In Emacs, you normally set customizable variables (user options) using
the M-x customize interface (see Easy Customization in GNU Emacs Manual). We recommend this method for most users.
However, it is also possible to set them in your use-package
declarations by using the :custom keyword.
(use-package comint :defer t :custom (comint-buffer-maximum-size 20000 "Increase comint buffer size.") (comint-prompt-read-only t "Make the prompt read only."))
This is better than using setq in a :config block, as
customizable variables might have some code associated with it that
Emacs will execute when you assign values to them. (In Emacs 29 and
later, there is also the new setopt macro that does this for
you.)
Note that the values customized using :custom are not
saved in the standard Emacs custom-file (see Saving
Customizations in GNU Emacs Manual). You should therefore set
each user option using either the :custom keyword or
M-x customize-option command; the latter will save
customized values in the Emacs custom-file. Do not use both
for the same variable, as this risks having conflicting values in your
use-package declaration and your custom-file, which can lead to
problems that are both tricky and tedious to debug.