Unbind `M-o` in diff-mode with use-package

Question

I'm trying to unbind M-o in diff-mode with use-package. To do that I've written:

(use-package diff
  :defer t
  :config
  (unbind-key "M-o" diff-mode-map))

Restarted Emacs 25, open a diff file, but M-o still seems to be mapped to diff-goto-source.

I checked diff-mode.el and found out that there is diff-mode-shared-map which seems to have M-o. So I did changed the map in the previous code to:

(use-package diff
  :defer t
  :config
  (unbind-key "M-o" diff-mode-shared-map))

But, it still does not work.

I've used the same snippet for other modes and they seem to work as I expected. Is diff-mode special?

Answer 1

The package^* name is diff-mode, not diff. The use-package package symbol depends on what is provided, see near the bottom of diff-mode.el:

;; provide the package
(provide 'diff-mode)

I checked diff-mode.el and found out that there is diff-mode-shared-map which seems to have M-o.

diff-mode-shared-map binds o, diff-mode-map adds an ESC prefix which is equivalent to M-o. Therefore diff-mode-map is the map you want to modify.

(use-package diff-mode
  :defer t
  :config
  (unbind-key "M-o" diff-mode-map))

Or you could unbind o in diff-mode-shared-map:

(use-package diff-mode
  :defer t
  :config
  (unbind-key "o" diff-mode-shared-map))

This would unbind C-c = o in diff-minor-mode as well.

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^{* This is not the same kind of package as a package.el package)}