custom.el

(defun copy-current-line-position-to-clipboard ()
  "Copy current line in file to clipboard as '</path/to/file>:<line-number>'"
  (interactive)
  (let ((path-with-line-number
         (concat (buffer-file-name) ":" (number-to-string (line-number-at-pos)))))
    (x-select-text path-with-line-number)
    (message (concat path-with-line-number " copied to clipboard"))))

(define-key global-map (kbd "M-l") 'copy-current-line-position-to-clipboard)