Add new function 'string-pixel-width'

* doc/lispref/display.texi (Size of Displayed Text): Mention it.
* lisp/emacs-lisp/shortdoc.el (string): Mention it.

* lisp/emacs-lisp/subr-x.el (string-pixel-width): New function.

1 files changed, 19 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 8d6bb19fd49..6f01209574d 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -441,6 +441,25 @@ is inserted before adjusting the number of empty lines."
      ((< (- (point) start) lines)
       (insert (make-string (- lines (- (point) start)) ?\n))))))
 
+;;;###autoload
+(defun string-pixel-width (string)
+  "Return the width of STRING in pixels."
+  (with-temp-buffer
+    (insert string)
+    (save-window-excursion
+      (let ((dedicated (window-dedicated-p)))
+        ;; Avoid errors if the selected window is a dedicated one,
+        ;; and they just want to insert a document into it.
+        (unwind-protect
+            (progn
+              (when dedicated
+                (set-window-dedicated-p nil nil))
+              (set-window-buffer nil (current-buffer))
+              (car (window-text-pixel-size
+                    nil (line-beginning-position) (point))))
+          (when dedicated
+            (set-window-dedicated-p nil dedicated)))))))
+
 (provide 'subr-x)
 
 ;;; subr-x.el ends here