emacs-29 f571e8f1bb6: Improve c-ts-mode font-lock for function names (bug#63390)

[Yuan Fu]

branch: emacs-29
commit f571e8f1bb678c52839180e450c2415b31a70516
Author: Yuan Fu <casouri@gmail.com>
Commit: Yuan Fu <casouri@gmail.com>

    Improve c-ts-mode font-lock for function names (bug#63390)
    
    When a function definition has preproc directives in its body, it
    can't correctly parse into a function_definition.  This fix tries to
    recognize this case and highlight the function_declarator correctly.
    
    * lisp/progmodes/c-ts-mode.el:
    (c-ts-mode--font-lock-settings): New rule.
    (c-ts-mode--top-level-declarator): New function.
---
 lisp/progmodes/c-ts-mode.el | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
index b042782efa7..7a57d5adeb1 100644
--- a/lisp/progmodes/c-ts-mode.el
+++ b/lisp/progmodes/c-ts-mode.el
@@ -627,6 +627,13 @@ MODE is either `c' or `cpp'."
 
      (function_definition
       declarator: (_) @c-ts-mode--fontify-declarator)
+     ;; When a function definition has preproc directives in its body,
+     ;; it can't correctly parse into a function_definition.  We still
+     ;; want to highlight the function_declarator correctly, hence
+     ;; this rule.  See bug#63390 for more detail.
+     ((function_declarator) @c-ts-mode--fontify-declarator
+      (:pred c-ts-mode--top-level-declarator
+             @c-ts-mode--fontify-declarator))
 
      (parameter_declaration
       declarator: (_) @c-ts-mode--fontify-declarator)
@@ -750,6 +757,19 @@ For NODE, OVERRIDE, START, END, and ARGS, see
        (treesit-node-start identifier) (treesit-node-end identifier)
        face override start end))))
 
+(defun c-ts-mode--top-level-declarator (node)
+  "Return non-nil if NODE is a top-level function_declarator."
+  ;; These criterion are observed in
+  ;; xterm.c:x_draw_glyphless_glyph_string_foreground on emacs-29
+  ;; branch, described in bug#63390.  They might not cover all cases
+  ;; where a function_declarator is at top-level, outside of a
+  ;; function_definition.  We might need to amend them as we discover
+  ;; more cases.
+  (let* ((parent (treesit-node-parent node))
+         (grandparent (treesit-node-parent parent)))
+    (and (treesit-node-match-p parent "ERROR")
+         (null grandparent))))
+
 (defun c-ts-mode--fontify-variable (node override start end &rest _)
   "Fontify an identifier node if it is a variable.
 Don't fontify if it is a function identifier.  For NODE,