Allow customizing the Gnus summary thread indicators

* doc/misc/gnus.texi (Summary Buffer Lines): Document them.

* lisp/gnus/gnus-sum.el (gnus-summary-prepare-threads): Use them.

* lisp/gnus/gnus-sum.el (gnus-sum-opening-bracket)
(gnus-sum-closing-bracket, gnus-sum-opening-bracket-adopted)
(gnus-sum-closing-bracket-adopted): New variables.

3 files changed, 80 insertions, 10 deletions

diff --git a/doc/misc/gnus.texi b/doc/misc/gnus.texi
index 4aa07ce3887..cfd3ceda3ff 100644
--- a/doc/misc/gnus.texi
+++ b/doc/misc/gnus.texi
@@ -5028,10 +5028,37 @@ Nothing if the article is a root and lots of spaces if it isn't (it
 pushes everything after it off the screen).
 @item [
 Opening bracket, which is normally @samp{[}, but can also be @samp{<}
-for adopted articles (@pxref{Customizing Threading}).
+for adopted articles (@pxref{Customizing Threading}).  This can be
+customized using following settings:
+
+@table @code
+@item gnus-sum-opening-bracket
+@vindex gnus-sum-opening-bracket
+Opening bracket for normal (non-adopted) articles.  The default is
+@samp{[}.
+
+@item gnus-sum-opening-bracket-adopted
+@vindex gnus-sum-opening-bracket-adopted
+Opening bracket for adopted articles.  The default is @samp{<}.
+
+@end table
+
 @item ]
 Closing bracket, which is normally @samp{]}, but can also be @samp{>}
-for adopted articles.
+for adopted articles.  This can be customised using following settings:
+
+@table @code
+@item gnus-sum-closing-bracket
+@vindex gnus-sum-closing-bracket
+Closing bracket for normal (non-adopted) articles.  The default is
+@samp{]}.
+
+@item gnus-sum-closing-bracket-adopted
+@vindex gnus-sum-opening-bracket-adopted
+Closing bracket for adopted articles.  The default is @samp{>}.
+
+@end table
+
 @item >
 One space for each thread level.
 @item <
diff --git a/etc/NEWS b/etc/NEWS
index 26e4b8514fc..9aa735da726 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -511,6 +511,19 @@ tags to be considered as well.
 ** Gnus
 
 +++
+*** New user options to customise the summary line specs %[ and %].
+Four new options introduced in customisation group
+'gnus-summary-format'.  These are 'gnus-sum-opening-bracket',
+'gnus-sum-closing-bracket', 'gnus-sum-opening-bracket-adopted', and
+'gnus-sum-closing-bracket-adopted'.  Their default values are '[', ']',
+'<', '>' respectively. These variables control the appearance of '%['
+and '%]' specs in the summary line format.  '%[' will normally display
+the value of 'gnus-sum-opening-bracket', but can also be
+'gnus-sum-opening-bracket-adopted' for the adopted articles.  '%]' will
+normally display the value of 'gnus-sum-closing-bracket', but can also
+be 'gnus-sum-closing-bracket-adopted' for the adopted articles.
+
++++
 *** New user option 'gnus-paging-select-next'.
 This controls what happens when using commands like 'SPC' and 'DEL' to
 page the current article.  If non-nil (the default), go to the
diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index 9432eefcb4d..9488b324878 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -1460,8 +1460,8 @@ the normal Gnus MIME machinery."
     (?I gnus-tmp-indentation ?s)
     (?T (if (= gnus-tmp-level 0) "" (make-string (frame-width) ? )) ?s)
     (?R gnus-tmp-replied ?c)
-    (?\[ gnus-tmp-opening-bracket ?c)
-    (?\] gnus-tmp-closing-bracket ?c)
+    (?\[ gnus-tmp-opening-bracket ?s)
+    (?\] gnus-tmp-closing-bracket ?s)
     (?\> (make-string gnus-tmp-level ? ) ?s)
     (?\< (make-string (max 0 (- 20 gnus-tmp-level)) ? ) ?s)
     (?i gnus-tmp-score ?d)
@@ -3748,6 +3748,30 @@ buffer that was in action when the last article was fetched."
       (inline
         (gnus-summary-extract-address-component gnus-tmp-from))))))
 
+(defcustom gnus-sum-opening-bracket "["
+  "With %[ spec, used to identify normal (non-adopted) articles."
+  :version "28.1"
+  :type 'string
+  :group 'gnus-summary-format)
+
+(defcustom gnus-sum-closing-bracket "]"
+  "With %] spec, used to identify normal (non-adopted) articles."
+  :version "28.1"
+  :type 'string
+  :group 'gnus-summary-format)
+
+(defcustom gnus-sum-opening-bracket-adopted "<"
+  "With %[ spec, used to identify adopted articles."
+  :version "28.1"
+  :type 'string
+  :group 'gnus-summary-format)
+
+(defcustom gnus-sum-closing-bracket-adopted ">"
+  "With %] spec, used to identify adopted articles."
+  :version "28.1"
+  :type 'string
+  :group 'gnus-summary-format)
+
 (defun gnus-summary-insert-line (header level current undownloaded
                                  unread replied expirable subject-or-nil
                                  &optional dummy score process)
@@ -3805,8 +3829,14 @@ buffer that was in action when the last article was fetched."
                        (1+ (match-beginning 0)) (1- (match-end 0))))
            (t gnus-tmp-from)))
          (gnus-tmp-subject (mail-header-subject gnus-tmp-header))
-         (gnus-tmp-opening-bracket (if gnus-tmp-dummy ?\< ?\[))
-         (gnus-tmp-closing-bracket (if gnus-tmp-dummy ?\> ?\]))
+         (gnus-tmp-opening-bracket
+          (if gnus-tmp-dummy
+              gnus-sum-opening-bracket-adopted
+            gnus-sum-opening-bracket))
+         (gnus-tmp-closing-bracket
+          (if gnus-tmp-dummy
+              gnus-sum-closing-bracket-adopted
+            gnus-sum-closing-bracket))
          (inhibit-read-only t))
     (when (string= gnus-tmp-name "")
       (setq gnus-tmp-name gnus-tmp-from))
@@ -5439,10 +5469,10 @@ or a straight list of headers."
             (if (and (eq gnus-summary-make-false-root 'adopt)
                      (= gnus-tmp-level 1)
                      (memq number gnus-tmp-gathered))
-                (setq gnus-tmp-opening-bracket ?\<
-                      gnus-tmp-closing-bracket ?\>)
-              (setq gnus-tmp-opening-bracket ?\[
-                    gnus-tmp-closing-bracket ?\]))
+                (setq gnus-tmp-opening-bracket gnus-sum-opening-bracket-adopted
+                      gnus-tmp-closing-bracket gnus-sum-closing-bracket-adopted)
+              (setq gnus-tmp-opening-bracket gnus-sum-opening-bracket
+                    gnus-tmp-closing-bracket gnus-sum-closing-bracket))
             (if (>= gnus-tmp-level (length gnus-thread-indent-array))
                 (gnus-make-thread-indent-array
                  (max (* 2 (length gnus-thread-indent-array))