(defun my/buffer-has-major-mode-p (major-mode _sym buffer)
  "Return t if BUFFER has MAJOR-MODE set."
  (eq (buffer-local-value 'major-mode buffer) major-mode))

(defun my/restrict-symbol (mode symbols)
  "Restrict SYMBOLS to a certain major MODE.

Ideally, packages should restrict their own `interactive'
commands to their own mode (see the `interactive' help). However,
this is not common practice so this little facility makes it
easier to restrict symbols to a certain mode. Meaning, the
command will not appear in the M-x menu as a possible completion.

This may be handy if a command may be destructive for a major
mode it wasn't meant for.

Example:

    (my/restrict-symbol 'ledger-mode '(ledger-occur))"
  (dolist (sym symbols)
    (put sym 'completion-predicate
         (apply-partially #'my/buffer-has-major-mode-p mode))))