         (exts (when (or mlist lclist shlist lcshlist)
+                 (seq-union (seq-map #'car lclist)
+                            (seq-union (seq-map #'car mlist)
+                                       (seq-union (seq-map #'car lcshlist)
+                                                  (seq-map #'car shlist))))))
+         (ed-exts (when exts
+                    (seq-map
+                     (lambda (elt)
+                       (concat "*" (string-trim  elt "\\\\" "\\\\'")))
+                     exts))))
+    ed-exts))
+
+(defvar tex--buffers-list nil)
+(defvar-local tex--old-syntax-function nil)
+
+(cl-defmethod xref-backend-references ((_backend (eql 'tex-etags)) identifier)
+  "Find references of IDENTIFIER in TeX buffers and files."
+  (require 'semantic/symref/grep)
+  (defvar semantic-symref-filepattern-alist)
+  (let (bufs texbufs
+             (mode major-mode))
+    (dolist (buf (buffer-list))
+      (if (eq (buffer-local-value 'major-mode buf) mode)
+          (push buf bufs)
+        (when (string-match-p ".*\\.[tT]e[xX]" (buffer-name buf))
+          (push buf texbufs))))
+    (unless (seq-set-equal-p tex--buffers-list bufs)
+      (let* ((amalist (tex--collect-file-extensions))
+             (extlist (alist-get mode semantic-symref-filepattern-alist))
+             (extlist-new (seq-uniq
+                           (seq-union amalist extlist #'string-match-p))))
+        (setq tex--buffers-list bufs)
+        (dolist (buf bufs)
+          (when-let ((fbuf (buffer-file-name buf))
+                     (ext (file-name-extension fbuf))
+                     (finext (concat "*." ext))
+                     ((not (seq-find (lambda (elt) (string-match-p elt finext))
+                                     extlist-new)))
+                     ((push finext extlist-new)))))
+        (unless (seq-set-equal-p extlist-new extlist)
+          (setf (alist-get mode semantic-symref-filepattern-alist)
+                extlist-new))))
+    (let* (setsyntax
+           (punct (with-syntax-table (tex-find-references-syntax-table)
+                    (seq-positions identifier (list ?w ?_)
+                                   (lambda (elt sycode)
+                                     (not (memq (char-syntax elt) sycode))))))
+           (end (and punct
+                     (memq (1- (length identifier)) punct)
+                     (> (length identifier) 1)
+                     (concat "\\("
+                             (regexp-quote
+                              (string (elt identifier
+                                           (1- (length identifier)))))
+                             "\\)")))
+           (beg (and punct
+                     (memq 0 punct)
+                     (concat "\\("
+                             (regexp-quote (string (elt identifier 0)))
+                             "\\)")))
+           (text-mode-hook
+            (if (or end beg)
+                (progn
+                  (tex-xref-syntax-function identifier beg end)
+                  (setq setsyntax (lambda ()
+                                    (setq-local syntax-propertize-function
+                                                tex--xref-syntax-fun)
+                                    (setq-local TeX-style-hook-applied-p t)))
+                  (cons setsyntax text-mode-hook))
+              text-mode-hook)))
+      (unless (memq 'doctex-mode (derived-mode-all-parents mode))
+        (setq bufs (append texbufs bufs)))
+      (when (or end beg)
+        (dolist (buf bufs)
+          (with-current-buffer buf
+            (unless (local-variable-p 'tex--old-syntax-function)
+              (setq tex--old-syntax-function syntax-propertize-function))
+            (setq-local syntax-propertize-function
+                        tex--xref-syntax-fun)
+            (syntax-ppss-flush-cache (point-min)))))
+      (unwind-protect
+          (xref-backend-references nil identifier)
+        (when (or end beg)
+          (dolist (buf bufs)
+            (with-current-buffer buf
+              (when buffer-file-truename
+                (setq-local syntax-propertize-function
+                            tex--old-syntax-function)
+                (syntax-ppss-flush-cache (point-min))))))))))
+
 (make-obsolete-variable 'tex-mode-load-hook
                         "use `with-eval-after-load' instead." "28.1")
 (run-hooks 'tex-mode-load-hook)