emacs: Add "c" key binding to copy a button link.

* emacs/guix-info.el (guix-info-button-map): New variable.
  (guix-info-button-copy-label): New procedure.
* doc/emacs.texi (Emacs Info buffer): Mention a "c" key binding.
  (Emacs Keymaps): Add 'guix-info-button-map'.

doc/emacs.texi

@@ -290,6 +290,9 @@ emacs, The GNU Emacs Manual}) which can be used to:
 
 @end itemize
 
+It is also possible to copy a button label (a link to an URL or a file)
+by pressing @kbd{c} on a button.
+
 
 @node Emacs Configuration
 @subsection Configuration
@@ -397,6 +400,9 @@ Keymap with specific keys for ``output-info'' buffers.
 @item guix-generation-info-mode-map
 Keymap with specific keys for ``generation-info'' buffers.
 
+@item guix-info-button-map
+Keymap with keys available when a point is placed on a button.
+
 @end table
 
 @node Emacs Appearance

emacs/guix-info.el

@@ -313,7 +313,15 @@ VAL is a list, call the function on each element of this list."
 
 ;;; Buttons
 
+(defvar guix-info-button-map
+  (let ((map (make-sparse-keymap)))
+    (set-keymap-parent map button-map)
+    (define-key map (kbd "c") 'guix-info-button-copy-label)
+    map)
+  "Keymap for buttons in info buffers.")
+
 (define-button-type 'guix
+  'keymap guix-info-button-map
   'follow-link t)
 
 (define-button-type 'guix-action
@@ -350,6 +358,14 @@ VAL is a list, call the function on each element of this list."
             (guix-get-show-entries guix-profile 'info guix-package-info-type
                                    'name (button-label btn))))
 
+(defun guix-info-button-copy-label (&optional pos)
+  "Copy a label of the button at POS into kill ring.
+If POS is nil, use the current point position."
+  (interactive)
+  (let ((button (button-at (or pos (point)))))
+    (when button
+      (kill-new (button-label button)))))
+
 (defun guix-info-insert-action-button (label action &optional message
                                              &rest properties)
   "Make action button with LABEL and insert it at point.