Make font locking work in mercury-mode

* lisp/progmodes/prolog.el (prolog-font-lock-keywords): Work in
all modes derived from prolog-mode (bug#45747).
(mercury-mode): Set up variables based on the Prolog system (bug#45747).

Copyright-paperwork-exempt: yes

diff --git a/lisp/progmodes/prolog.el b/lisp/progmodes/prolog.el
index c8f6c12a3f00a3d63cf3aa4c3f163c1db579763e..9f5f9ed6d3da5ca60f709019f55ef670e521cf14 100644 (file)
--- a/lisp/progmodes/prolog.el
+++ b/lisp/progmodes/prolog.el
@@ -1201,7 +1201,9 @@ Commands:
 (define-derived-mode mercury-mode prolog-mode "Prolog[Mercury]"
   "Major mode for editing Mercury programs.
 Actually this is just customized `prolog-mode'."
-  (setq-local prolog-system 'mercury))
+  (setq-local prolog-system 'mercury)
+  ;; Run once more to set up based on `prolog-system'
+  (prolog-mode-variables))
 
 \f
 ;;-------------------------------------------------------------------
@@ -2082,7 +2084,7 @@ Argument BOUND is a buffer position limiting searching."
     (delq
      nil
      (cond
-      ((eq major-mode 'prolog-mode)
+      ((derived-mode-p 'prolog-mode)
        (list
         head-predicates
         head-predicates-1