Support setting region from secondary selection and vice versa

* lisp/mouse.el (secondary-selection-exist-p): New function to
allow callers to tell existence of the secondary selection
in current buffer.
(secondary-selection-to-region): New function to set
beginning and end of the region from those of the secondary
selection.
(secondary-selection-from-region): New function to set
beginning and end of the secondary selection from those of
the region.  (Bug#27530)

* etc/NEWS: Mention the new functions.

2 files changed, 35 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index a685a9fbe92..280ab64f37c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1819,6 +1819,13 @@ can be replicated simply by setting 'comment-auto-fill-only-comments'.
 ** New pcase pattern 'rx' to match against a rx-style regular expression.
 For details, see the doc string of 'rx--pcase-macroexpander'.
 
+---
+** New functions to set region from secondary selection and vice versa.
+The new functions 'secondary-selection-to-region' and
+'secondary-selection-from-region' let you set the beginning and the
+end of the region from those of the secondary selection and vise
+versa.
+
 
 * Changes in Emacs 26.1 on Non-Free Operating Systems
 
diff --git a/lisp/mouse.el b/lisp/mouse.el
index 3f448f018a4..4a4fe52c872 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -1916,6 +1916,34 @@ CLICK position, kill the secondary selection."
          (> (length str) 0)
          (gui-set-selection 'SECONDARY str))))
 
+(defun secondary-selection-exist-p ()
+  "Return non-nil if there is the secondary selection in current buffer."
+  (memq mouse-secondary-overlay (overlays-in (point-min) (point-max))))
+
+(defun secondary-selection-to-region ()
+  "Set beginning and end of the region to those of the secondary selection.
+This puts mark and point at the beginning and the end of the
+secondary selection, respectively.  This works when the secondary
+selection exists and the region does not exist in current buffer;
+the secondary selection will be deleted afterward.
+If the region is active, or the secondary selection doesn't exist,
+this function does nothing."
+  (when (and (not (region-active-p))
+             (secondary-selection-exist-p))
+    (let ((beg (overlay-start mouse-secondary-overlay))
+          (end (overlay-end mouse-secondary-overlay)))
+      (push-mark beg t t)
+      (goto-char end))
+    ;; Delete the secondary selection on current buffer.
+    (delete-overlay mouse-secondary-overlay)))
+
+(defun secondary-selection-from-region ()
+  "Set beginning and end of the secondary selection to those of the region.
+When there is no region, this function does nothing."
+  (when (region-active-p) ; Create the secondary selection from the region.
+    (delete-overlay mouse-secondary-overlay) ; Delete the secondary selection even on a different buffer.
+    (move-overlay mouse-secondary-overlay (region-beginning) (region-end))))
+
 
 (defcustom mouse-buffer-menu-maxlen 20
   "Number of buffers in one pane (submenu) of the buffer menu.